Nichollsemys baieri Brinkman et al., 2006

Menon, Juliette C. L., Brinkman, Donald B., Hermanson, Guilherme, Joyce, Walter G. & Evers, Serjoscha W., 2024, New insights into the early morphological evolution of sea turtles by re-investigation of NiCHollSemYS baieri, a three-dimensionally preserved fossil stem chelonioid from the Campanian of Alberta, Canada, Swiss Journal of Palaeontology (27) 143 (1), pp. 1-51 : 6-37

Nichollsemys baieri Brinkman et al., 2006

Type material: TMP 1997.99 .1 (holotype), a skull preserved in a concretionary nodule (Brinkman et al., 2006: Figs. 2–5 ) ( Fig. 1 ) .

Type locality: Along Chin Coulee , South of Taber, southern Alberta, Canada (Brinkman et al., 2006) .

Type horizon: A glacial till containing concretions typical of the Bearpaw Formation, Bearpaw Shale, Late Campanian, Late Cretaceous.

Revised diagnosis

Nichollsemys baieri can be identified as belonging to Pan-Chelonioidea by having a roughly triangular ventral exposure of the parabasisphenoid, with both ventral pterygoid ridges forming a reverse “V” pattern alongside the parabasisphenoid and basioccipital ventral exposures; a posteriorly retracted processus inferior parietalis (Gaffney, 1979); a high dorsum sellae with a distinct dorsomedian process between both clinoid processes.

Nichollsemys baieri shares with early protostegids (e.g., Santanachelys gaffneyi , Bouliachelys suteri , Rhinochelys pulchriceps ) and some crown chelonioids (e.g., Allopleuron hofmanni ) the presence of a splenial, which is well-developed with a surface area equivalent to that of the prearticular. It differs from protostegids by having a laterally closed foramen palatinum posterius; the presence of foramina praepalatina; by having a large medial jugal process contacting the palatine; and having a dorsoventral elongate foramen nervi trigemini.

Nichollsemys baieri shows many similarities with the stem chelonioid Toxochelys spp. It resembles the species by displaying the same striated ornamentation pattern of skull roof elements such as the parietal and postorbital (see Williston, 1903, pl. 18; Matzke, 2009); a robust processus pterygoideus externus terminated by a large flange; a deep temporal emargination that opens anteriorly to the foramen stapedio-temporale in dorsal view; and the presence of foramina praepalatina. However, Nichollsemys baieri clearly differs from Toxochelys latiremis and Toxochelys moorevillensis by showing an elongated rod-like rostrum basisphenoidale (the rostrum is flat and short in Toxochelys ) and by lacking an epipterygoid (which is present in Toxochelys spp. ). Nichollsemys baieri shares with the stem chelonioids Toxochelys latiremis , Toxochelys moorevillensis and Porthochelys laticeps a narrow interorbital bar in dorsal view, and large foramina palatinum posterius, the latter of which is likely a symplesiomorphic feature as it is present in outgroups but absent in crown chelonioids. Nichollsemys baieri differs from Porthochelys laticeps by having a much narrower cranium and narrower dentary triturating surface.

Nichollsemys baieri shares several features with crown chelonioids that differentiate it from protostegids, Toxochelys spp. and Porthochelys laticeps , specifically by lacking nasals. Te aforementioned presence of a rod-like rostrum basisphenoidale of the parabasisphenoid is also a typical crown chelonioid feature, although it is present in at least some protostegids (e.g., Rhinochelys pulchriceps ). It differs from many crown chelonioids (e.g., Allopleuron hofmanni , cheloniids) and also the indeterminate, possible crown chelonioid Ctenochelys spp. by lacking the inclusion of the vomer in the triturating surface and showing larger flanges on the external process of the pterygoid, and from crown chelonioids by the presence of an elongate anterior frontal process. Similarities of Nichollsemys baieri with the dermochelyids Dermochelys coriacea and Allopleuron hofmanni that are not shared with cheloniids include having a "T"-shaped quadratojugal. Nichollsemys baieri resembles Dermochelys coriacea by having strongly anteriorly protruding "V" shaped processes of the frontals into the prefrontals. It shares with Allopleuron hofmanni an anteroposteriorly very short processus inferior parietalis that contacts the crista pterygoidei posteriorly to the anterior end of the basisphenoid; the presence of a large fenestra from the internal carotid artery canal into the canalis cavernosus; and a deep cheek emargination that reaches the height of the ventral third of the orbit. Nichollsemys baieri differs from Dermochelys coriacea by lacking tooth-like processes along the cranial labial ridge and by the presence of the coronoid. It differs from Dermochelys coriacea and Allopleuron hofmanni by lacking an extensive contact between jugal and squamosal and showing a postorbital-quadratojugal contact.

Nichollsemys baieri is unique among pan-chelonioids by having by a very small ventral exposure of the parabasisphenoid, a strongly reduced sella turcica, and a dorsally notched labial margin in the symphysis of the dentary.

Description and comparisons

General appearance

Te skull of TMP 1997.99.1 is fairly complete, with nearly the entire cranium and the mandible being preserved

( Fig. 1 ; Brinkman et al., 2006). Most sutures are clear in the slice data so that we could produce a full digital reconstruction of each cranial bone ( Fig. 2 ). Te posterior part of the parietal and the crista supraoccipitalis are broken. Te squamosals are not articulated anymore and still floating in the matrix surrounding the upper temporal emargination (see below). On the right side, the posterior part of the jugal and postorbital bones are not preserved. In addition, most of the right quadratojugal is not preserved. Only a fragment of it remains in disarticulation still floating in the matrix. Te cranium has a length of 111.5 mm from the tip of the snout to the base of the occipital condyle and a maximum width of 96.5 mm between the quadratojugals, as measured with the straight-line measurement tool of Mimics. Cranial scales sulci are not visible despite good preservation of the external surface on the skull roof that bears a radiated ornamentation pattern (Brinkman et al., 2006) similar to the condition found in Toxochelys latiremis ( Fig. 1 ; Williston, 1903: pl. 18). TMP 1997.99.1 shows potential internal “soft tissue” preservation, particularly the preservation of a semicircular duct (see below). Te snout is slightly elongated but blunt. In dorsal view, the interorbital bar is narrow like in Toxochelys latiremis ( Matzke, 2009) and the orbits are dorsolaterally oriented ( Fig. 1A ). Te maxilla shows a slightly grainy exterior surface below the ascending process, possibly indicating the extent of the rhamphotheca. Te cheek emargination, which reaches the lower third of the orbit, is deep for a marine turtle. Te braincase and the otic region are well preserved (see below). Tere is no evidence for the presence of ossified epipterygoids, as any corresponding sutures that would indicate their presence are not visible, even though all other sutures are easily discerned. Te specimen has a pair of small and anteroposteriorly elongated foramina praepalatina, a pair of large and anteroposteriorly elongated foramina palatinum posterius on the palate (Brinkman et al., 2006), and a pair of large and rounded foramina orbito-nasale between the orbit and the nasal cavity. A secondary palate is absent ( Fig. 1 ). Te mandible is almost completely preserved, showing a small abraded area at its posterior left end. Te anterior tip of the dentary shows unusual symmetrical notch (see below). Te mandible is still in articulation with the cranium ( Fig. 1 ).

Nasal

Te dorsal margins of the external nares of TMP 1997.99.1 are fully intact ( Figs. 2 , 3 ). It is therefore apparent that nasals are absent (Brinkman et al., 2006). Te absence of nasals contrasts with the condition found in the stem chelonioids Toxochelys latiremis and Porthochelys laticeps ( Matzke, 2009; Zangerl, 1953a) and early protostegids (Evers et al., 2019a; Hooks, 1998; Kear & Lee, 2006; Raselli, 2018).

Prefrontal

Te prefrontals of TMP 1997.99.1 are fairly complete

( Figs. 2 , 3 , 4 ). Te posterior part of the dorsal surface of the right prefrontal is damaged at the articulation with the frontal along the orbit. In addition, the anterolateral border of the right prefrontal is damaged at the suture with the maxilla. Nevertheless, the full morphology of the prefrontal can be appreciated. Te prefrontal contacts the maxilla laterally, the frontal posteriorly, its counterpart medially, and the descending process contacts the vomer and the palatines posteroventrally ( Figs. 2 , 3 , 4 ). Te prefrontal forms the dorsal border of the foramen orbito-nasale and participates anterodorsally in the orbital margin ( Fig. 4 ). Te prefrontal forms the dorsal border of the external nares

( Fig. 2A, B ). Te prefrontals jointly contribute to the anterior two thirds of the interorbital bar. In dorsal view, the prefrontals are elongated but narrower than in moderns cheloniids ( Fig. 2A, B ). A narrow interorbital bar occurs in Toxochelys latiremis ( Matzke, 2009) , Porthochelys laticeps ( Williston, 1903) , and in Mexichelys coahuilaensis (Brinkman et al., 2009) . Te medial suture between both prefrontals of TMP 1997.99.1 is less than half of the total length of these bones, as a full midline contact is hindered by a deep median process formed by the frontals ( Fig. 2A, B ). Te contribution of the prefrontal to the external naris is greatly reduced in comparison to other chelonioids without nasals, in which the prefrontal often forms a small part of the lateral margin of the external naris. Te prefrontal of TMP 1997.99.1 forms the lateral margin of the keyhole shaped fissura ethmoidalis ( Fig. 3A, B ).

Frontal

Te left frontal of TMP 1997.99.1 is completely preserved but the right one is damaged near the orbit ( Figs. 2 , 3 , 4 , 5 ). Te frontal participation to the skull roof is just slightly smaller than that of the parietal ( Fig. 2A, B ), in contrast to modern chelonioids that exhibit a significantly larger parietal contribution than their frontal one. Te frontal of TMP 1997.99.1 contacts the prefrontal anteriorly, meets the postorbitals posterolaterally and the parietals posteriorly, and its counterpart at the midline ( Fig. 2A, B ). Te frontal laterally makes an important contribution to the orbit about equal in size to that of the postorbital ( Figs. 2 , 3 , 4 ). Tis is similar to the early protostegid Rhinochelys pulchriceps and stem chelonioids Toxochelys latiremis ( Matzke, 2009) and Porthochelys laticeps , but different to the frontal contribution to the orbit in cheloniids, which is often small (e.g., Chatterji et al., 2021; Jones et al., 2012). Te frontals of TMP 1997.99.1 jointly form a pointed anterior process that deeply inserts between the prefrontals ( Fig. 5 ). Te process is longer ventrally than dorsally, such that the frontal deeply underlaps the prefrontal and extends through the fissura ethmoidalis into the nasal capsule. Te pointed anterior process is longer and narrower than in other chelonioids. On the ventral surface of the frontal of TMP 1997.99.1, low and blunt ridges, the crista cranii, are present that anteriorly connect to the fissura ethmoidalis and posteriorly to the descending process of the parietal ( Figs. 4 , 5 ). Te crista cranii form the sulcus olfactorius, a ventrally open trough that allows the transmission of the olfactory nerve from the braincase to the nasal capsule (e.g., Evers et al., 2019b; Ferreira et al., 2022; Gaffney, 1979). Te crista cranii of Nichollsemys baieri resemble those of most pan-chelonioids, including Toxochelys latiremis (USNM 11558), but are higher than those of Dermochelys coriacea and lack the midline contact that is seen in some early protostegids (Evers et al., 2019a).

Parietal

Te parietals of TMP 1997.99.1 are generally well preserved, but their posterior margins show signs of damage, particularly the left one ( Figs. 2 , 4 , 5 ). As preserved, the parietals initially suggest the presence of deep upper temporal emarginations that expose the foramen stapedio-temporale on the otic process in dorsal view ( Fig. 2A, B ), but the entire bone margin, and the articulation with the supraoccipital, is too damaged to document the depth of the emargination with certainty. However, despite these damaged margins of the parietal, there is good evidence for deep posterior emarginations from the squamosals, which are completely preserved but disarticulated (see below). Te dorsal margin of the squamosal is strongly anteromedially curved, as is only the case in taxa with relatively deep emarginations. Tus, we interpret that the damage along the posterior parietal margins is only relatively superficial and that the posterior emargination reached beyond the level of the foramen stapedio-temporale, as also is the case in Toxochelys latiremis ( Matzke, 2009; see the description of the squamosal below for a more detailed justification). Tis greatly contrasts with the condition of TMP 2000.55.1, another skull referred to Nichollsemys baieri by Brinkman et al., (2006: Fig. 3 ), which seems to lack any trace of temporal emarginations and which is one of the reasons why we believe this specimen to belong to a different taxon. We therefore here and elsewhere focus on the descriptions of TMP 1997.99.1, the holotype of Nichollsemys baieri , and do not consider TMP 2000.55.1 as part of the paradigm for Nichollsemys baieri .

Te parietal of TMP 1997.99.1 is a large bone that is constituted of two plates ( Figs. 2 , 4 , 5 ). Te horizontal plate contributes to the dorsal skull roofing and the upper temporal emargination, while the descending plate forms the processus inferior parietalis ( Figs. 5 , 6 ). Anteriorly, the horizontal plate contacts the frontal along an oblique suture, which traverses posterolaterally from the skull midline to the triple junction with the postorbital

( Fig. 2A, B ). Te parietal contacts the postorbital laterally and meets the other parietal medially alongside the interparietal suture ( Fig. 2A, B ). Due to the disarticulation of the squamosals and to the damaged posterior margins of the postorbitals and parietals, it is not possible to determine with certainty if the parietal contacted the squamosals posterolaterally. However, when the squamosals are rotated back into what we believe to be their original position, a contact is absent by a short distance (see squamosal, below). Nevertheless, we cannot exclude that this contact was present, as the probably closely related stem chelonioid Toxochelys latiremis has a peculiar posterolaterally directed parietal process that extends along the posterior postorbital margin ( Matzke, 2009), thereby excluding the latter from the temporal emargination. In Toxochelys latiremis , this process establishes a contact with the squamosal (e.g., Matzke, 2009), but the broken margin of the parietal in TMP 1997.99.1 does not allow us to verify if this process may have been present. Te horizontal plate of the parietal of TMP 1997.99.1 is short, roughly trapezoidal, with its longest edge forming the interparietal suture ( Fig. 2A, B ). A pineal foramen is clearly absent. However, fine striations that dorsally cover the parietals radiate from the midpoint of the interparietal suture ( Fig. 1A ). Te parietals fully overlap the supraoccipital in the skull roof for their preserved posterior length ( Fig. 2A, B ). Anterior to the supraoccipital, the parietals are ventrally strongly constricted inside the cavum cranii, forming a deep, midline trough ( Figs. 5 , 6 ). In an endocast of the braincase, this would show as a positively protruding, dorsal medial ridge, the ‘cartilaginous rider’ of Zangerl (1960) ( Fig. 5 ), which has shown to be a space for the anterior, cartilaginous parts of the supraoccipital ( Werneburg et al., 2021).

