Axestemys infernalis Joyce, Brinkman and Lyson, 2019

Ponstein, Jasper, Wallaard, Jonathan J. W., Rijke, Maarten de & Fraaije, René H. B., 2024, The first cranium of Axestemys infernalis (Testudines: Trionychidae) from the Lance Formation of Wyoming, USA; an updated description and phylogenetic analysis, Palaeontologia Electronica (a 31) 27 (2), pp. 1-33 : 4-14

Axestemys infernalis Joyce, Brinkman and Lyson, 2019

Figures 1–8

.2012 Axestemys splendida Hay ; Vitek, p. 16, pl. 8, p. 17, pl. 9, figs 1, 2, 3, 4

Taxonomic Assignment

MAB 13742 resembles the three other known skulls of Axestemys infernalis in having blunt and deep maxillae, triturating surfaces that do not form a secondary palate, and a postorbital that extends over the entire anteroposterior length of the postorbital bar. These traits are also present in the other species of Axestemys with well-preserved crania; Axestemys splendida (TMP 90.59.01 and UALVP 5000) from the Campanian of Montana ( Gardner et al. 1995), A. montinsana Vitek, 2012 ( UMMP 27029) from the Palaeocene of Montana ( Vitek, 2012) and A. vittata Pomel, 1847 (BMNH R8694) from the Eocene of England ( Walker and Moody, 1985, therein Eurycephalochelys fowleri ). We thus assign MAB 13742 to A. infernalis , following the assignment of all Lance and Hell Creek material of Axestemys to A. infernalis by Joyce et al. (2019).

Overall Preservation

MAB 13742 comprises a relatively complete skull. The bones of the skull roof are fairly smooth ( Figure 1 ), in contrast to the sculptured skull roof in NSM PV24650 ( Joyce et al., 2019). Although the skull surface appears ribbed on the scan images ( Figures 2-7 ), this is an artefact of the scan. Overall, the right half of MAB 13742 is better preserved, and lies slightly anterior to the left half. The posterodorsal region is poorly preserved. Notably, the posterodorsal portion of the crista supraoccipitalis is broken off. The dorsal surface of the processus trochlearis oticum bears various cracks, so that bone contacts are not discernible. Both zygomatic arches are broken posteriorly. The basioccipital, squamosal, and quadratojugal are either missing or severely damaged.

Overall Proportions

Although MAB 13742 is relatively large (snout – occipital condyle length 9.8 cm [occipital condyle is missing, measured snout – midline of quadrate condyles]), it is significantly smaller than DMNH 98814 (14.5 cm), the smallest skull reported by Joyce et al. (2019). As in trionychids ( Gaffney, 1979b), the temporal emargination is well-developed. The anterior portion of the skull is roughly triangular in outline ( Figure 3 ). The orbits are large, roughly two-thirds of the skull in dorsoventral height, and appear to project anteriorly ( Figures 2 , 4 ).

Osteology

The prefrontal is the anteriormost bone of the skull roof, comprising the dorsal margin of the apertura narium externum and the anteromedial wall of the orbit ( Figures 2 , 3 ). The apertura narium externum is weakly excavated laterally to a similar degree as DMNH 98814 ( Vitek, 2012), but it is not as extensive as in DMNH 130951 ( Joyce et al., 2019). Both prefrontals suture at the midline, which is discernible as a straight line through most of its course. The prefrontal contacts the maxilla anteroventrally, anterior to the orbit, yet seems to be fused to the frontal dorsally. Through a descending process, it contacts the dorsal side of the vomer within the orbit.

The frontal continues roofing the orbit and includes the dorsalmost point of the orbit ( Figures 2 , 3 ). Similar to the prefrontals, the frontals meet along a straight suture at their midline. In dorsal view, the frontal sutures with the parietal halfway the postorbital bar and contacts the anterodorsal portion of the postorbital. The anteroventral suture with the prefrontal is not visible.

Only a small portion of the parietals is preserved, as the anteromedial margin of the temporal emargination and the anteromedial portion of the processus trochlearis ( Figure 3 ). On the skull roof, the parietals are slightly depressed safe for a modest ridge that defines their medial suture. The parietal forms a straight suture with the frontal anteriorly and with the postorbital anterolaterally. On the processus trochlearis, it has a lateral contact with the prootic. The posterior portion of the parietal, that would have contributed to the crista supraoccipitalis, is broken off.