Te processus inferior parietalis ( Figs. 5 , 6 ) forms nearly the entire secondary lateral braincase wall. Te processus inferior parietalis has a part anterior to the trigeminal foramen, which extends to the pterygoid ventrally

( Figs. 5 , 6 ). In most turtles, including extant chelonioids, the processus inferior parietalis articulates with the pterygoid along a ridge called the crista pterygoidei, which is a dorsally ascending plate of the pterygoid that also forms parts of the secondary lateral braincase wall. In TMP 1997.99.1, the crista pterygoidei is extremely low ( Fig. 6 ), such that the parietal directly contacts the dorsal surface of the central part of the pterygoid. Te processus inferior parietalis anterior to the trigeminal foramen is anteroposteriorly short, creating a large foramen interorbitale ( Fig. 5 ), as in all chelonioids except Dermochelys coriacea , which lacks an osseous processus inferior parietalis completely. Te anterior part of the processus inferior parietalis is mediolaterally thin and its lateral surface bears no ridge ( Fig. 6B ). Tis contrasts with extant chelonioids, in which the lateral surface of this part of the processus inferior parietals is laterally thickened. Te anterior part of the processus inferior parietalis of TMP 1997.99.1 also forms the anterior margin of the large, ovoid, and almost vertically oriented trigeminal foramen ( Fig. 6B ). Te posterior rim of the foramen is formed by the prootic, the ventral rim by the pterygoid, as in protostegids and chelonioids (Chatterji et al., 2021; Evers et al., 2019a; Jones et al., 2012; Raselli, 2018). A posteroventral process of the parietal along the posterodorsal margin of the trigeminal foramen is absent in TMP 1997.99.1 ( Fig. 6B ). Posterodorsal to the trigeminal foramen, the processus inferior parietalis overlaps the prootic and supraoccipital ( Fig. 6B ).

Postorbital

Te posterior part of the postorbitals is difficult to differentiate from the jugals in the CT scans, due to the shingled nature of the contact of these bones. However, the suture lines are quite distinct in external view of the specimen ( Fig. 1C, D ), in part due to the striations on both bones, which have different trajectories. Tus, we tried to find and model the sutures in the CT scan along the expected sutural trajectory from the external specimen’s view.

Te postorbital of TMP 1997.99.1 ( Figs. 2 , 3 , 4 ) is an elongated, roughly rectangular bone ( Fig. 2A, B ). Its surface is sculptured by fine striations that originate from a point just behind the orbit ( Fig. 1C, D ). It contributes to the skull roof, from the posterior rim of the orbit to the anterior rim of the squamosal ( Figs. 2 , 3 , 4 ). Te postorbital contacts the jugal anterolaterally, the frontal anteromedially, the parietal posteromedially, and the squamosal posteriorly ( Figs. 2 , 3 , 4 ). A contribution to the rim of the upper temporal emargination is not preserved, but it likely existed as the squamosal probably lacked a parietal contact (see parietal and squamosal). A small gap separates the ventral margin of the posterior process of the postorbital and the dorsal margin of the quadratojugal on both skull sides ( Fig. 4 ), but the edges of these bones all appear slightly broken or eroded. It is possible, and maybe even likely, that the postorbitalquadratojugal contact was originally present. However, it is also possible that this contact, if existent, was limited to the internal side of the cheek, and not expressed on the lateral skull surface. Tis would suggest a small jugal–squamosal contact, which is potentially indicated on the right side of the skull by a shallow facet on the ventral part of the posterior process of the postorbital, which seems to indicate the former extent of the now broken right jugal. Te postorbital contributes to the posterior orbital margin and constitutes approximatively one quarter of the orbital perimeter. Te orbital margin formed by the postorbital is well rounded ( Figs. 2 , 3 , 4 ) and lacks an “eyebrow” process that is seen in many extant chelonioids (e.g., Lepidochelys olivacea : SMNS 11070; Chelonia mydas : NHMUK 1969.766). Te posterior margin of the orbit is slightly thickened on the visceral side of the postorbital ( Fig. 3A, B ).

Jugal

Te jugals are intact with the exception of damage to the posterior process of both bones ( Figs. 2 , 3 , 4 ). Te jugal of TMP 1997.99.1 is an elongate bone that forms much of the cheek region ( Fig. 4 ). It externally contacts the maxilla anteroventrally, the postorbital dorsally, and the quadratojugal posteroventrally ( Fig. 4 ). We are unable to establish a clear contact between the quadratojugal and postorbital despite the general articulation and exquisite preservation of the skull (see postorbital above). If this absence of contact is genuine, then the squamosal and jugal must have had a small contact. A squamosal– jugal contact is rare among turtles, but typically occurs in dermochelyids, including the probable fossil dermochelyid Allopleuron hofmanni ( Mulder, 2003) . Its potential presence in TMP 1997.99.1 may thus have systematic relevance. However, the squamosal–jugal contact in dermochelyids is at least partially formed by an anteroventral process of the squamosal, which inserts anteriorly between the quadratojugal and postorbital along the lateral skull surface. Such a process is clearly absent in the well-preserved squamosals of TMP 1997.99.1 (see below). Tus, even if a small squamosal– jugal contact was originally present, this may not be homologous with the dermochelyid condition. Although the contact is usually absent in stem chelonioids such as Toxochelys latiremis ( Matzke, 2009) and extant cheloniids, it can occur in some extant specimens as individual variation (e.g., Caretta caretta : MHNLM EMV 2004.3.22; Supplementary file S1: Fig. S1 ).

Te anterior half of the jugal of TMP 1997.99.1 forms a narrow jugal arch posteroventral to the orbit ( Fig. 4 ). Tis arch participates in the cheek emargination ventrally and forms the posteroventral orbital rim dorsally. Te jugal participation to the orbital rim is about one quarter of the total perimeter ( Fig. 4 ) and thus more extensive than in extant cheloniids and Dermochelys coriacea (Chatterji et al., 2021, 2022; Gaffney, 1979; Jones et al., 2012; Seago, 1979), which already have comparatively large jugals. In the stem chelonioid Toxochelys latiremis , the jugal participation to the orbit is much smaller ( Matzke, 2009), and more similar to non-chelonioid turtles. Te ventral rim of the jugal arch of TMP 1997.99.1 forms the anterior two thirds of the cheek emargination ( Fig. 4 ), which is moderately deep in comparison to most turtles generally, but notably deep for a marine taxon more widely, or pan-chelonioids specifically. Te cheek emargination of TMP 1997.99.1 stretches from the posterior end of the jugalmaxilla contact to the posterior end of the quadratojugal, forming a wide concave margin that extends dorsally to the level of the lower third of the orbit. Tis condition is similar to Toxochelys latiremis (see Matzke, 2009: Figs. 3 , 4 ) and somewhat deeper than in dermochelyids, including Allopleuron hoffmanni ( Mulder, 2003) and Dermochelys coriacea .

Medially, the jugal of TMP 1997.99.1 is extended by a broad and hooked-shaped medial process ( Fig. 2C, D ). Tis medial process is large, which is typical for most turtles, contacts the maxilla anteriorly, has a broad contact with the pterygoid posteriorly ( Fig. 2C, D ), but a contact with the palatine is clearly absent. Te later contact is prevented both by the presence of a large foramen palatinum posterius and a pterygoid–maxilla contact along the posterior margin of this foramen

( Fig. 2C, D ). Tese contacts are basically identical to the morphology of Toxochelys latiremis (AMNH 5118). Tis can even be appreciated by the figures of Matzke (2009: Fig. 2a, b ), although the author attests the contact to be present, citing their reconstruction drawing as evidence ( Matzke, 2009: Fig. 22). Te presence of a plesiomorphically large medial process of the jugal in Toxochelys latiremis and TMP 1997.99.1 contrasts with the condition in cheloniids, in which this process is generally less developed than in TPM 1997.99.1 and in which a contact with the palatine is present, due to the absence of a foramen palatinum posterius. In protostegids and dermochelyids, the medial process of the jugal is entirely reduced (e.g., Evers & Benson, 2019) and its absence prevents the possibility of the jugal contacting the palatine or pterygoid.

Quadratojugal

Te right quadratojugal of TMP 1997.99.1 is poorly preserved ( Fig. 4C, D ). Only a little fragment is preserved floating in the matrix. Te left quadratojugal, by contrast, is essentially complete ( Fig. 4A, B ). Te quadratojugal of TMP 1997.99.1 is situated in the posterior region of the cheek area and participates in the posterior half of the cheek emargination ( Figs. 2 , 3 , 4 ). Te bone is composed of two principal parts, an expanded dorsal plate that forms parts of the temporal region and an elongated, ventrally descending process ( Fig. 4 ). Te dorsally expanded part is anterolaterally overlapped by the jugal. Posterior to the jugal contact, the quadratojugal certainly contacted the disarticulated squamosal, but a contact with the postorbital dorsally is less clear ( Fig. 4 ). Tis contact is not preserved on neither side of the skull, making it possible that this represents a genuine absence. In this case, a squamosal–jugal contact would be present instead. In Allopleuron hofmanni and Dermochelys coriacea , the quadratojugal is excluded from contacting the postorbital by an extensive contact between the squamosal and the jugal ( Mulder, 2003). In addition, the quadratojugal of TMP 1997.99.1 contacts the quadrate along its descending process, which overlays the quadrate laterally ( Fig. 3A, B ), thereby forming the anterolateral margin of the cavum tympani ( Fig. 4 ). Te vertical ventral quadratojugal process of TMP 1997.99.1 is rod-shaped, resulting in an overall “T-shaped” quadratojugal that is also found in Allopleuron hofmanni ( Mulder, 2003) and Dermochelys coriacea (Gaffney, 1979; Nick, 1912; Seago, 1979), which contrasts with the reconstruction of the ventral quadratojugal process of the original description (Brinkman et al., 2006). Tis shape is due to the absence of the anterior prolongation of the anterior part of the quadratojugal that occurs in cheloniids and protostegids, and which gives a “L” shape to this bone. Te deep ventral process of the quadratojugal also contributes to the cheek emargination ( Fig. 4 ), which is generally deeper than in most other chelonioids. Despite the similarity in the “T-shape”, the quadratojugal of TMP 1997.99.1 differs from those of dermochelyids in being less clearly integrated into the cavum tympani. Although it certainly forms its anterior margin, it does not extend medially to form parts of the internal, anterior surface of the cavum tympani, as is the case in Allopleuron hofmanni (NHMUK R4213) and Dermochelys coriacea (UMZC R3031). All Toxochelys latiremis specimens that preserve the quadratojugal are severely crushed, making it difficult to fully interpret this morphology (e.g., Matzke, 2009). However, it seems possible that its original shape was similar to that of Nichollsemys baieri , based on our examinations of 3D models of AMNH 5118. Tis specimen preserves a thin ventral quadratojugal process as in TMP 1997.99.1. Additionally, the medial side of the cheek region of AMNH 5118 shows that the quadratojugal is anterodorsally expanded underneath the jugal to a similar degree and shape as TMP 1997.99.1. However, possible contributions to the actual cavum tympani of AMNH 5118 are not discernable based on the fossil.

Squamosal

In TMP 1997.99.1, the squamosals are not preserved in articulation ( Figs. 2A, B , 3C, D ; Brinkman et al., 2006). Tey are floating in the surrounding matrix within the upper temporal fossa ( Fig. 3C, D ). Tis preservation prevents the identification of all contacts with certainty. Indeed, there is particular ambiguity regarding the possible contacts with the jugal, the parietal, and opisthotic, all of which are discussed below. Contacts with the quadrate, the quadratojugal, and postorbital, however, were certainly present.

Te squamosal of TMP 1997.99.1 has an elongated, tapering anterior process ( Fig. 2A, B ), which extends along the posterior margin of the upper temporal emargination. Te lateral surface of the anterior process hereby underlaps the postorbital, and, to a lesser extent, the quadratojugal. Tis strong overlapping articulation between postorbital and squamosal differs from the articulation seen in extant chelonioids, in which the squamosal is anteriorly broad (instead of tapering), and in which the contacts with the postorbital and quadratojugal are simple contacts that abut along their respective margins.

We produced a partial skull reconstruction by digitally re-articulating the squamosal into its original position

( Fig. 7 ). As there is uncertainty to reconstructions, we produced two alternatives, which primarily differ in the angle with which the squamosal caps the cavum tympani

( Fig. 7 ). Tis leads to different interpretations regarding the upper (= posterodorsal) temporal emargination. In particular, when the anterior process is relatively strongly downturned ( Fig. 7A, C, D ), the emargination becomes quite deep ( Fig. 7C ). Tis implies a deeper emargination than in Toxochelys latiremis ( Matzke, 2009) , but still a less deep emargination than in extant or fossil members of the sister clade of chelonioids, Chelydroidea (e.g., Emarginachelys cretacea : Whetstone, 1978; Leiochelys tokaryki : Brinkman et al., 2022; Protochelydra zangerli : Erickson, 2010; extant chelydrids: Joyce, 2016; extant kinosternids: Joyce & Bourque, 2016). Tis reconstruction, however, also implies a slightly larger gap in the capping of the cavum tympani along the quadrate contact ( Fig. 7E ). An alternative orientation of the squamosal of TMP 1997.99.1 ( Fig. 7B, D, F ), which minimizes the aforementioned cavum tympani gap ( Fig. 7F ), results in a strongly dorsally oriented anterior squamosal process

( Fig. 7B ). Tis reconstruction implies that more bone of the postorbital (and parietal) is posteriorly missing in comparison to the other reconstruction. It also implies a less deep upper temporal emargination ( Fig. 7D ). Tis is closer to the condition of Toxochelys latiremis ( Matzke, 2009) . However, this also implies the upper temporal emargination margin to be strongly posteroventrally inclined ( Fig. 7B ), which is generally not observed in turtles, including Toxochelys latiremis ( Matzke, 2009) . Tus, we prefer the first-described reconstruction with a lower angle of the anterior process.

A number of morphological statements can be made about the squamosal of TMP 1997.99.1 regardless of exact orientation. Te potential jugal–squamosal contact depends on the margins of the jugal, quadratojugal, and postorbital in this area and cannot be determined with certainty. A contact with the parietal is extremely unlikely, when a “regular” parietal morphology is assumed as the tapering tip of the squamosal of TMP 1997.99.1 seems fully preserved. However, there is one way in which a squamosal–parietal contact could have existed: Toxochelys latiremis has a highly unusual parietal morphology ( Matzke, 2009) with a very long posteromedial process that reaches the squamosal. We can verify its presence based on examinations of the fossil and 3D models of AMNH FARB 5118. Given the many similarities between Toxochelys latiremis and TMP 1997.99.1, it is certainly possible that the same unusual process existed in the latter.

Te ventral surface of posterior part of the squamosal of TMP 1997.99.1 certainly capped the cavum tympani formed by the quadrate ( Figs. 4 , 7A, B ). However, although this surface is slightly concavely curved, it has no deeper cavity of excavation that would suggest the presence of an antrum postoticum distinct from the cavum tympani. Tis posterodorsal expansion of the cavum tympani is typically present in the squamosal of turtles (Gaffney, 1979), but reduced in chelonioids. A distinct antrum postoticum is already absent in Toxochelys latiremis , although the definition of this structure varies slightly between authors. For example, Matzke (2009) already accepts the capping of the quadrate by the squamosal as indicating the presence of a ‘true’ antrum postoticum in Toxochelys latiremis , even when the finger-like extension into the bone is absent.

Te squamosal of TMP 1997.99.1 forms a relatively short, medially projecting, horizontally oriented sheet of bone that lies on the dorsal surface of the quadrate within the floor of the upper temporal fossa ( Fig. 7C, D ). Tis medial sheet, however, does not project over the entire quadrate surface to reach the opisthotic. We are unaware of any extant or fossil turtle that lacks this contact and the morphology of TMP 1997.99.1 specifically contrasts with the condition of extant chelonioids, in which there always is a broad squamosal–opisthotic contact within the temporal fossa. Tis morphology constitutes another similarity to Toxochelys latiremis , in which the squamosal facets on the quadrate (e.g., AMNH 1496) or the articulated squamosals (e.g., AMNH 5118) show that the contact with the opisthotic was a lot smaller than in extant chelonioids, and limited to the posterior tip of the paroccipital process, as we also propose was the case for TMP 1997.99.1. Lastly, the posterior end of the squamosal of TMP 1997.99.1 is again similar to Toxochelys latiremis , but differs from the morphology of extant chelonioids. In TMP 1997.99.1 and extant cheloniids, the dorsal margin of the upper temporal emargination forms a rim that curved posteromedially toward the paraoccipital process

( Fig. 7E, F ). Tis morphology defines a posterolateral surface of the squamosal. In TMP 1997.99.1 and Toxochelys latiremis (AMNH 5118), this surface bears a single, shallow fossa ( Fig. 7A, B ). In extant cheloniids, this surface is additionally crossed by at least one robust, vertically trending ridge, creating at least two fossae. Tese ridges and fossae are usually interpreted to be muscle attachment sites, and the simpler morphology of TMP 1997.99.1 and Toxochelys latiremis suggests differences in muscle attachment between these Cretaceous species and their extant relatives.

Premaxilla

Te premaxillae of TMP 1997.99.1 are fairly complete, but their labial ridge is slightly damaged, particularly that of the right one ( Fig. 3A, B ).

Te premaxilla of TMP 1997.99.1 ( Figs. 2 , 3 , 5 ) is a small, paired bone. It forms the most anterior part of the snout ( Fig. 3A, B ), the anterior part of the triturating surface ( Fig. 2C, D ), and the ventral surface of the fossa nasalis ( Fig. 5 ). Te distinct labial ridge, which is jointly formed with the maxilla, lacks even the hint of notches or processes ( Figs. 2C, D , 3A, B ), in contrast to Dermochelys coriacea , which exhibits tooth-like tomiodonts ( Nick, 1912). Te premaxilla of TMP 1997.99.1 meets the maxilla laterally, the vomer posteriorly, and its counterpart medially ( Fig. 2 ). Posteriorly, the premaxilla forms the anterior rim of a thin and anteroposteriorly elongated foramen praepalatinum, the posterior margin being formed by the vomer ( Fig. 2C, D ). Te dorsal opening of the foramen praepalatinum is associated with an anteriorly extending, dorsally open groove in the floor of the nasal cavity ( Fig. 2A, B ). Tis groove leads anteriorly into an additional, unnamed foramen at the base of the ascending part of the premaxilla that forms the bone wall ventral to the external naris ( Fig. 2A, B ). Paired foramina praepalatina are generally present in Toxochelys latiremis ( Matzke, 2009) , and these are dorsally also associated with a groove (e.g., AMNH 5118). Although foramina praepalatina are absent in protostegids (Evers et al., 2019a), extant cheloniids (e.g., Lepidochelys olivacea : SMNS 11070), and Dermochelys coriacea ( Nick, 1912) , the dorsal groove and anterior, unnamed foramina seen in TMP 1997.99.1 and Toxochelys latiremis (e.g., AMNH 1497, USNM 11560) are also present in these taxa with the exception of Dermochelys coriacea . On their dorsal side, the premaxillae of TMP 1997.99.1 jointly form a low median ridge that connects posteriorly with that of the vomer. At the anterior end of this ridge, a pocket or small cavity appears to be present that leads anteroventrally into the region between both premaxillae

( Fig. 2A, B ). Tis has also been described for Rhinochelys pulchriceps (Evers et al., 2019a) , a protostegid, and can furthermore be observed in Toxochelys latiremis (AMNH 5118). Extant cheloniids also have a small gap in this area, but it is not as broad as in TMP 1997.99.1 or Toxochelys latiremis . Te ventral side ( Fig. 3C, D ) of the interpremaxillary region of TMP 1997.99.1 is characterized by a broad, but shallow pit ( Fig. 2C, D ) that accommodates the dorsal labial structures of the dentary (see dentary for peculiarities in TMP 1997.99.1).

Maxilla

Te maxilla of TMP 1997.99.1 ( Figs. 2 , 3 , 4 , 5 ) is a large, paired bone that forms most of the upper jaw. It meets the premaxilla and vomer anteromedially ( Fig. 2C, D ), the prefrontal dorsally ( Fig. 2A, B ), and contacts the palatine medially between the foramen orbito-nasale and foramen palatinum posterius ( Fig. 2C, D ). Te maxilla shows, in addition, a contact with the pterygoid and jugal at its posterior end ( Fig. 2C, D ), lateral to the posterior rim of the foramen palatinum posterius ( Fig. 3C, D ). Te maxilla forms the anterior quarter of the orbital rim, and forms large parts of the floor of the orbital cavity between the palatine and the jugal ( Fig. 2A, B ). Tis part of the maxilla tapers posteriorly and contacts the pterygoid lateral to the foramen palatinum posterius. Tis contact, as well as the large size and oval shape of the foramen palatinum posterius are basically identical with the morphology in Toxochelys latiremis (AMNH 5118; partly contrasting Matzke, 2009). Te maxilla of TMP 1997.99.1 forms parts of the lateral margin of the foramen orbito-nasale ( Fig. 2A, B ), which is otherwise formed by the prefrontal and palatine ( Fig. 4 ). Anterior to the foramen orbito-nasale, the maxilla floors the nasal passage and the lateral third of the fossa nasalis. At its posterolateral end, the maxilla forms a posteriorly tapering process that underlaps the jugal and participates in the anterior portion of the cheek emargination ( Fig. 4 ). Te labial ridge of TMP 1997.99.1 is distinct and sharp

( Fig. 2C, D ), but lacks tooth-like processes or notches, as are present in Dermochelys coriacea ( Nick, 1912) . Te profile of the labial ridge in lateral view is slightly downturned along the central parts of the maxilla

( Fig. 4 ). Tis curvature continues onto the premaxillae. Te external maxillary surface below the orbit shows the presence of numerous neurovascular foramina that probably indicate the former presence and extent of a keratinous rhamphotheca ( Figs. 3 , 4 ). Tese foramina connect internally with a central canal that traverses most of the maxilla anteroposteriorly, the canalis alveolaris superior. Tis canal is connected to a relatively large foramen alveolare superius, which is located within the lateral margin of the foramen orbito-temporale. Internally, the canalis alveolaris superior is additionally connected to a posteriorly directed canal, the canalis infraorbitalis. Tis canal is smaller in diameter than the canalis alveolaris superior, and also has a smaller opening within the dorsal surface of the maxilla. Tis foramen, the foramen supramaxillare, can only be seen in the CT scans with certainty on the right side and opens centrally on the dorsal surface of the maxilla that forms the floor of the orbital fossa ( Fig. 2A, B ). Te maxilla forms most of the triturating surface. A low, but distinct lingual ridge is formed jointly with the palatine that runs parallel to the labial ridge ( Fig. 2C, D ).

Vomer

Te vomer of TMP 1997.99.1 ( Figs. 2 , 3 , 5 ) is an unpaired element medially situated in the palatal area. Te vomer meets the premaxillae anteriorly, the maxillae anterolaterally, the palatines laterally, the prefrontal anterodorsally, and the pterygoids posteriorly ( Figs. 2C, D , 5 ). Te latter contact is absent in protostegids, but present in the stem chelonioid Toxochelys latiremis ( Matzke, 2009) and all extant chelonioids. For the purpose of this description, we distinguish three parts of the vomer of TMP 1997.99.1: a mediolaterally broad part anterior to the internal naris, a central part that supports the dorsal vomerine structures, and a posterior part. Te vomer is mediolaterally broadest anterior to the internal nares, where it has two ventrally projecting lateral processes that contact the premaxillae anteriorly and the maxillae anterolaterally ( Fig. 2C, D ). Te central area between these processes on the ventral surface of the vomer is mediolaterally concave. Tis concavity is posteriorly continuous with the deep fossa surrounding the central part of the vomer, in which the internal nares are located. Tis part of the vomer is very similar to Toxochelys latiremis ( Matzke, 2009) and other taxa that lack secondary palates, but differs strongly form modern cheloniids and other chelonioids with secondary palates, in which the anterior part of the vomer generally articulates to the same bones, but in which it is developed as a horizontal plate that is integrated into the triturating surfaces of the maxilla (Brinkman et al., 2006). Te anterior part of the vomer of TMP 1997.99.1 forms the posterior rim of the foramina praepalatina ( Fig. 2C, D ). More posteriorly, the ventrolateral processes form the medial wall and floor of the nasal passages. Te vomer does not participate in the medial rim of the foramen orbito-nasale, which occurs in several chelonioids (e.g., Lepidochelys olivacea : SMNS 11070) but is not universally present within the group. On the dorsal surface that forms parts of the floor of the nasal cavity, the vomer of TMP 1997.99.1 forms a low median ridge that continues anteriorly onto the premaxillae.

Te central part of the vomer is dorsally slightly arched with respect to the anterior part and the posterior process

( Fig. 5 ). Te dorsal surface of the central part is covered by two anterodorsally directed columnar processes that articulate with the prefrontal to form the medial wall and dorsal roof to the nasal passage ( Figs. 3A, B , 5 ). Tese dorsal processes are medially separated by a narrow and deep median groove called the sulcus vomeri, which represents the ventral third of the keyhole-shaped fissura ethmoidalis ( Fig. 3A, B ). Tis mimics the morphology of most turtles, with the exception of Dermochelys coriacea among cryptodires, in which the dorsal processes are greatly reduced, instead forming two low ridges on either side of the sulcus vomeri. In moderns cheloniids and in Allopleuron hofmanni , the processes are similar in height but due to the formation of a secondary palate, the nasal passages are considerably larger than in TMP 1997.99.1, making the vomer dorsoventrally higher.

Te posterior part of the vomer forms an anteroposteriorly elongated and slightly posteriorly descending posterior process that lies between the palatines and posteriorly articulates with the pterygoids

( Fig. 2C, D ; Brinkman et al., 2006). Te dorsal surface of this part of the vomer shows a sharp median crest that is posteriorly continued by an interpterygoid ridge and that likely anchored the interorbital septum. Tere is no posterior extension of the sulcus vomeri as a dorsally open trough. Te ventral surface of the vomer is flat and a ventral keel is absent ( Fig. 2C, D ). Tese conditions strongly differ from those found in the protostegids, which possess a posteriorly less elongated vomer with a slight keel (Evers et al., 2019a). In modern chelonioids a keel is variously present (e.g., Chelonia mydas , Dermochelys coriacea ; but see Lepidochelys olivacea for the absence of a keel), but usually restricted to the part of the vomer that connects the central column between the nasal passages with the posterior vomer process.

Palatine

Te palatine of TMP 1997.99.1 is a paired bone forming the lateral parts of the palate adjacent to the vomer

( Figs. 2 , 3 , 4 , 5 ). Te palatine contacts the maxilla anterolaterally, the vomer medially, the pterygoid posteriorly ( Fig. 2C, D ), and the descending process of the prefrontal anterodorsally ( Fig. 4 ). A lateral contact with the jugal and a median interpalatine contact are clearly absent ( Fig. 2 ). Te palatine consists of a horizontally flat sheet forming large parts of the floor of the orbital cavity and an anteroventrally projecting process that contributes to the triturating surface ( Fig. 2C, D ). Te horizontal sheet of the palatine shows anteriorly a dorsally elongated contact with the descending process of the prefrontal and forms the dorsal roof of the meatus choanae. It also forms the medial rim of the foramen orbito-nasale ( Figs. 2A, B , 4 ). Te medial border of the horizontal sheet of the palatine strongly overlaps the vomer, reducing the dorsal exposure of the latter.

Our 3D models of TMP 1997.99.1 show slight irregularities along the palatine–pterygoid suture

( Fig. 2C, D ). Tis is because both bones are tightly interfingered and the individual struts and pockets of this articulation are difficult to separate with great precision due to the high bone porosity and fine suture lines. Nevertheless, the position and orientation of the suture is clear and can furthermore be verified in photographs

( Fig. 1B ). Te palatine and vomer jointly meet the pterygoid in a roughly straight, transverse suture at the posterior level of the foramen palatinum posterius

( Fig. 2C, D ). Te medial margin of the relatively large, anteroposteriorly oval foramen palatinum posterius is formed by the palatine ( Fig. 2C, D ; Brinkman et al., 2006), whereas the posterior border is largely formed by the pterygoid, which contacts the maxilla in this area as the bone that forms the lateral margin of the foramen. Tis is comparable to Toxochelys latiremis , in which well-preserved specimens also show the same condition (e.g., AMNH 5118; Matzke, 2009). In Toxochelys latiremis , there may be some variation to this morphology, as some specimens (e.g., USNM 11560; Matzke, 2009) show that the palatine have a short, laterally recurved process along the posterior border of the foramen palatinum posterius, which minimizes the pterygoid’s contribution to its margin, but which generally does not seem to reach the maxilla. Te foramen palatinum posterius is completely absent in extant chelonioids and laterally open in protostegids.

Te base of the ventrolateral process of the palatine of TMP 1997.99.1 is constricted between the foramen orbito-nasale and foramen palatinum posterius but broadens laterally along its contact with the maxilla in both anterior and posterior directions ( Fig. 2C, D ). Along this contact, it reaches anteriorly to contact the posterior tip of the anteroventral process of the vomer in the lateral rim of the internal naris ( Fig. 2C, D ). Tis vomer–palatine contact anterior to the internal naris is typical of chelonioids and also present in Toxochelys latiremis ( Matzke, 2009) , but largely absent among other cryptodires. Te ventral surface of the anterolateral process of the palatine of TMP 1997.99.1 contributes narrowly, but distinctly, to the triturating surface ( Fig. 2C, D ). A similar contribution is also present in Toxochelys latiremis ( Matzke, 2009) and modern chelonioids, but it is absent in some protostegids, such as Desmatochelys lowii ( Raselli, 2018) . Together with the maxilla, the palatine of TMP 1997.99.1 forms a low and blunt lingual ridge that medially frames the triturating surfaces ( Fig. 2C, D ).