The postorbital forms both the posterior margin of the orbit and the anterior margin of the temporal emargination ( Figures 3 , 4 ). It extends ventrally to near the ventral margin of the orbit, contacting the maxilla ventrally and overlying the jugal – thus excluding the jugal superficially from the orbit. Expansion of the bone makes up most of the postorbital bar in lateral view, forming the dorsal most two-thirds of the posterior margin of the orbit. The lateral exposure of the postorbital is thereby much greater than in DMNH 130951 ( Joyce et al., 2019, figure 18) and the other known crania of Axestemys ( Walker and Moody, 1985; Vitek, 2012). Dorsally, the postorbital forms a broad contact with the frontal anteriorly and the parietal posteriorly. The lateral surface of the postorbital features some shallow anteroventral-posterodorsal oriented grooves.

The jugal is present as a tall but mediolaterally thin bone along the anterolateral margin of the skull and comprises part of the zygomatic arch ( Figures 3 , 4 ). Anteriorly, it is overlain by the postorbital dorsally and a posterior process of the maxilla ventrally. This configuration causes a superficial exclusion of the jugal to the orbit in lateral view unlike the other specimens of Axestemys infernalis ( Vitek, 2012; Joyce et al., 2019). Whether the jugal continues anteriorly underneath the maxilla and postorbital to reach the orbit could not be determined. Only the anterior portion of the zygomatic arch is preserved, so its relation to either the quadratojugal, squamosal, or pterygoid are unknown.

The presence of the quadratojugal in MAB 13742 is uncertain, as the posterior portions of the zygomatic arches are missing, and the dorsal surface of the quadrates is damaged. If the quadratojugal is present, it is merely by small fragments that do not yield useful morphological information.

The squamosal is likely missing or at least severely broken. In other Axestemys infernalis specimens, the squamosal forms an elongate posterior process that extends beyond the occipital condyle ( Joyce et al., 2019). No such structure is preserved in MAB 13742. Potential areas of contact, such as the dorsal portion of the quadrate and the posterior portion of the quadratojugal are also missing or damaged.

There is no ossified premaxilla present in MAB 13742 ( Figure 2 ). This bone is present and ossified in DMNH 98814 and NSM PV24650 ( Joyce et al., 2019), and the known crania of Axestemys splendida ( Gardner et al. 1995) , A. montinsana ( Vitek, 2012) , and A. vittata ( Moody and Walker, 1970; Walker and Moody, 1985).

The maxilla is a dorsoventral deep bone that composes the entire ventral margin of orbit, as well as the lateral and ventral wall of apertura narium externum ( Figures 2 , 4 ). In anterior view the maxillae form an arch ventral to the apertura narium externum, a consequence of the absence of the premaxilla. Inside the nasal capsule, the medial side of the maxilla bears a posteriorly oriented supraalveolar foramen ( Figure 5 ). The maxilla contacts the prefrontal anteriorly to the orbit along a straight suture. In contrast to other Axestemys infernalis specimens, where the maxilla-jugal contact is positioned ventral to the orbit ( Vitek, 2012; Joyce et al., 2019), in MAB 13742 the maxilla-jugal is achieved posteroventral to the orbit by a posterior process of the maxilla. This process overlies the jugal and contacts the postorbital posterodorsally. The slightly concave lateral surface is pitted and few shallow anteroposteriorly oriented grooves stretch, ventral to the orbit, from its anterior margin to the posterior process of the maxilla. The ventral margin of the maxilla curves gently posterodorsally in lateral view. Ventrally the maxilla forms a moderately wide, flat, triturating surface that is pitted with foramina ( Figure 6 ). These triturating surfaces do not meet at the midline, so that the vomer remains exposed. The triturating surfaces lack a prominent lingual margin, instead terminating in a modestly rounded edge. Sutures on the ventral side are not discernible, so its contribution to the foramen palatinum posterius could not be assessed.

In ventral view, the vomer is visible between the triturating surfaces of the maxillae ( Figure 6 ). Anteroventrally, it has a broad concave surface, its dorsal most point midway, which forms the anterior portion of the internal nares. The vomer bears a mediolaterally thin posterior process that extends one third into the anteroposterior length of the palatines. A thin groove runs along the midline of this process. Further posterior, a low ridge continues the trajectory of this process until the dorsal margin of the internal nares, but it is unclear whether this is a continuation of the vomer or merely in between the palatines. The vomer contacts the maxilla anterolaterally, the descending process of the prefrontal anterodorsally, and the palatines posterodorsally. Due to the resolution of the scan, the presence of foramina such as the praepalatine foramina, is unclear.