Quadrate

Te quadrate of TMP 1997.99.1 ( Figs. 2 , 3 , 4 , 6 , 7 ) is a paired bone situated in the posterolateral skull region. It forms the cavum tympani, the incisura columellae auris, and the processus articularis ( Figs. 4 , 7 ). Te ventral end of the processus articularis is finished by the facet for the mandibular articulation ( Fig. 2C, D ). Te quadrate meets the prootic anteromedially ( Figs. 2 , 6 , 7 ), the opisthotic posteromedially ( Figs. 2 , 7C, D ), the pterygoid ventromedially ( Figs. 2 , 3C, D , 6 ), and the quadratojugal anteriorly ( Fig. 4 ). Even if both squamosals are disarticulated, it is apparent that the quadrates contacted them posterodorsally, but also that the contact was not very tight ( Fig. 7 ). Instead, the quadrate surface that faces the squamosal is basically flat and any type of interdigitation of these bones must have been absent. Tis is also observed in crown chelonioids, but not apparent in non-chelonioid outgroups. A processus epipterygoideus of the quadrate is absent ( Fig. 6 ).

Te quadrate forms the lateral half of the distinct, anteriorly directed processus trochlearis oticum

( Fig. 6A ). Te process is well rounded and occupies a relatively small part of the mediolateral width of the otic chamber, so that it is well separated laterally from the internal surface of the cheek and medially well separated from the braincase. Te processus trochlearis oticum of TMP 1997.99.1 is similar of that of Toxochelys latiremis (see in Matzke, 2009: Figs. 4 , 5 ), although somewhat more prominent. Tis process is weaker and projects less prominently into the temporal fossa in Allopleuron hofmanni (see Mulder, 2003: pls. 11 and 12) and is absent in Dermochelys coriacea (see Nick, 1912: pls. 1 and 2), whereas the development of the process is variable among extant cheloniids.

Te quadrate of TMP 1997.99.1 forms the lateral rim of the foramen stapedio-temporale ( Figs. 2A, B , 7C, D ), which is a very large opening as is typical for chelonioids ( Rollot et al., 2021). Tis foramen represents the dorsal aperture of the canalis stapedio-temporale, which is otherwise formed by the prootic and traverses the otic capsule dorsoventrally and into the anterior region of the cavum acustico-jugulare. Te quadrate also forms the lateral wall of the cavum acustico-jugulare and the lateral rim of its posterior opening, the fenestra postotica

( Fig. 3C, D ). Within the cavum acustico-jugulare, the quadrate minorly contributes to the posterolateral wall of the canalis cavernosus, which is otherwise formed by the prootic and pterygoid.

Te cavum acustico-jugulare is connected to the cavum tympani via the incisurae columellae auris, which appears as a posteroventral notch in the cavum tympani

( Fig. 3C, D ) that funneled the stapes through the middle ear region toward the fenestra ovalis of the inner ear and which partially includes the Eustachian tube. Te surface of the funnel-shaped cavum tympani of TMP 1997.99.1 is formed by the quadrate, but its anterior margin is formed by the quadratojugal and its dorsal margin by the squamosal ( Figs. 4 , 7 ). In the posterodorsal part of the cavum tympani, the quadrate is anterolaterally recurved to form a hook-like process that surrounds the posterodorsally open aperture of the antrum postoticum ( Fig. 4A, B ). Tis part of the quadrate was topped by the posterior part of the squamosal ( Fig. 7 ), without the formation of a distinct antrum postoticum (see squamosal). Despite the deformation that affects specimens of Toxochelys latiremis , their quadrate shows a similar overall shape to that of TMP 1997.99.1, particularly in that the quadratojugal participates to the anterior rim of the cavum tympani ( Matzke, 2009). In protostegids, the antrum postoticum condition is variable, and can be widely open in Protostega gigas to absent in some of the Rhinochelys pulchriceps specimens, showing intraspecific variation in the latter (Evers et al., 2019a). In Dermochelys coriacea (see pls. 1 and 2 in Nick, 1912), as in modern cheloniids, the antrum postoticum is absent or strongly reduced (see squamosal).

Te processus articularis of TMP 1997.99.1 is located ventral to the incisura columellae auris ( Fig. 3C, D ). Its posterior surface is smooth and lacks an infolded ridge of the quadrate ( Figs. 3C, D , 7E, F ). Te foramen chorda tympani quadrati is not visible in the posterior surface of the processus articularis. Te processus articularis projects strongly ventrally below the level of the palate

( Figs. 3C, D , 4 ), as in all chelonioids, and is situated at a level that is anterior to the foramen magnum ( Fig. 2C, D ). Te condylus mandibularis of TMP 1997.99.1 shows a shallow and wide anteroposterior groove that separates the articular surface into lateral and medial subfacets ( Fig. 2C, D ). On the left side of the specimen, the pterygoid marginally participates in the medial facet of the condylus mandibularis ( Figs. 2C, D , 5D , 6A ). Tis contact is not preserved on the right side of the specimen, but here the process still extends far down the quadrate’s articular process, similar to the condition in the early protostegid Rhinochelys pulchriceps (Evers et al., 2019a) . Generally, this does not seem to occur in other Cretaceous pan-chelonioids, including Toxochelys latiremis ( Matzke, 2009) , in which the pterygoid ends before reaching the ventral end of the processus articularis of the quadrate.

Pterygoid

Both pterygoids of TMP 1997.99.1 ( Figs. 2 , 3 , 4 , 5 , 6 ) are complete and well-preserved, allowing the identification of many internal canals and structures. Te pterygoid is a large, complex, paired bone that connects the palate to the basicranium. Tis bone can be divided into an anterior part that is in connection with the palate and that forms the external (transverse) pterygoid process, and a posterior process that floors the basicranium. Te pterygoid contacts the exoccipital, basioccipital, parabasisphenoid, prootic, quadrate, vomer, palatine, jugal, maxilla, as well as the other pterygoid ( Figs. 2 , 3 , 4 , 5 , 6 ).

Te anterior part of the pterygoid is strongly expanded medially and separated from the posterior process by a constriction in the center of the bone that forms a deep lateral notch facing the subtemporal fenestra ( Fig. 2C, D ). Te lateral expansion of the pterygoid forms a well-defined external pterygoid process, which contributes to the posterior rim of the foramen palatinum posterius and contacts the maxilla and the jugal anterolaterally ( Fig. 2C, D ; Brinkman et al., 2006). Te process expands deeply posterolaterally into the subtemporal fossa. Te anterior morphology of the pterygoid is nearly identical to that of Toxochelys latiremis ( Matzke, 2009) , but important differences to other pan-chelonioids exist. Specifically, the transverse pterygoid process is strongly reduced to absent in cheloniids and does not occur in Dermochelys coriacea (Gaffney, 1979) , whereas it does not contact the maxilla anterolaterally in protostegids for which this region is well known, so that it becomes a “free” process that projects laterally into the subtemporal fenestra (e.g., Evers et al., 2019a; Hirayama, 1998; Kear & Lee, 2006; Raselli, 2018). Te lateral margin of the external pterygoid process of TMP 1997.99.1 is capped by a well-developed, obliquely oriented, vertical flange that is ovoid in outline ( Figs. 4 , 6 ). Tis condition is similar to Toxochelys latiremis ( Matzke, 2009) . Anteriorly, the pterygoid of TMP 1997.99.1 contacts the palatine and, more medially, the vomer along a broad suture ( Fig. 2C, D ; Brinkman et al., 2006).

Te pterygoids of TMP 1997.99.1 are in contact with each other along a median suture that extends along the anterior half of the bone and that exceeds the length of the anteroposterior exposure of the parabasisphenoid

( Figs. 2C, D , 5 ; Brinkman et al., 2006). Te ventral interpterygoid suture does not form a very deep ridge, but is nevertheless slightly raised and is V-shaped in coronal cross section of the CT slices ( Fig. 2C, D ). Brinkman et al. (2006) considered this ridge to be homologous with that seen in more derived chelonioids, and the morphology indeed differs from that of Toxochelys spp. , in which a midline ridge of any kind is absent. For example, Toxochelys mooreviliensis (FMNH PR 219) with its near perfectly preserved basicranium has a completely flat interpterygoid surface. However, we here note that sea turtles in which the presence of a median ridge is unambiguous, have ridges that are much deeper, forming often a lamina-like crest, again therefore displaying a different morphology. Tis can for example be exemplified by Allopleuron hofmanni (NHMUK R4213) and Argillochelys antiqua (NHMUK R41636). Although the weak ridge in TMP 1997.99.1 is somewhat similar to the derived morphology of the aforementioned presumed fossil cheloniids in the sense that a ridge is apparent, we score the respective character (ch. 104) as state “0”, as Nichollsemys baieri fulfills the “incipient” presence of the ridge that is detailed in the character state definition. In future versions of the matrix, it may be good to revise this character into an ordered multistate character, in which the incipient presence of the ridge is not coded with its absence, but as an intermediate state. However, as we do not revise other characters of our baseline matrix, we only suggest this change here for the future. In the posterior part of the pterygoid of TMP 1997.99.1, the bone forms a posterior process that diverges posterolaterally from the midline to accommodate the triangular ventral exposure of the parabasisphenoid and basioccipital ( Fig. 2C, D ). To the right and left, these two bones are bordered by low, but sharp crests ( Fig. 2C, D ) that are formed by the pterygoid and have sometimes been called ventral pterygoid ridges (Evers et al., 2019a). Tese ridges terminate near the posterior end of the pterygoids. Te ventral pterygoid ridges of Nichollsemys baieri show a reverse “V” pattern (Brinkman et al., 2006), which is observed in many chelonioids (Gaffney, 1979). Te ventral surface of the pterygoid in TMP 1997.99.1 shows also a pterygoid fossa that is moderately deep and laterally bordered by a distinct and rounded ventrolateral ridge formed by the ramus of the pterygoid that extends to the mandibular articulation process of the quadrate ( Fig. 2C, D ). Te pterygoid has a deep extension along this process of the quadrate and closely approximates the articulation surface ( Figs. 2C, D , 5 ). Tis is better preserved on the left side of the skull, where the pterygoid even seems to make a small contribution to the articulation surface itself. On the right side, this cannot be verified, as the most of the ventral extend of the pterygoid seems to be broken.

Te posterior pterygoid process of TMP 1997.99.1 has a broad exposure on the posterior surface of the skull ( Fig. 3C, D ), where it forms the ventral margin of the fenestra postotica. Besides contacting the basioccipital, the posterior process of the pterygoid also has a very broad contact with the exoccipital medial to the fenestra postotica ( Fig. 3C, D ). Tis broad exposure is unusual, especially as it partly separates the basioccipital and exoccipital lateral to the occipital condyle ( Fig. 3C, D ). Te broad posterior exposure of the pterygoid forms a plateau-like, flat area ventromedial to the openings for the hypoglossal nerve, which are close to the exoccipitalpterygoid suture ( Fig. 3C, D ). In extant chelonioids (i.e., cheloniids and Dermochelys coriacea ), the basioccipital and exoccipital are closely occluded in this area and the pterygoid is far laterally removed from the hypoglossal foramina. As in all pan-chelonioids, the fenestra postotica of TMP 1997.99.1 is coalescent with the foramen jugulare posterius ( Fig. 3C, D ). Te foramen posterius canalis carotici interni opens within the posterior process of the pterygoid (Brinkman et al., 2006), in a position between the posterior end of the ventral ridge that parallels the contact with the basioccipital and the margin of the fenestra postotica ( Figs. 2C, D , 8 ). Te foramen is relatively small compared to the foramen stapedio-temporale, which is approximately four times the diameter of the foramen posterius canalis carotici interni. Te internal carotid canal then traverses anterodorsally through the pterygoid ( Fig. 8 ). Slightly anterior to the level of the labyrinth, the internal carotid artery pierces through the dorsal pterygoid surface where it is overlain by the prootic ( Fig. 8A ) and the canal then continues anteriorly along the pterygoid-prootic suture. As the internal carotid canal approaches the parabasisphenoid suture, it forms a large internal cavity between the pterygoid, prootic, and parabasisphenoid ( Fig. 9 ). From this cavity, there are two openings. Medially, the internal carotid artery canal enters the sutural region of the parabasisphenoid and pterygoid and continues farther anteriorly

( Fig. 8 ), where it separates into a medial cerebral artery canal within the parabasisphenoid and an anteriorly continuing palatine artery canal. Laterally, the cavity opens into the canalis cavernosus via an unusually large foramen pro ramo nervi vidiani (the palatine artery foramen of Brinkman et al. (2006)), which is further described below ( Figs. 8A , 9 ).

Te dorsal surface of the posterior process of the pterygoid forms the majority of the floor of the cavum acustico-jugulare ( Fig. 3C, D ), including the recessus scalae tympani. Tere is no contact with the ventral tip of the processus interfenestralis of the opisthotic, so that a hiatus postlagenum remains. Tus, the pterygoid also floors the posterior parts of the cavum labyrinthicum, whereas anteriorly, the pterygoid surface is overlain by the ventral process of the prootic. At the anterior end of the cavum acustico-jugulare, the pterygoid and prootic form the posterior opening into the canalis cavernosus. Te pterygoid forms the floor and lateral and medial margins of this canal along raised ridges. Te medial of these ridges is unnamed and its medial side is bordered by the canalis caroticus interni. Both ridges are interrupted along their course by foramina ( Fig. 9 ). Te medial ridge bears the aforementioned, unusual, large foramen, which is much larger than the foramen posterius canalis carotici interni, but smaller than the foramen stapedio-temporale and which opens between the internal carotid canal and the canalis cavernosus ( Fig. 9 ; Brinkman et al., 2006). Te foramen is located in the prootic-pterygoid suture and is positioned just anteroventrally to the lateral foramen of the facial nerve canal within the ventral process of the prootic (see prootic) and thus just posterior to the level of the trigeminal foramen and foramen cavernosus ( Fig. 9 ). In this area, one would usually expect the foramen (and canalis) pro ramo nervi vidiani, which carries the vidian nerve from the canalis cavernosus into the canal for the internal carotid artery (Gaffney, 1979; Rollot et al., 2021). As no separate foramen pro ramo nervi vidiani is evident in TMP 1997.99.1, we propose that the vidian nerve passed through the large foramen, essentially making it an enlarged foramen pro ramo nervi vidiani. A similarly enlarged foramen pro ramo nervi vidiani is absent in modern cheloniids ( Rollot et al., 2021), and in the protostegid Rhinochelys pulchriceps (Evers et al., 2019a) . However, a large foramen between the canalis cavernosus and carotid arterial system is also present in Allopleuron hofmanni (NHMUK R4213) as evident from CT-scan derived 3D models that we have of this specimen (Evers, 2021), although this morphology, indeed this area, has not previously been described for Allopleuron hofmanni ( Mulder, 2003) . As the foramen of TMP 1997.99.1 is extraordinarily large for a nerve foramen, the possibility that other structures also passed through the foramen (e.g., the mandibular artery or an unnamed artery) cannot be excluded and is potentially supported by strange, burrow-like structures that extend through parts of the fossil and that may represent partial “soft tissue” preservation (see below). Brinkman et al. (2006) identified the large foramen as the passage for the palatine artery. However, given the identification of an alternative palatine artery canal (see below), and given that the mandibular artery of extant cheloniids branches off the palatine artery in a position anterior to the processus inferior parietalis (personal observation on diceCTed specimen of Chelonia mydas , UF-herp-51413), we herein favor an interpretation in which the large foramen only serves as the passage for the vidian nerve, while we cannot provide a satisfactory explanation of the large size of the foramen pro ramo nervi vidiani. Possibly, the close spacing of the canalis cavernosus and internal carotid artery canal led to a broad coalescence of these canals in the form of a ‘window’, similar to the coalescence of some other openings such as the foramen jugulare posterius and fenestra postotica in chelonioids. In TMP 1997.99.1, the vidian nerve enters the internal carotid canal by the foramen pro ramo nervi vidiani and, likely, follows the path of the carotid artery, before entering the canal for the palatine artery. Te absence of a separate exiting foramen for the vidian nerve suggests the absence of a canalis nervus vidianus, as in all other chelonioids ( Rollot et al., 2021).

Te pterygoid ridge to the lateral side of the canalis cavernosus of TMP 1997.99.1 is deeply notched dorsally to form the ventral margin of the trigeminal foramen

( Figs. 6 , 9 ). Tis foramen is dorsoventrally elongated in TMP 1997.99.1 ( Fig. 6 ) and ventrally extends nearly to the floor of the sulcus cavernosus. Tis posteroventral expansion of the foramen has been interpreted to be an osteological correlate for the mandibular artery leaving the trigeminal foramen (Evers & Benson, 2019). Anterior to the trigeminal foramen, the crista pterygoidea is formed as an extremely low, nearly absent ridge ( Figs. 6 , 9 ). Instead, the ventral process of the parietal is very deep and is weakly attached to the dorsal pterygoid surface

( Figs. 5 , 6 , 9 ). Tere is no trace of an epipterygoid. Te sulcus cavernosus extends anteroposteriorly between the medial surface of the ventral process of the parietal and the rostrum basisphenoidale of the parabasisphenoid ( Fig. 9 ). Te sulcus becomes anteriorly strongly constricted, such that the parietal nearly has a contact with the rostrum basisphenoidale, limiting the pterygoid exposure to a thin band between the two ( Fig. 9 ).

Te exiting foramen of the palatine branch of the carotid artery (foramen anterius canalis caroticus palatinum of Rabi et al., 2013; foramen anterius canalis carotici lateralis of Rollot et al., 2021) opens anteriorly into the floor of the sulcus cavernosus ( Figs. 6 , 8 ), in a small foramen between the pterygoid and the parabasisphenoid. Tis foramen was unnoticed in the original description (Brinkman et al., 2006). Te position of the foramen is approximately at the same level as the exiting foramen for the cerebral branch of the carotid artery (foramina anterius canalis caroticus cerebralis sensu Rabi et al., 2013; foramina anterius canalis caroticus basisphenoidalis sensu Rollot et al., 2021)

( Fig. 8 ). Tis condition differs from that of Toxochelys mooreviliensis (FMNH PR 219 and 27338), in which the foramen anterius canalis caroticus palatinum is situated at the midlength of the foramen anterius canalis carotici cerebralis and the anterior limit of the crista pterygoidei, so that is situated farther anteriorly than in TMP 1997.99.1. All Toxochelys spp. specimens for which we could evaluate this feature show several supernumerary foramina in the area of the foramen anterius canalis carotici palatinum, but CT scans of FMNH PR 219 show that these smaller foramina are not connected to the internal carotid arterial system. Despite some differences, the internal carotid artery canal branching system of TMP 1997.99.1 is a lot more similar to Toxochelys latiremis and Toxochelys mooreviliensis (not known for Toxochelys browni , which has been suggested to represent a different species) than originally described (Brinkman et al., 2006).

Te anterior end of the rostrum basisphenoidale of TMP 1997.99.1 rests on the suture of the right and left pterygoids, which forms a raised dorsal median ridge

( Figs. 5 , 6 , 9 ). Te morphology of Toxochelys spp. is again slightly different in that the rostrum is not formed as a rod-like process and in that the interpterygoid suture forms more of an elevated platform than an elevated median ridge (e.g., Brinkman et al., 2006; Matzke, 2009). Te dorsal surface of the pterygoid of TMP 1997.99.1 lateral to the contact with the parietals show no anterior exiting foramina for the vidian nerve, suggesting that these canals could not be identified in the CT scan or that the nerve course is fully associated with that of the palatine artery.

Epipterygoid

An epipterygoid is absent in TMP 1997.99.1. Tere are several indications that this absence is genuine, as opposed to the epipterygoid being fused or falsely not identified by us. Te first indication is that the CT scan of TMP 1997.99.1 generally shows all sutures relatively clearly, but an epipterygoid suture is undetectable in the area where the bone is expected to be if it were present. Tis could, in theory, be explained by fusion of the epipterygoid with the surrounding bones. However, the epipterygoid of presumably closely related taxa, particularly Toxochelys mooreviliensis , is banana-shaped, and lies on the surface of the pterygoid at the base of the secondary lateral braincase wall (e.g., FMNH PR 219). In Toxochelys mooreviliensis , the central aspect of the crescentic element overlaps the processus inferior parietalis with its dorsal margin (FMNH PR 219). Tus, assuming a similar epipterygoid shape for TMP 1997.99.1 the descending process of the parietal would either be expected to have anteroventrolateral and posteroventrolateral processes that expand onto the pterygoid surface or the pterygoid would be expected to form a dorsally rounded overlap onto the lateral surface of the descending process of the parietal. Neither of these morphologies are observed. TMP 1997.99.1 also lacks an epipterygoid process of the quadrate or clearly developed fossa cartilaginis epipterygoidei. Tus, we interpret the evidence as indicating the true absence of the epipterygoid. Te absence of the epipterygoid is insofar interesting, as many fossil chelonioids do have epipterygoids, including Cretaceous species (e.g., Toxochelys latiremis , Ctenochelys spp. , Allopleuron hoffmanni ) and much younger Eocene stem cheloniids (e.g., Argillochelys cuneiceps , Eochelone brabantica ), indicating that the epipterygoid may have been lost in TMP 1997.99.1 independently to the phylogenetically shallower loss in cheloniids.

Supraoccipital

Te ventral part of the supraoccipital that is integrated into the braincase is complete in TMP 1997.99.1, but the dorsal and posterior regions of the crista supraoccipitalis are broken ( Figs. 2 , 3 , 4 , 5 , 9 ). Te supraoccipital is an unpaired bone that dorsally roofs the posterior braincase. Tis bone is prolonged dorsally and posteriorly by a prominent sagittal crest called the crista supraoccipitalis. Te length of this structure in TMP 1997.99.1 is unclear due to the aforementioned breakage. Nevertheless, it seems evident that the crest must have been quite long posteriorly, as the base of the crest at the break line is quite massive and as the preserved anterior portion of the crest already overlaps the full occipital condyle and expands beyond the level of the posterior squamosal tips ( Fig. 2C, D ). Tus, the crest was probably similar to that of Toxochelys latiremis ( Matzke, 2009) . Anteriorly, the supraoccipital contacts the parietals along a strait vertical suture that expands to the top of the crista supraoccipitalis ( Fig. 5 ), which is completely overlain by the parietals and thus seem to lack any expression in the dorsal skull roof ( Fig. 2A, B ). Te ventral part of the supraoccipital forms two ventrally directed lateral processes that roof the braincase in the shape of a narrow, reversed U ( Fig. 3C, D , 5 , 9 ). On both sides, the ventral rim of the lateral processes contacts the prootics anteriorly and the opisthotics more posteriorly ( Figs. 2A, B , 6 , 9 ). In cross section, the braincase has the shape of a reversed keyhole at the level of the supraoccipital– opisthotic contact, the dorsal constriction being formed by the supraoccipital. At their facet for the contact with the prootic and the opisthotic, these ventral processes show a small recess that house the dorsal ends of anterior and posterior semicircular canals of the labyrinth. At their posterior ventral ends the lateral processes contact the exoccipitals ( Fig. 3C, D ). Te supraoccipital participates to the foramen magnum, and its posteroventral border is forming the dorsal rim of this aperture ( Fig. 2C, D ).

Exoccipital

Both exoccipitals of TMP 1997.99.1 are complete ( Figs. 3 , 5 ). Te exoccipital is a paired bone that forms the lateral rim of the foramen magnum ( Fig. 3C, D ). Te exoccipital meets the supraoccipital dorsally, the opisthotic dorsolaterally, the posterior process of the pterygoid ventrolaterally, and the basioccipital medioventrally

( Fig. 2C, D ). A contact with the parabasisphenoid is clearly absent ( Fig. 5 ). A median point contact with its counterpart is apparent above the condylus occipitalis.

Te anterior surface of the exoccipital posteriorly encloses the recessus scalae tympani. Te exoccipital contributes to both the lateral opening of the recessus scalae tympani, the fenestra postotica, the medial opening toward the cavum cranii, and the foramen jugulare anterius ( Fig. 5 ). Te ventral exoccipital process that abuts the para basisphenoid is anteriorly expanded in this region to form the ventral margin of the foramen jugulare anterius. Tis anterior process of the exoccipital just about contacts the processus interfenestralis of the opisthotic. Te exoccipital forms a lateral expansion that contributes to the medial rim of the fenestra postotica

( Fig. 3C, D ). Te exoccipital is further laterally expanded along the dorsal margin of the fenestra postotica and extends along the paroccipital process that is formed by the opisthotic ( Fig. 3C, D ).

Te exoccipital forms a dorsal process that extends along the posterior surface of the opisthotic and forms the lateral wall of the foramen magnum. Dorsally, it reaches the supraoccipital ( Fig. 3C, D ). Te medioventral part of the exoccipital laterally overlaps the basioccipital and forms a posteriorly directed process that forms the dorsolateral part of the condylus occipitalis ( Fig. 2C, D ; Brinkman et al., 2006). Te exoccipital parts of the occipital condyle project further posteriorly than the medioventral basioccipital part ( Fig. 2C, D ). Although the basioccipital is expressed in the floor of the foramen magnum itself, the exoccipitals come to touch each other in a point contact along the dorsal surface of the occipital condyle ( Fig. 3C, D ; contra Brinkman et al., 2006). On the posterolateral surface, near the contact with the pterygoid, the exoccipital has two foramina nervi hypoglossi ( Fig. 2C, D ).

Basioccipital

Te basioccipital of TMP 1997.99.1 is complete ( Figs. 2 , 3 , 5 , 6 ). It is a small, median, unpaired, roughly triangular bone that contributes to the braincase posteroventrally

( Fig. 2C, D ). It consists of a main body, that floors the posteromedial braincase, and a posterior process that forms the condylus occipitalis. Te main body of the basioccipital contacts the para basisphenoid anteriorly and the pterygoids laterally ( Fig. 2C, D ). Te posterior process of the basioccipital contacts the exoccipitals along its lateral surface. Te dorsal exposure of the main body of the basioccipital is roughly triangular and floors the posterior part of the braincase ( Fig. 5 ). Together with the parabasisphenoid, it forms a high median ridge, the crista dorsalis basioccipitalis, that separates the base of the cavum cranii into two deep lateral pits ( Fig. 6 ). Te basis tuberculi basalis is located at the middle of the crista. Laterally, the basioccipital and parabasisphenoid jointly form the ventral margin of the hiatus acusticus. On the ventral surface, the basioccipital forms indistinct, rounded bulges that we interpret as the basioccipital tubercles ( Fig. 2C, D ). Te tubercles are low and separated by a shallow, but wide median pit that is situated on the ventral surface of the basioccipital. Tis condition reminds of that of modern cheloniids, which, however, possess much deeper pits. In Dermochelys coriacea , the basioccipital tubercles are higher. Te posterior process of the basioccipital of TMP 1997.99.1 contacts the exoccipitals dorsolaterally and forms the median third of the smooth and rounded occipital condyle ( Fig. 3C, D ). Te dorsal border of the posterior process forms the ventral rim of the foramen magnum.

Prootic

Te prootic of TMP 1997.99.1 ( Figs. 2 , 3 , 5 , 6 , 7 , 9 ) is a paired bone situated in the anteromedial part of the otic capsule. Tis bone contacts the parietal anterodorsomedially, the supraoccipital posterodorsomedially, the quadrate posterolaterally, the opisthotic posteriorly, the pterygoid ventrally, and the parabasisphenoid medially ( Fig. 6 ). Te prootic has an anterolateral process that overlaps the anterior surface of the quadrate at the anterior end of the otic capsule, which, together with the quadrate forms the anteriorly bulged processus trochlearis oticum ( Fig. 6 ). Posterior to the processus trochlearis oticum, the prootic forms the medial part of the foramen stapedio-temporale ( Figs. 2A, B , 7C, D ). Te latter represents the opening of the canalis stapedio-temporale. Tis large canal, which is jointly formed with the quadrate, is deeply incised within the lateral surface of the prootic and is only capped laterally by the quadrate ( Fig. 7C, D ). Te canal ends ventrally in a foramen, the aditus canalis stapedio-temporalis, which is jointly formed by the prootic and quadrate as the opening for the stapedial artery from within the cavum acustico-jugulare.

Te prootic extends dorsomedially towards the parietal, contacting the processus inferior parietalis of the latter posterior to the trigeminal foramen ( Fig. 6 ). Tis large, ovoid opening is formed posteriorly by the prootic, anteriorly by the parietal, and ventrally by the pterygoid ( Fig. 6 ).

Posterior to the trigeminal foramen, the prootic forms the dorsal half of the foramen cavernosum ( Fig. 6C, D ), which constitutes the anterior opening of the canalis cavernosus that holds the lateral head vein (see pterygoid section), and which is medially, laterally, and dorsally framed by the prootic. Te medial wall of the canalis cavernosus is part of the medioventral process of the prootic that contacts the parabasisphenoid and the pterygoid ventrally ( Fig. 6 ). In the pterygoid–prootic contact, there is a large foramen that opens into the canal for the carotid artery ( Fig. 9 ). As described in the pterygoid section above, we interpret this as an enlarged foramen pro ramo nervi vidiani. Dorsally above this enlarged foramen pro ramo nervi vidiani, there is the lateral opening of the facial nerve canal ( Fig. 9 ), which projects mediolaterally through the prootic towards the medial wall of the prootic ( Fig. 6 ). Tis medial wall of the ventral prootic process contributes to the lateral wall of the braincase and its surface shows a small yet distinct fossa called the fossa acustico-facialis ( Figs. 5 , 6 ). Tis fossa preserves two foramina ( Fig. 5A, B ). Te anterior foramen, the foramen nervi facialis, connects to the canalis cavernosus via the aforementioned facial nerve canal. Te posterior foramen within the fossa acustico-facialis, the foramen nervi acusticus, connects to the cavum labyrinthicum. Turtles generally are expected to show two additional acoustic foramina (e.g., Evers et al., 2019b; Ferreira et al., 2022) to accommodate the three nerve rami, but these are absent in the specimen TMP 1997.99.1. Teir absence can likely be explained by a taphonomic or a segmentation artifact, as the foramina are often small and their separating bone struts are small and fragile.

Posterior to the fossa acustico-facialis, the prootic contributes to the hiatus acusticus, an irregular opening between the cavum labyrinthicum and the cavum cranii

( Fig. 5 ). Te prootic forms a deep cavity posteriorly that forms the anterior region of the inner ear cavities, which are otherwise formed by the opisthotic and supraoccipital. Laterally, toward the cavum acustico-jugulare, the prootic forms the anterior margin of the fenestra ovalis, to which the stapes would have articulated. Te fenestra ovalis remains open posteroventrally, as the prootic and opisthotic have no contact in its ventral margin, which is thus exposed toward the pterygoid. Between the fenestra ovalis and internal, posterior opening into the canalis cavernosus, the prootic forms a posteriorly oriented surface that is part of the anterior wall of the cavum acustico-jugulare. Tis surface is excavated by a posterior prootic recess (e.g., Evers & Joyce, 2020), which can be easier appreciated on the right side.