The palatine is a sheet-like bone that floors the nasal canal ( Figure 6 ). In ventral view, it forms the posterior part of the internal nares and lies further dorsal to the triturating surfaces and the vomer. A ridge runs along its midline, but it is unclear whether this ridge is an extension of the posterior process of the vomer or is formed by the palatines themselves. It contacts the vomer ventrally and reaches the parietals on the skull roof through an ascending process. Laterally, it contacts the maxillae, but the precise suture between the palatine and the maxilla is not visible. Posteriorly it contacts the pterygoid, but the precise location of this suture, as well as a potential contact with the basisphenoid, is unclear.

The pterygoid forms a major part of the posteroventral side of the cranium ( Figure 6 ). Its lateral margin is characterised by a slight ventrally curved ridge, and a lateral flange is absent. Posteriorly, at the level of the base of the foramen magnum, the pterygoid bears the opening of the foramen posterius canalis carotici interni ( Figure 7 ). Posteromedially, the pterygoid wraps around the basisphenoid, and it contacts the quadrate posterolaterally. The anterior sutures of the pterygoid are not visible, nor is it clear whether the pterygoids have a midline contact anteriorly to the basisphenoid.

Joyce et al. (2019) could not assess the presence of an epipterygoid due to the crushed trigeminal region in all three available specimens. Unfortunately, its presence in MAB 13742 is similarly unclear due to preservation.

The quadrate is recognisable as a roughly triangular bone at the posterolateral side of the cranium, that forms the deep cavum tympani and houses the incisura columellae auris ( Figure 4 ). This bone sits relatively far posteriorly, further than the foramen stapedio-temporale. Ventrally, the quadrate forms the bipartite articular condyle, posterior to the basisphenoid ( Figure 6 ). The lateral facet of the articular condyle is slightly more anteroposteriorly elongate and deeper than the medial facet. While it is clear the quadrate has a broad dorsal contact with the processus trochlearis, the exact sutures are not discernible.

The prootic borders the anterior margin of the processus trochlearis ( Figure 3 ). It has a medial contact with the parietal. The lateral margin, where it would have contacted the anterior portion of the quadrate, is broken off. Moreover, as the dorsal surface of the processus trochlearis is shattered, the distal sutures of the prootic are not discernible. Nonetheless, the foramen stapedio-temporale remains visible, and is roughly triangular in shape.

The opisthotic sits at the posterior side of the processus trochlearis. It is visible in dorsal and occipital views ( Figures 3 , 7 ), but due to the fragmentary nature of the dorsal side of this process, its sutures are not discernible.

The supraoccipital forms posterodorsal portion of skull, and roofs the foramen magnum ( Figures 3 , 4 ). The crista supraoccipitalis is mostly broken off, yet the supraoccipital still forms the posterior most portion of the cranium of MAB 13742. Anterolaterally, the supraoccipital bears two seemingly symmetrical depressions, which are more apparent on the CT rendering than on the actual specimen ( Figure 3 ). Both depressions are the result of damage.

The exoccipital forms the lateral wall of the foramen magnum and is visible in occipital view ( Figure 7 ). It bears the small foramen jugulare posterius, but no further details could be inferred about the shape and contacts of the exoccipital in MAB 13742.

The basioccipital is not preserved in MAB 13742.

The foramen magnum is floored by the basisphenoid ( Figure 7 ). The basisphenoid is a relatively small, triangular-shaped unpaired element visible at the posteroventral side of the palate. It connects to the pterygoids laterally, but it is unclear whether its anterior margin sutures to the pterygoids or the palatines.

Neuroanatomy

The neuroanatomy has not been described for any Axestemys species yet. Few studies considered variation in the morphology of the turtle brain endocast using geometric morphometrics ( Lautenschlager et al., 2018; Ferreira et al., 2022) but found little differentiation between adults of major extant clades. No trionychid braincase endocast has been described in detail. Here we compare the braincase endocast with that of the extant trionychine trionychids Apalone spinifera emoryi Agassiz, 1857 FMNH 21178 and Pelodiscus sinensis Wiegmann, 1835 IW 576-2 ( Werneburg et al., 2021a). The resolution of the scan of MAB 13742 unfortunately did not permit individual segmentation of the semicircular canals, instead showing the endosseous labyrinth as an undifferentiated single cavity. However, general statements about its proportions can be made. The endosseous labyrinth of MAB 13742 is compared to the described endosseous labyrinth of the plastomenid Plastomenus thomasii AMNH FR 6015 ( Evers et al., 2023), and to the extant trionychids Apalone spinifera emoryi FMNH 21178 (Media ID 00037651, Ferreira et al., 2021), Amyda cartiliginea Boddaert, 1770 FMNH 244117 (Media ID 000372714, Evers, 2021) and P. sinensis IW 576-2 (Media ID 000373752; Evers, 2021) that were available on MorphoSource. Additionally, the carotid artery and facial nerve canal systems could not be segmented.