Opisthotic

Te opisthotic of TMP 1997.99.1 ( Figs. 2 , 3 , 5 , 7 ) is a paired bone situated in the posteromedial part of the otic capsule. Anteriorly, the opisthotic contacts the prootic, laterally the quadrate, dorsally the supraoccipital

( Fig. 2A, B ), and posteriorly the exoccipital ( Fig. 3C, D ). Te opisthotic does not contact the pterygoid ventrally. Te opisthotic posterolaterally encloses the braincase and roofs the posterior region of the cavum acustico-jugulare ( Fig. 3C, D ).

Te anterior wall of the opisthotic shows a deep and large recess ( Fig. 6 ), which constitutes the posterior half of the cavum labyrinthicum. Te opisthotic additionally forms an anteriorly recurved ventral process, the processus interfenestralis ( Fig. 5 ), which separates the cavum labyrinthicum anteriorly from the recessus scalae tympani posteriorly. Te fenestra perilymphatica forms a large foramen between both of these spaces in the medial part of the processus interfenestralis. Te part of the opisthotic that is medial to the fenestra perilymphatica forms the anterior margin of the foramen jugulare anterius ( Fig. 5 ). Tis part of the opisthotic does not contact the basioccipital ventrally. Laterally, the processus interfenestralis forms the posterior margin of the fenestra ovalis, which remains open ventrally. Te processus does not contact the pterygoid ventrally, leaving a large hiatus postlagenum between the labyrinth and recessus scalae tympani. Te base of the processus interfenestralis is laterally pierced by the foramen nervi glossopharyngei.

Posterolaterally, the opisthotic is prolonged into a paraoccipital process ( Figs. 3C, D , 7E, F ), that extends posteriorly to contact the quadrate and, likely, the squamosal (see squamosal). Te paroccipital process contributes to the dorsal wall of the fenestra postotica

( Fig. 3C, D ), but is also strongly overlapped by the exoccipital, which somewhat reduces its surficial contribution to this large opening.

Parabasisphenoid

Te parabasisphenoid of TMP 1997.99.1 ( Figs. 2 , 3 , 4 , 5 , 6 , 8 , 9 ) is an unpaired median bone that forms the anteroventral floor of the braincase. It contacts the pterygoid anterolaterally, the prootic laterally, and the basioccipital posteriorly ( Fig. 6 ). Te parabasisphenoid is composed of two principal structures, a central part with a dorsal surface called the parabasisphenoid cup that is bowl-like, and a rod-like and anteriorly directed process called rostrum basisphenoidale ( Fig. 6 ). Ventrally, the parabasisphenoid has a small triangular exposure between the pterygoids and basioccipital (Brinkman et al., 2006).

Te parabasisphenoid cup of TMP 1997.99.1 is relatively deep, as its surrounding walls are high. Tere is a thin dorsal crest, the crista tuberculi basalis, that traverses the parabasisphenoid cup in the midline, and which continues posteriorly on the basioccipital ( Fig. 6 ; Brinkman et al., 2006). Te rostrum basisphenoidale of TMP 1997.99.1 projects anteriorly from the parabasisphenoid cup as a long rod with a nearly circular cross-section ( Figs. 5 , 6 , 8 , 9 ; Brinkman et al., 2006). An internal suture of the rostrum basisphenoidale was reported by Brinkman et al. (2006), but we cannot confirm the presence of this suture based on the new CT scans. Te rostrum basisphenoidale projects slightly dorsally with its anterior end such that its posterior half is embedded between the pterygoids, whereas its anterior half rest upon the interpterygoid contact dorsally ( Figs. 5 , 6 ). Te rostrum basisphenoidale extends slightly anterior to the level of the anterior margin of the processus inferior parietalis ( Fig. 4 ) and can thus be seen in lateral view. Te trabeculae are only developed as extremely shallow, indistinct ridges on the dorsolateral aspect of the rostrum and are slightly stronger developed anteriorly ( Fig. 6 ). Te sella turcica usually is a fossa that invades the base of the dorsum sellae and rostrum basisphenoidale, in which the anterior foramina for the cerebral artery are situated and which holds the pituitary gland (e.g., Ferreira et al., 2022; Gaffney, 1979). In TMP 1997.99.1, the sella turcica is not developed as a fossa or depression ( Fig. 6D ). Instead, the anterior foramina for the cerebral artery (foramina anterius canalis caroticus cerebralis sensu Rabi et al., 2013; foramina anterius canalis caroticus basisphenoidalis sensu Rollot et al., 2021) are located at the intersection of the rostrum basisphenoidale and the dorsum sellae on an anterodorsally facing surface ( Fig. 6D ). Right and left foramina are separated by a thin strut of bone that projects slightly anteriorly as a weak ridge on the anterior surface of the dorsum sellae and are thus only minorly spaced across the midline ( Figs. 6D , 9 ). Dorsally, the ridge between the anterior cerebral artery foramina continues as a median spike on the dorsal margin of the dorsum sellae ( Figs. 6D , 9 ). Te dorsum sellae itself is a steep and high, vertical wall of bone between the clinoid processes. Tese are developed as thin, steeply dorsolaterally projecting rods ( Figs. 6 , 9 ). Retractor bulbi pits are absent. Instead, the anterior foramina for the abducens nerve just open in the shallowly concave surface ventral to the base of each clinoid process ( Figs. 6 , 9 ). Posteriorly, the abducens canal traverses through the anterior wall of the parabasisphenoid and opens in the parabasisphenoid cup ( Fig. 6D ).

Te rostrum basisphenoidale and sella turcica show much variation among pan-chelonioids. Te sella turcica plesiomorphically is a relatively deep and broad fossa at the base of a broad, plate-like rostrum basisphenoidale. Te trabeculae are quite distinct plesiomorphically and highlight the sella turcica further, and the anterior cerebral foramina are widely spaced and situated within the sella turcica. Tis plesiomorphic combination of traits can be observed in Toxochelys latiremis (e.g., AMNH 1042; Matzke, 2009). A broad, plated rostrum basisphenoidale is present in Dermochelys coriacea , but the trabeculae are reduced, and the cerebral foramina strongly modified. Extant cheloniids universally have a rod-like rostrum basisphenoidale and the sella turcica is reduced to a shallow, anteroposterior elongate but narrow groove, in which the anterior cerebral foramina are closely spaced or even convergent. In many ways, the rostrum basisphenoidale morphology of TMP 1997.99.1 thus appears to be derived with regard to Toxochelys spp. , particularly in the clear formation of a rod-like rostrum, the reduction of the trabeculae, and the narrow midline spacing of the anterior cerebral foramina ( Fig. 6 ; Brinkman et al., 2006). Te strong reduction of a sella turcica may be autapomorphic to Nichollsemys baieri , based on the specimen TMP 1997.99.1.

Te parabasisphenoid internally houses parts of the internal carotid arterial system (see also pterygoid and prootic sections) ( Figs. 5 , 6 , 8 ; Brinkman et al., 2006). Te internal carotid canal first comes in contact with the parabasisphenoid at the position of the large cavity between pterygoid, prootic, and parabasisphenoid that is described above (see pterygoid section) ( Fig. 8 ). From here, the canal extends within the pterygoid-parabasisphenoid suture. Slightly more anteriorly, just posteriorly to the level of the clinoid process base, the internal carotid canal splits in two subordinate canals

( Fig. 8 ). Te more medial of these two canals corresponds to the canalis caroticus basisphenoidalis of Rollot et al. (2021), through which the cerebral branch of the carotid artery extends. Tis canal enters the parabasisphenoid and exits dorsally in the sella turcica at the foramen anterius canalis carotici basisphenoidalis, which is described above. Te second canal extends further anteriorly within the pterygoid-parabasisphenoid suture and corresponds to the palatine artery canal (the canalis caroticus lateralis of Rollot et al., 2021) ( Fig. 8 ). Tis is slightly smaller than the canal for the cerebral artery, as in Toxochelys mooreviliensis (FMNH PR 219). Te palatine artery canal of TMP 1997.99.1 exits dorsally into the sulcus cavernosus as a foramen formed in the pterygoid-parabasisphenoid suture (foramen anterius canalis carotici palatinum of Rabi et al., 2013; foramen anterius canalis caroticus lateralis of Rollot et al., 2021), which is situated at the same level as the foramen anterius canalis carotici basisphenoidalis (see pterygoid section) ( Figs. 6 , 8 ). Tus, TMP 1997.99.1 has a fully developed internal carotid artery split into palatine and cerebral arteries (as in extant chelonioids), but this arterial system is fully embedded in bone. Tis embedding contrasts the condition of extant chelonioids, in which the undivided internal carotid artery exits into the sulcus cavernosus before it splits into the cerebral and palatine arteries, the latter of which is never covered by bone (Evers & Benson, 2019; Evers et al., 2019b; Zangerl, 1953b). Although this situation has been known since Zangerl (1953b), it was never well documented. Here, we can confirm that this arterial pattern is observable in extant cheloniids, as evidenced by a dice-CT scan of Chelonia mydas (UF herp 51413), which nicely shows the arterial pattern. Te arterial pattern of TMP 1997.99.1 with a fully enclosed carotid split is the plesiomorphic condition, as it can also be observed in chelydrids (Albrecht, 1976; Rollot et al., 2021), as well as in Toxochelys moorevillensis (FMNH PR219). A covered split is further observable in in Allopleuron hofmanni (NMUK R4213), protostegids (Evers et al., 2019a), as well as Eocene cheloniids such as Eochelone brabantica (Evers & Benson, 2019; Evers et al., 2019a), such that the uncovered carotid split of extant cheloniids seems to be a derived feature that evolved fairly late within chelonioid evolution.

Stapes

Te medial portions of both stapes of TMP 1997.99.1 are preserved ( Fig. 10G–K ). Tey are in approximately their original position though slightly pushed into the labyrinth cavity on each side ( Fig. 10G ). As in other turtles, the stapes has the shape of a trumpet with a medially placed stapedial footplate attached to a thin shaft that connects with the tympanic membrane via the cartilaginous extrastapes (e.g., Foth et al., 2019; Rollot et al., 2024)

( Fig. 10 ). In TMP 1997.99.1, the stapedial footplate is nearly circular and medially excavated by a deep recess

( Fig. 10K ). Te rim of the stapedial footplate is delicate, but slightly thicker in the dorsal aspect ( Fig. 10H ) compared to the anterior, ventral, and posterior region. Te bone forming the stapedial footplate is very thin, which makes it hard to segment it consistently, as in some slices the bone is only a few voxels thick. In TMP 1997.99.1, as currently segmented, both stapedial footplates are penetrated by irregular but fairly large openings ( Fig. 10K ). It is unclear if any of these represent genuine morphology, or if they are segmentation or taphonomic artefacts of the thin bone layer forming this part of the stapes.

Laterally, the stapes continues as a very thin rod-like shaft, which is notably curved. ( Fig. 10H–I ). Here, the curvature is such that the stapedial rod bows downwards in the central area of the cavum acustico-juglare. On each side, the stapedial shaft is incomplete, and broken before entering the incisura columellae auris.

Endosseous and partial membranous labyrinth

We segmented the right endosseous labyrinth of TMP 1997.99.1. In addition, we noticed a dark halo is apparent inside parts of the endosseous labyrinth cavity, which seems to extend through parts of the posterior semicircular canal. Interestingly, segmentation of this halo suggests that the structure in question might represent “soft tissue” preservation of the membranous posterior semicircular duct, as we argue below.

As in most turtles, the endosseous labyrinth cavity of TMP 1997.99.1 is comprised of a series of intersecting canals formed by the prootic, opisthotic, and supraoccipital (Evers et al., 2019b; Ferreira et al., 2022). Te vertical semicircular canals are nearly symmetrical

( Fig. 10A ), as is typical for turtles (Evers et al., 2022a). Again, as in other turtles, many details of the underlying membranous labyrinth morphology are poorly reflected in the endosseous endocast (Evers et al., 2019b, 2022a; Ferreira et al., 2022), as the ampullae of TMP 1997.99.1 are poorly visible and as the intersection of the posterior and lateral semicircular canal form a large confluent cavity, the secondary common crus ( Fig. 10D ). Only the lateral ampulla is visible as a thickening toward the anterior end of the lateral semicircular canal ( Fig. 10A ). Te lateral semicircular canal is gently bowed outwards along its course, whereas the vertical semicircular canals are relatively straight in their mid-sections ( Fig. 10A ). Toward the common crus, the anterior and posterior semicircular canals become strongly curved ventrally, such that their intersection in the common crus describes a strong dorsal embayment ( Fig. 10A ). Such embayments have been observed and described for a range of different extant and fossil turtles (e.g., Evers et al., 2019b, 2021; Joyce et al., 2021c; Lautenschlager et al., 2018; Martín-Jiménez & Péréz-García 2023a, 2023b, 2023c; Rollot et al., 2021; Smith et al., 2023), although they are not always as deep as in TMP 1997.99.1 and also not universally present among turtles (e.g., Evers et al., 2019b, 2022a; Lautenschlager et al., 2018). Interestingly, the protostegid Rhinochelys pulchriceps has a similarly deep dorsal embayment (Evers et al., 2019b), whereas embayments are entirely absent in most fossil and extant crown chelonioids (e.g., Argillochelys cuneiceps , Lepidochelys olivacea , Dermochelys coriacea , Allopleuron hofmanni ; Evers et al., 2019b, 2022a), or much shallower (e.g., Puppigerus camperi , Eochelone brabantica ; Evers et al., 2022a). However, another early protostegid, Bouliachelys suteri , lacks a deep common crus embayment (Evers et al., 2022a).

Te fenestra ovalis of TMP 1997.99.1 has a lateral orientation ( Fig. 10A ) that is seen in most turtles, including chelydrids and protostegids. Tis contrasts with the derived condition of crown chelonioids, in which the fenestra ovalis is posteroventrally oriented (e.g., Evers et al., 2019b). Te fenestra perilymphatica shows as a relatively small, roundish opening at the posterior end of the endosseous labyrinth and below the semicircular canals ( Fig. 10D ).