Overall, the braincase endocast of MAB 13742 is anteroposteriorly elongate and tubular ( Figure 8 ). As in other turtles ( Ferreira et al., 2022), there is no clearly defined boundary between the braincase endocast and nasal cavity in MAB 13742 ( Figure 8B ). In all three specimens, the cerebral region is the most transversely wide. In Apalone spinifera emoryi and Pelodiscus sinensis , the width decreases gradually anteriorly towards the sulcus olfactorius, giving it roughly an arrowhead shape in dorsal view. In Axestemys infernalis , on the other hand, the decrease in width is more pronounced posteriorly than anteriorly, so that the area directly posterior to the sulcus olfactorius is medially constricted ( Figure 8A ). Moreover, paired spike-like projections are present ventrally at the posterior portion of the sulcus olfactorius in A. infernalis ( Figure 8C ) as in P. sinensis , which are missing in A. spinifera . In all three turtles, the optic and cerebellar region is medially constricted to make room for the endosseous labyrinth. This region widens posteriorly, but the widening is more pronounced in A. infernalis than in A. spinifera and P. sinensis . MAB 13742 bears an elongate ‘cartilaginous rider’ ( Figure 8B ), similar to what Werneburg et al. (2021b) observed in the extant trionychids A. spinifera and P. sinensis . In lateral view, the sulcus olfactorius is more dorsoventrally tall in A. infernalis than A. spinifera and P. sinensis . The entire braincase endocast is oriented horizontally in P. sinensis , while the sulcus olfactorius is deflected dorsally in MAB 13742 and in A. spinifera . Ventrally, the endocast of the pituitary fossa of MAB 13742 forms a relatively wide cone that is directed anteriorly ( Figure 8C ), and is much more pronounced than in A. spinifera , while this structure is rectangular in P. sinensis .

The foramen ovalis is relatively extensive ventrally in MAB 13742 ( Figure 8B ) compared to the extant trionychids as it is deeper than the semicircular canals; in Plastomenus thomasii AMNH FR 6015 the foramen ovalis was damaged so that its ventral extend could not be determined ( Evers et al., 2023). Moreover, the foramen ovalis in MAB 13742 curves lateroventrally, so that it is visible from dorsal view ( Figure 8A ), unlike in the other trionychians. In MAB 13742, the cavity for the anterior semicircular canal is significantly farther extended anteriorly than the posterior semicircular canal is extended posteriorly, as in P. thomasii ( Evers et al., 2023) . The depression of the common crus is minor and the dorsal most point of both the anterior and posterior semicircular are roughly at the same level, while the anterior semicircular canal is clearly more dorsally expanded in the other trionychians ( Figure 8B ).

Phylogeny

The parsimony analysis under K = 12 yielded three trees with a score of 15.94107, which is slightly lower than the score of 15.97 reported by Evers et al. (2023). Figure 9A shows the strict-consensus topology of these three trees, which is overall similar to the topology of the parsimony analysis of Evers et al. (2023). Similar to their analysis, Axestemys infernalis is placed as the sister-taxon to a clade composed of (Cyclanorbinae + Helopanoplia distincta Hay, 1902 ) + Plastomenidae ( Figure 9A ). By contrast, A. infernalis does not represent the earliest branch of Pan-Cyclanorbinae in our analysis. We recover the clade Atoposemys superstes Russell, 1930 + Asipederetoides foveatus Gardner et al., 1995 , as the earliest branch of Pan-Cyclanorbinae instead, rather than nested within Plastomenidae . This placement of A. superstes and A. foveatus is supported by one unambiguous apomorphy; 1(4). The position of A. infernalis as sister-taxon to Cyclanorbinae + Plastomenidae is then supported by a single apomorphy; 24(1).