Te structure that might represent “soft tissue” preserved remnants of the posterior semicircular duct extends through the entire endosseous posterior semicircular canal of TMP 1997.99.1, including the common crus region dorsally, and into the secondary common crus ventrally ( Fig. 10B, C, E, F ). Within the secondary common crus region, the duct curves posteroventrally below the horizontal plane of the lateral semicircular canal ( Fig. 10E, F ). Tis course matches the expectation of the posterior semicircular duct, which curves posteroventrally around the lateral semicircular duct in extant turtles (Evers et al., 2019b, 2022a; Ferreira et al., 2022). Te posterior semicircular duct of TMP 1997.99.1 has a smaller internal diameter than the semicircular canal, whereby the outer aspect of the duct membrane seems attached to the outer wall of the semicircular canal ( Fig. 10B ), creating perilymphatic spaces toward the inner perimeter of the posterior semicircular canal ( Fig. 10B, E ). Tis, again, is expected for a membranous semicircular duct, as these are suspended from the outer semicircular canal walls in extant reptiles (Baird, 1970), which has also been observed and figured for extant turtles (Evers et al., 2019b). Te internal diameter of the posterior semicircular duct of TMP 1997.99.1 is unusually large, and much larger than the internal diameters that have been reported for membranous turtle labyrinths (Evers et al., 2019a, 2019b; Ferreira et al., 2022), including sea turtles (Evers et al., 2022a). Te functional interpretations of this are discussed below.

Potential arterial “soft tissue” preservation

Te sedimentary matrix of the nodule in which TMP 1997.99.1 is preserved is not homogeneous, but rather traversed by likely burrows of varying internal diameters and varying grey value differences to the surrounding matrix in the CT slices. Some of the potential burrows show as lighter colored structures, whereas other are darker. Most of these potential burrows have a seemingly random spatial distribution and traverse the internal hollows of the fossil from all directions. However, we noticed that a few of these structures follow the expected course of arterial structures. Particularly, there is a continuous system of structures within the bony canals of the internal carotid artery system that potentially represent “soft tissue” preservation of the carotid arteries. Tis inference is supported by the approximate right-left symmetry of structures to either skull side (Supplementary File 1: Fig. S2 ). However, it is also possible that these structures arise from bioturbators that follow the path of the arterial canals. Besides tracing the internal carotid artery canal, the palatine artery canal and the cerebral artery canal (Supplementary File 1: Fig. S2 ), the proposed “soft tissue” system shows an additional, posterior branch of the internal carotid artery, which extends through the floor of the cavum acustico-jugulare (Supplementary File 1: Fig. S2 ). Tis posterior branch is asymmetric on both skull sides, as the left sided structure has an additional subbranch we could not find in the right one (Supplementary File 1: Fig. S2 ). Yet, the proposed “soft tissue” system additionally shows an arterial branch in the area of the enlarged foramen pro ramo nervi vidiani (Supplementary File 1: Fig. S2 ). On the right side, this branch is clearly exiting this foramen as well as the trigeminal foramen into the adductor fossa (Supplementary File 1: Fig. S2 ). On the left side, this branch is shorter and less clearly associated with the foramen pro ramo nervi vidiani (Supplementary File 1: Fig. S2 ). Although we are careful in inferring any novel arterial conclusions from these observations, they at least provide tentative additional evidence for the “soft tissue” preservation we infer for the membranous labyrinth.

Dentary

Te dentary of TMP 1997.99.1 ( Figs. 11 , 12 ) is well preserved. Te dentary is a large, unpaired bone resulting from the fusion of the left and right dentaries, which is seen in all cryptodires (e.g., Evers et al., 2022b; Gaffney, 1979). Te dentary of TMP 1997.99.1 contacts the coronoid, the surangular, the angular, and the splenial. Contacts with the prearticular and articular are absent

( Figs. 11 , 12 ). Te dentary represents the main part of the lower jaw and dominates the mandible in lateral view, overlapping most of the coronoid and surangular, such that only a small band of the latter is visible dorsally along the lateral margin of the dorsal foramen into the fossa Meckelii ( Fig. 12A–D ). Posteriorly, the dentary overlaps the surangular up to a level shortly before the foramen auriculotemporalis. Tis jaw configuration is similar in Toxochelys latiremis ( Matzke, 2009) but in contrast to that of modern cheloniids, in which the surangular has a large lateral exposure and an anteriorly directed process that separates the dentary into a posterodorsal and posteroventral process (Evers et al., 2022b).

Te dentary rami of TMP 1997.99.1 are robust and anteroposteriorly elongated processes like in all pan-chelonioids. In contrast to extant cheloniids (Evers et al., 2022b), but similar to Toxochelys latiremis ( Matzke, 2009) , the dentary rami of TMP 1997.99.1 retain approximately the same width across their length and show no expansion of the triturating surfaces toward the symphysis ( Fig. 11A, B ; Brinkman et al., 2006). Te anterior tip of the dentary in the symphyseal area is deeply notched dorsally, whereas the notch is laterally framed by a small spike-like projection to either side

( Fig. 11A–C ). Tis morphology is highly unusual and not seen in any other turtle that we are aware of, but we are confident that it is genuine, as there are no external signs of damage, as the spike-like projects are symmetric, and as the mandible was found in articulation with the cranium. Tere is no trace of a median symphyseal ridge in TMP 1997.99.1 ( Fig. 11A–B ). Te triturating surfaces are moderately narrow and decorated by a low, but sharp labial ridge ( Fig. 11A–C ; Brinkman et al., 2006). Te lingual ridge is absent, but the lingual margin is nevertheless raised enough for the triturating surfaces to form a slightly depressed dorsal platform ( Fig. 11A, B ; Brinkman et al., 2006). Te lingual margin is rounded toward the medial (= lingual) surface of the dentary

( Fig. 12E–H ). Te triturating surface bears a line of anteroposteriorly aligned, large neurovascular foramina

( Fig. 11A, B ). In addition, such foramina of various sizes cover the lateral surface of the dentary anterior to the coronoid process and adductor fossa. Tey are particularly dense close to the labial margin of the jaw

( Figs. 11C , 12A–D ).

In lateral view, the dentary is anteriorly high at the symphysis, becomes lower in central parts of the dentary ramus and then becomes slightly higher again toward the low coronoid process ( Fig. 12A–D ). Te adductor fossa is well imprinted onto the lateral dentary surface below the externally visible articulation area with the coronoid (Brinkman et al., 2006). Te adductor fossa is ventrally bound by a relatively strong ridge ( Fig. 12A–D ; Brinkman et al., 2006). Te foramen dentofaciale majus is located just anterior to the distinct adductor fossa ( Figs. 12A–D ). Although still a large opening, the foramen dentofaciale majus of TMP 1997.99.1 is smaller than in many extant cheloniids (Evers et al., 2022b). Tis is possibly because there is a smaller, additional foramen just anterior to the ‘major’ foramen dentofaciale majus, which also opens into the canalis alveolaris inferior. Te canalis alveolare inferior has a very large diameter in TMP 1997.99.1, and can be traced anteriorly until the symphyseal notch, where it penetrates the surrounding bone. Posteromedially, the canal opens into the large foramen alveolare inferius, which is concealed in medial view by the prearticular and splenial where it opens into the fossa Meckelii. Te fossa Meckelii anteriorly becomes a shallow sulcus cartilaginis Meckelii on the lingual surface of the dentary ( Fig. 12E–H ). It becomes shallower toward the symphysis, where the left and right sulci meet below the lingual margin of the symphysis. In addition, there is a fine, narrow, yet deep groove directly ventral to the lingual margin. Te groove is coalescent between right and left dentary rami, and continues posteriorly until approximately half the anteroposterior length of each dentary ramus ( Figs. 12E–H ).

Surangular

Te surangular of TMP 1997.99.1 is a paired bone that is situated at the posterior end of the lateral side of the mandible ( Figs. 11 , 12 ). It has two principal structures: an anterodorsally directed process that contributes to the dorsal foramen into the fossa Meckelii, and a posterior part that is included in the jaw articulation surface. Te surangular contacts the dentary anteriorly, the angular ventromedially, the articular posteriorly, the coronoid anterodorsally, and possibly the prearticular medially along its recurved lamina ( Figs. 11 , 12 ).

Te anterodorsal processes of the surangular forms the lateral margin of the dorsal foramen into the fossa Meckelii ( Fig. 11A–E ). Tis lateral margin of the foramen extends dorsally much higher than its medial margin (as formed by the prearticular), resulting in a broad medial exposure of the surangular and a dorsomedial orientation of the foramen into the fossa Meckelii ( Fig. 12E–H ). Tis is unusual for chelonioids, as the lateral and medial margins are equally high in extant cheloniids (Evers et al., 2022b). At the posterior end of the fossa Meckelii and the base of its anterodorsal process, the surangular of TMP 1997.99.1 shows a strong anteromedially directed recurved lamina (Evers & Benson, 2019) ( Fig. 11D, E ). In the fossil, the lamina comes close to the lateral wall of the fossa Meckelii as formed by the prearticular, but an actual contact is just about absent ( Fig. 11D, E ). However, given how close both bones come, the contact may well be inferred based on external observation of the specimen. Te recurved lamina of TMP 1997.99.1 encases a deep anteromedial recess of the surangular, which extends to the fossa Meckelii posterolaterally. At the posterior surface of the fossa, there is a large auriculotemporal canal, which extends posterolaterally through the surangular and opens on the lateral surface below the level of the articular surfaces as a small and anteroposteriorly elongated foramen auriculotemporalis

( Fig. 12A–D ). A dorsal surangular foramen is absent. Te foramen auriculotemporalis is absent in Dermochelys coriacea and extant cheloniids (Evers et al., 2022b), but appears in protostegids (e.g., Rhinochelys pulchriceps ; Evers et al., 2019a; Protostega gigas : FMNH P27385) and in Toxochelys latiremis ( Matzke, 2009) .

Posteriorly to the fossa Meckelii, the surangular becomes laterally expanded, forming a roughly crescentic, posterodorsally exposed surface that forms the anterolateral part of the area articularis mandibularis

( Fig. 11D, E ). Te lateral expansion is caused by a moderately wide ectocondylar flange (Evers et al., 2022b), which is well rounded. Tis flange overhangs the foramen auriculotemporalis laterally. Te articular surface of the surangular is nearly flat, but angled slightly laterally, so that the medial margin that is facing the prearticular and articular bones is gently raised. Tis creates a subtle division of ecto- (lateral) and endocondylar (medial) subfacets ( Fig. 11D, E ), which are common in cryptodires (Evers et al., 2022b).

Coronoid

Te coronoid of TMP 1997.99.1 ( Figs. 11 , 12 ) is a paired bone situated at the dorsal end of the coronoid process. It contacts the dentary anteroventrally and laterally, the splenial anteromedially, the prearticular posteromedially, and the surangular posterolaterally ( Figs. 11 , 12 ).

Te coronoid consists of a central part that is dorsally raised to a low, rounded coronoid process ( Fig. 12A–D ). Te process only extends dorsally slightly beyond the dentary, so that most of the coronoid is concealed in lateral view. Posteriorly, the coronoid forms a medial and a lateral process which arches over the fossa Meckelii to contact the prearticular and surangular, respectively

( Fig. 11D, E ). Te coronoid thus forms the anterior rim of the dorsal foramen into the fossa Meckelii. Whereas the posterolateral process is relatively short, the posteromedial one extends for nearly half the length of the foramen along the side of the prearticular ( Fig. 11D, E ).

Te coronoid of TMP 1997.99.1 has a well-developed, relatively long anteroventral process ( Fig. 12E–H ), which extends along the posteromedial surface of the dentary above the level of the fossa Meckelii, thereby minutely contributing to the triturating surfaces. Together with the splenial and dentary, the coronoid forms the foramen intermandibularis medius, i.e., the anterior opening of the fossa Meckelii ( Fig. 12E–H ). A coronoid foramen is absent in TMP 1997.99.1.

Angular

Te angular of TMP 1997.99.1 ( Figs. 11 , 12 ) is a paired, elongate bone situated ventrally at the posterior half of the medial wall of the mandibular ramus. It meets the splenial anterodorsally, the prearticular dorsally, the surangular posterolaterally, the dentary ventrolaterally, and the articular posteriorly ( Figs. 11 , 12 ). Te angular floors the fossa Meckelii, thereby prohibiting a dentary– prearticular contact ( Fig. 11D, E ). Te posterior part of the angular is mediolaterally broader than the anterior parts of the bone, which becomes extremely thin and rod-like ( Figs. 11F, G , 12E–H ). Te posterior end of the bone is more or less horizontally oriented

( Fig. 11F, G ), underlapping the ventral margins of the surangular, articular, and prearticular. Te contact with the articular is small, so that much of the latter bone was ventrally exposed and not covered by the angular. As the angular begins to taper anteriorly, it becomes medially twisted until it lies vertically within the medial wall of the mandibular ramus ( Fig. 12E–H ). In this area, it is underlapped by the dentary, so that it has no ventral exposure ( Fig. 11F, G ). Close to the triple junction with the splenial and prearticular, the angular forms the ventral margin of a relatively small posterior intermandibular foramen, which is dorsally closed by the prearticular ( Fig. 12E–H ). Posterior intermandibular foramina are completely reduced in most extant cheloniid species (Evers et al., 2022b), but are clearly present in Toxochelys latiremis (e.g., AMNH 14221).

Prearticular

Te prearticular of TMP 1997.99.1 ( Figs. 11 , 12 ) is a paired, vertically oriented sheet of bone in the posterior part of the medial side of the mandibular ramus. Te prearticular contacts the splenial anteroventrally, the coronoid anterodorsally, the articular posteriorly, the angular ventrally, and likely the surangular ( Figs. 11 , 12 ). Te prearticular has no contact with the dentary. Te anterior end of the bone forms parts of the medial wall of the fossa Meckelii ( Fig. 11D–E ). As a splenial is present in TMP 1997.99.1, the prearticular only has a single, anterodorsally directed process (Evers et al., 2022b)

( Fig. 12E–H ). Its dorsal margin forms parts of the medial margin of the dorsal foramen into the fossa Meckelii, but its contribution to this margin is reduced by a moderately long posteromedial process of the coronoid ( Fig. 11D– E ). Along the contact with the angular, the prearticular forms the posterior intermandibular foramen ( Fig. 12E– H ). A prearticular foramen is absent. Just anterior to the articular surface of the mandible, the prearticular nearly contacts the surangular along its vertical, recurved lamina (see surangular) ( Fig. 11D–E ). Posterior to this near contact, the prearticular and surangular enclosed a posterior, pocket-like expansion of the fossa Meckelii. Tis cavity is posteriorly closed by the articular, which is wedged between the prearticular and surangular. Te articular of TMP 1997.99.1 lacks an anterior articular process, which is present in many turtles although it seems to ossify very late in ontogeny (Evers et al., 2022b). It is thus possible that the posterior prolongation of the fossa Meckelii housed the cartilaginous anterior end of the articular of TMP 1997.99.1. Te prearticular participates in the formation of the anteroventral part of the articular facet of the mandible in the form of a small flange that is medially expanded and becomes wider posteroventrally ( Fig. 11D–E ).