We present a new Bayesian topology in Figure 9B (the Bayesian log file is available in Appendix 5). As in Evers et al. (2023), the Bayesian topology is markedly different from the parsimony topology concerning the contents of the Trionychidae stem-group ( Figure 9 ). This stem is formed successively by a clade containing Perochelys lamadongensis Li, Joyce, and Liu, 2015 , P. hengshanensis Brinkman, Rabi, and Zhao, 2017 , Kuhnemys maortuensis Yeh, 1965 , and Petrochelys kyrgyzensis Nessov, 1995 , and Gobiapalone orlovi as the sister-taxon to crown-Trionychidae ( Figure 9B ). Notably, and contrary to the Bayesian topology of Evers et al. (2023), we do not recover Axestemys infernalis as the sister-taxon to crown-Cyclanorbinae. Instead, we find reasonably strong support that the clade containing Hutchemys spp. Joyce et al., 2009, and Helopanoplia distincta is closer to Cyclanorbinae than A. infernalis ( Figure 9B ). A. infernalis is recovered as closer to Cyclanorbinae than to Plastomenidae , but support for this placement is weak. As in the parsimony tree, the position of A. infernalis is supported by a single apomorphy; 24(1).

Figures

FIGURE 1. Photographs of the cranium of Axestemys infernalis MAB13742 embedded in matrix. A anterior view, B right lateral view, C oblique overview, D dorsal view. Both scale bars equal 1 cm; scale bar A applies to the anterior view; scale bar B applies to both right lateral and dorsal view.

FIGURE 2. Reconstruction of the cranium of Axestemys infernalis MAB13742 in anterior view. A shows the digital rendering of the CT-scan and B the interpretative line drawing. Dashed lines are observed sutures, dotted lines indicate damage. Scale bar equals 1 cm. Abbreviations: ane. apertura narium externum, orb. orbit, fr. frontal, jug. jugal, max. maxilla, par. parietal, po. postorbital, prf. prefrontal, quad. quadrate.

FIGURE 3. Reconstruction of the cranium of Axestemys infernalis MAB13742 in dorsal view. A shows the digital rendering of the CT-scan and B the interpretative line drawing. Dashed lines are observed sutures, dotted lines indicate damage. Scale bar equals 1 cm. Abbreviations: cso. crista supraoccipitalis, fst. foramen stapedio-temporale, pt. processus trochlearis, fr. frontal, jug. jugal, par. parietal, po. postorbital, prf. prefrontal, pro. prootic, quad. quadrate, so. supraoccipital.

FIGURE 4. Reconstruction of the cranium of Axestemys infernalis MAB13742 in right lateral view. A shows the digital rendering of the CT-scan and B the interpretative line drawing. Dashed lines are observed sutures, dotted lines indicate damage. Scale bar equals 1 cm. Abbreviations: cso. crista supraoccipitalis, ica. incisura columellae auris, jug. jugal, max. maxilla, par. parietal, po. postorbital, quad. quadrate, so. supraoccipital.

FIGURE 5. Reconstruction of the cranium of Axestemys infernalis MAB13742, digital rendering of the CT-scan showing the internal view of right lateral side and the supraalveolar foramen of the maxilla. Scale bar equals 1 cm. Abbreviations: saf. supraalveolar foramen, max. maxilla. Mirrored.

FIGURE 6. Reconstruction of the cranium of Axestemys infernalis MAB13742 in ventral view. A shows the digital rendering of the CT-scan and B the interpretative line drawing. Scale bar equals 1 cm. Abbreviations: fpp. foramen palatinum posterius, bs. basisphenoid, max. maxilla, pal. palatine, pter. pterygoid, vom. vomer.

FIGURE 7. Reconstruction of the cranium of Axestemys infernalis MAB13742 in posterior view. A shows the digital rendering of the CT-scan and B the interpretative line drawing. Scale bar equals 1 cm. Abbreviations: fjp. foramen jugulare posterius, fm. foramen magnum, fpcci. foramen posterius canalis carotici interni, ex. exoccipital, op. opisthotic, pter. pterygoid, quad. quadrate, so. supraoccipital.

FIGURE 8. Digital reconstruction of the braincase endocast of Axestemys infernalis MAB13742 in A dorsal, B right lateral and C ventral views. Scale bar equals 1 cm. Abbreviations: acs. anterior semicircular canal, cc. common crus, cr. cartilaginous rider, el. endosseous labyrinth, fo. foramen ovalis, nc. nasal cavity, ocr. optic and cerebellar region, pf. pituitary foramen, pcs. posterior semicircular canal, suo. sulcus olfactorius.

FIGURE 9. Cladograms showing A the results of the parsimony analysis in TNT, using a concavity constant of K = 12. B shows the results of the Bayesian analysis. Support values are indicated for each node.