Splenial

Te splenial of TMP 1997.99.1 ( Figs. 11 , 12 ) is a paired, vertically oriented, sheet-like bone that forms the anterior part of the medial wall of the fossa Meckelii. Te splenial meets the coronoid dorsally, the prearticular posterodorsally, the angular posteroventrally, and the dentary anteroventrally ( Figs. 11 , 12 ). Te posterior part of the splenial is sheet-like, filling the entire space between angular and prearticular. Anterior to the prearticular, the splenial has a short contact with the coronoid ( Fig. 12E–H ), which differs from the long splenial–coronoid contact of chelids (Evers et al., 2022b), plesiochelyids (Gaffney, 1976), pleurosternids (Evers, 2023), baenids (Gaffney, 1982), helochelydrids (Joyce et al., 2014), but also from the plesiomorphic condition of turtles in which this contact is entirely absent ( Proganochelys quenstedtii : Gaffney, 1990). Anterior to its coronoid contact, the splenial of TMP 1997.99.1 gradually reduces its height ( Fig. 12E– H ). Te resulting anterodorsal border of the splenial forms the posteroventral margin a the large and anteriorly directed foramen intermandibularis medius, which is the opening from the fossa Meckelii into the sulcus Meckelii. Te anterior splenial process continues anteriorly in the ventral margin of the sulcus Meckelii, where it contacts the dentary ventrally ( Fig. 12E–H ). Tere is a clearly developed splenial foramen near the base of the anterior process ( Fig. 12E–H ). Tis foramen is also seen in those specimens of Toxochelys latiremis which clearly preserve a splenial (e.g., AMNH 1496; Supplementary file 1: Fig. S3 ). Splenials are absent in extant chelonioids and have also not been reported to occur in Toxochelys latiremis ( Matzke, 2009) . However, we identified several specimens in which splenials are undoubtedly present (AMNH 1496, AMNH 14221, AMNH 5118, YPM 3609, YPM 3611, Supplementary file 1: Fig. S3 ). It seems possible that previous descriptions of Toxochelys latiremis (i.e., Matzke, 2009) have overlooked the suture between angular and splenial. For instance, AMNH 5119 is figured in Matzke (2009: Fig. 13 ), and the angular in the interpretative line drawing increases its height anteriorly, which is unusual for angulars generally (Evers et al., 2022b). Ten, the anterior end becomes a thin process, and there is a clearly developed foramen associated with the base of this process. Te morphology of the anterior part of the angular in Matzke (2009) thus mirrors the splenial morphology of TMP 1997.99.1. Tus, it seems likely that splenials are plesiomorphically present within pan-chelonioids. Besides in TMP 1997.99.1 and Toxochelys latiremis , splenials also occur in early protostegids (e.g., Rhinochelys pulchriceps : Evers et al., 2019a; Santanachelys gaffneyi : pers. obs. SWE), and in Allopleuron hofmanni ( Mulder, 2003) , which is usually interpreted as a Cretaceous crown group chelonioid.

Articular

Te articular of TMP 1997.99.1 ( Figs. 11 , 12 ) is a small, posteriorly situated, paired bone that mostly forms the area articularis mandibularis, i.e., the articular facet with the quadrate. Te left articular is complete, but the right one is abraded posteroventrally. Te articular contacts the surangular anterolaterally, the prearticular anteromedially, and the angular anteroventrally ( Figs. 11 , 12 ). Te articular is positioned between the posterior processes of the prearticular and surangular ( Fig. 11F– G ), where it closes the posterior extension of the fossa Meckelii that lies between these bones. Te articular of TMP 1997.99.1 lacks an anterior process, which may have been formed as a cartilaginous process within this recess. Te articular is a bone that ossifies late in ontogeny, resulting in the common absence of an ossified anterior process in many turtles (Evers et al., 2022b).

Te area articularis mandibularis is the dorsally exposed surface of the articular ( Fig. 11D–E ). In TMP 1997.99.1, this surface of the articular contributes to roughly one fourth of the total articulation area, with one half formed by the surangular and another fourth by the prearticular ( Fig. 11D–E ). Along the contact with the surangular, the articular is slightly raised, resulting in a posterior tubercle that finishes the articular surface posteriorly, and separates it from the small, lip-like retroarticular process. Tis is as in most pan-chelonioids, in which an elongate retroarticular process is absent, with the exception of some Late Cretaceous protostegids such as Terlinguachelys fischbecki (Lehman and Tomlinson, 2004) . Posterior chorda tympani foramina are absent in the articular (or elsewhere) of TMP 1997.99.1.

Cornu branchiale I

Te preserved record of the hyoid apparatus of TMP 1997.99.1 consists only of a fragment of one hyobranchial element still floating in the matrix ( Figs. 1C , 13 ). Te position of this element close to the left quadratojugal could mean that it formerly belonged to the cornu branchiale I, probably its anterior part. However, it is also possible that this element is not preserved in its original anatomical position. For the purpose of this description, we use directional terms (e.g., “lateral”) with regard to the currently preserved position of the element.

Te element consists of a slightly curved rod ( Fig. 13 ), which has a thinner posterior end with a near circular cross-section, and a dorsoventrally broader but mediolaterally thinner anterior end with an elliptical cross-section. Tis anterior expansion is often observed in cornu branchiale I of turtles (e.g., Matzke, 2009; Mulder, 2003; Siebenrock, 1898). A specimen of Toxochelys latiremis (AMNH 5118) that preserves the first hyobranchial was previously reported ( Matzke, 2009; Zangerl, 1953a) and seems to show a similar morphology as in TMP 1997.99.1. However, due to the deformation that this specimen has undergone, it is difficult to confirm. Another record for this element occurs in NHMM 000001, a specimen of Allopleuron hofmanni , and shows a more squarish lateral profile for its anterior tip (see pl. 19 in Mulder, 2003).

Figures

Fig. 2 Three-dimensional renderings of the cranium of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A dorsal view; B interpretative line-drawing; C ventral view; D interpretative line-drawing.Note that bones are labelled in bold and features labelled in regular font. an apertura naris, boc basioccipital, ex exoccipital,f frontal, fon foramen orbito-nasale, fpcci foramen posterius canalis caroticus interni, fpp foramen palatinum posterius, fprp foramen praepalatinum, fsm foramen supramaxillare, fst foramen stapedio-temporale, fti inferior temporal fossa,in internal nares, j jugal, labr labial ridge, linr lingual ridge, mc mandibular condyle, mx maxilla,mp median pocket, op opisthotic,pal palatine, par parietal, pbs parabasisphenoid, pf prefrontal,pfo pterygoid fossa, pmx premaxilla, po postorbital, pr prootic, pt pterygoid, pto processus trochlearis oticum, q quadrate,qj quadratojugal, soc supraoccipital,sq squamosal, uf unnamed foramen,v vomer, Vc “V” shaped ventral crest

Fig. 3 Three-dimensional renderings of the cranium of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A anterior view; B interpretative line-drawing; C posterior view; D interpretative line-drawing. Note that bones are labelled in bold and features labelled in regular font. an apertura naris, boc basioccipital, ex exoccipital,f frontal, fap foramen antrum postoticum, fet fissura ethmoidalis, XII foramen nervi hypoglossi, fm foramen magnum, fop fenestra postotica, ica incisura columella auris,j jugal, mx maxilla, op opisthotic,pal palatine, par parietal,pbs parabasisphenoid, pf prefrontal,pmx premaxilla,po postorbital, pop paraoccipital process, pr prootic, pt pterygoid, qj quadratojugal, q quadrate, soc supraoccipital, sq squamosal, v vomer

Fig. 4 Three-dimensional renderings of the cranium of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A left lateral view; B interpretative line-drawing; C right lateral view; D interpretative line-drawing. Note that bones are labelled in bold and features labelled in regular font. Also note that on the right skull side, the jugal facets on the postorbital and quadratojugal indicate an originally larger extent of the bone, indicated in D by hashed color and a dashed outline. ap antrum postoticum, crcr crista cranii, csoc crista supraoccipitalis, f frontal, fap foramen antrum postoticum, fon foramen orbito-nasale, ica incisura columella auris, j jugal, mc mandibular condyle,mx maxilla, pal palatine, par parietal, pbs parabasisphenoid, pf prefrontal, pmx premaxilla, po postorbital, pt pterygoid, ptf pterygoid flange, q quadrate, qap quadrate articular process, qj quadratojugal, soc supraoccipital, sq squamosal, v vomer, vqjp ventral quadratojugal process

Fig. 5 Three-dimensional renderings of the cranium of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A right internal view; B interpretative line-drawing; C left internal view; D interpretative line-drawing. Note that bones are labelled in bold and features labelled in regular font. an apertura naris, avfp anteroventral process of the frontal,boc basioccipital,carr cartilaginous rider, ccb canalis caroticus cerebralis, co occipital condyle, csoc crista supraoccipitalis, ex exoccipital,f frontal, faf fossa acustico-facialis, fet fissura ethmoidalis, fnt foramen nervi trigemini, fon foramen orbito-nasale,iof interorbital fenestra,j jugal, mc mandibular condyle, mx maxilla, op opisthotic, pal palatine, par parietal, pbs parabasisphenoid, pf prefrontal, pif processus interfenestralis, pip processus inferior parietalis, pmx premaxilla, po postorbital,pr prootic, pt pterygoid, qj quadratojugal, q quadrate, rbs rostrum basisphenoidale,soc supraoccipital, sol sulcus olfactorius,sq squamosal, XII foramen nervi hypoglossi,v vomer, VII foramen nervi facialis, VIII foramen nervi acusticus, vppt ventral process of the pterygoid

Fig. 1 Photographs of the holotype skull of Nichollsemys baieri (TMP 1997.99.1). A dorsal view. B ventral view. C left lateral view. D right lateral view. E anterior view. F posterior view

Fig. 6 Three-dimensional renderings of the partial braincase of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1),A anterolateral view onto the trigeminal area of the left skull side; B anterolateral view of the internal basicranium; C close-up on the parabasisphenoid area based on B; D interpretative line-drawing. Note that bones are labelled in bold and features labelled in regular font. avr anterior ventral ridge, boc basioccipital, btb basis tuberculum basalis,cci canalis caroticus internus, clp clinoid process, faccc foramen anterius canalis caroticus cerebralis, faccp foramen anterius canalis carotici palatinum, faf fossa acustico-facialis, fcav foramen cavernosum, fnt foramen nervi trigemini, par parietal, pbs parasphenoid, pip processus inferior parietalis,pr prootic, pt pterygoid, q quadrate,rbs rostrum basisphenoidale, scav sulcus cavernosus, soc supraoccipital, ‘st’ vestigial sella turcica, VI foramen nervi abducentis

Fig. 7 Three-dimensional renderings of the partial left lateral skull side of Nichollsemys baieri (TMP 1997.99.1) with the squamosal rotated into alternative positions. A, C, E squamosal with low inclination. B, D, F squamosal with high inclination.A, B left lateral view; C, D dorsal view; E, F posterior view.Note that bones are labelled in bold and features labelled in regular font. aspq anterior squamosal tapering process, fst foramen stapedio-temporale, j jugal, op opisthotic, plf posterolateral fossa,po postorbital, pop paraoccipital process prootic, q quadrate, qj quadratojugal, sq squamosal, sqms squamosal medial sheet, sqpp squamosal posterior process

Fig. 8 Three-dimensional reconstruction of the partial basicranium of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1) with inferred cranial circulation based on canal endocasts. A dorsal view, with left side rendered transparent; B ventral view,with left side rendered transparent; C interpretative line-drawing of the dorsal view;D lateral left view without lateral head vein; E lateral left view including lateral head vein. Note that bones are labelled in bold and features labelled in regular font. ac cerebral artery, ap palatine artery, faccc foramen anterius canalis caroticus cerebralis,faccp foramen anterius canalis carotici palatinum, fnt foramen nervi trigemini, fprnv foramen pro ramo nervi vidiani, ica internal carotid artery, lhv lateral head vein, pbs parabasisphenoid, pt pterygoid,rbs rostrum basisphenoidale, spica split of the internal carotid artery

Fig. 9 Three-dimensional renderings of the partial braincase of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1) in anterior and slight lateral view. A overview of braincase morphology; B close-up onto the parabasisphenoid and anterior canalis cavernosus area; C interpretative line-drawing based on A; D interpretative line-drawing based on B. Note that bones are labelled in bold and features labelled in regular font. aexp anterior (hooked) exoccipital process,boc basioccipital, carr cartilaginous rider, cpt crista pterygoidei, csc crista supraoccipitalis,dptr dorsal interpterygoid ridge, faccc foramen anterius canalis caroticus cerebralis, faf fossa acustico-facialis, fnt foramen nervi trigemini, fprnv foramen pro ramo nervi vidiani, fst foramen stapedio-temporale, par parietal, pbs parabasisphenoid,pip processus inferior parietalis, pr prootic,pt pterygoid, ptf pterygoid flange, pto processus trochlearis oticum, q quadrate, rbs rostrum basisphenoidale, scav sulcus cavernosus, soc supraoccipital,VI foramen nervi abducentis, VII foramen nervi facialis

Fig. 10 Three-dimensional renderings of right endosseous labyrinth,the stapes, and“soft tissue” preservation of the posterior semicircular duct of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1).A lateral view of the endosseous labyrinth; B lateral view of the endosseous labyrinth rendered transparent with the preserved posterior semicircular duct in solid; C lateral view of the posterior semicircular duct; D posterior view of the endosseous labyrinth; E posterior view of the endosseous labyrinth rendered transparent with the preserved posterior semicircular duct in solid; F posterior view of the posterior semicircular duct; G lateral view of labyrinth structures rendered transparent to see the position of the stapes,which is slightly medially displaced through the fenestra ovalis; H stapes in dorsal view; I stapes in anterior view; K stapes in medial view. Arrows in B show the attachment of the posterior semicircular duct to the outer (dorsal) wall of the semicircular canal.Asterisk in E shows the level of the lateral semicircular canal to highlight how the "soft tissue" bends ventrally around it. Asc anterior semicircular canal, cc common crus,ccem common crus embayment, fov fenestra ovalis, fpl fenestra perilymphatica, lab endosseous labyrinth, lsc lateral semicircular canal,psc posterior semicircular canal, psd posterior semicircular duct, scc secondary common crus, sfp stapedial footplate, st stapes, sts stapedial shaft

Fig. 11 Three-dimensional renderings of the mandible of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A dorsal view;B interpretative line-drawing; C anterior view;D close-up onto the posterior part of the left mandibular ramus in dorsal view; E interpretative line-drawing; F ventral view; G interpretative line-drawing. Note that bones are labelled in bold and features labelled in regular font. An angular,art articular, cor coronoid, den dentary, fM fossa Meckelii, pra prearticular, spl splenial, sur surangular, surl surangular lamina, syn symphyseal notch, tri triturating surface

Fig. 12 Three-dimensional renderings of the mandible of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A left lateral view; B interpretative line-drawing; C right lateral view; D interpretative line-drawing; E medial view onto right mandibular ramus; F interpretative line-drawing; G medial view onto left mandibular ramus; H interpretative line-drawing. Note that the dentary was artificially cut at the symphysis to get undisturbed views onto the medial mandibular sides.Also note that bones are labelled in bold and features labelled in regular font. an angular, art articular, cor coronoid, corp coronoid process, den dentary, fdfm foramen dentofaciale majus, fim foramen intermandibulare medius, fM fossa Meckelii, fnat foramen nervi auriculotemporalis,pra prearticular, sM sulcus Meckelii, spf splenial foramen, spl splenial, sur surangular

Fig. 13 Three-dimensional renderings of the left cornu branchiale I of the holotype specimen of Nichollsemys baieri (TMP 1997.99.1). A external lateral view; B dorsal view; C anterior view; D medial lateral view; E ventral view; F posterior view