Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008

Gheerbrant, Emmanuel, Billet, Guillaume & Pickford, Martin, 2025, New data on the earliest known arsinoitheriid embrithopod (Mammalia, Paenungulata), Namatherium Pickford, Senut, Morales, Mein & Sanchez, 2008 from the middle Eocene of Namibia, Geodiversitas 47 (8), pp. 343-368 : 347-359

Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008

( Figs 2-11 )

TYPE MATERIAL. — Holotype. Namibia • 1 specimen (partial skull); Namibia, Sperrgebiet , Black Crow; Lutetian; GSN BC 13 ’08 ( Pickford et al. 2008a).

DIAGNOSIS. — See Pickford et al. (2008a).

DESCRIPTION

We here describe the new cranial specimen GSN BC 21’19 from Black Crow, which is attributed to Namatherium blackcrowense based on shared characters (see Discussion). GSN BC 21’19 consists of a damaged basicranium and posterior part of a skull comprising the squamosal, occipital and petrosal that are broken apart into seven pieces of bone. Each of these pieces of bone was reassembled and mounted in association with the holotype (partial skull) using their 3D digital models, which allowed the reconstruction of a large part of the skull of Namatherium blackcrowense , with the exception of the snout, the dorsal part of the skull and the lower jaw ( Fig. 11 and Appendix 3).

We used anatomical terminology that generally follows the English equivalents of terms from the Nomina Anatomica Veterinaria, 5th edition ( Waibl et al. 2005). When this practice was not possible (see Wible 2010), terms were taken from the general comparative literature cited below. Anatomical comparisons were made and detailed especially with Arsinoitherium , among other paenungulates.

Squamosal

The right squamosal is preserved as two broken fragments that retain anatomical connection at the level of the zygomatic process ( Fig. 2 ). One piece of bone includes part of the zygomatic process with the posterior parts of the glenoid fossa and postglenoid process, and the most lateral part (roof) of the external auditory meatus ( Fig. 2A, B ). The other piece corresponds to the posterior part of the squamosal fragment with the external auditory meatus and the posttympanic process ( Fig.2 C-F). The squamous part (scale) of the squamosal is missing (broken).

The fragment of squamosal corresponding to the posterior part of the zygomatic process ( Fig. 2A, B ) perfectly matches in size and shape with the same area preserved in the holotype of N. blackcrowense ( Fig. 11 ). It preserves most of the postglenoid process, but only a small part of the glenoid fossa corresponding to its most posterior and lateral part ( Fig. 2B ). As in the holotype, the postglenoid process forms a smooth low and inflated ridge behind the glenoid fossa. It extends transversely for about 55 mm. The glenoid fossa is shallow and flat as in the holotype, and it is open laterally. The overall construction of the zygomatic process of the squamosal is stout, with a very thick bone supporting the glenoid fossa (height between postglenoid process and upper part of the zygomatic process = 42 mm). As in the holotype, the zygomatic process flares strongly laterally, in contrast to that of Arsinoitherium Beadnell, 1902 . Dorsal to the glenoid fossa, part of the crest of the zygomatic arch is preserved in the area close to the junction with the nuchal crest.

The other squamosal fragment ( Fig. 2 C-F) preserves the large and deep notch of the external auditory meatus, flanked by both the base of the postglenoid process and the posttympanic process. It also preserves on the dorsal side the posterior part of the temporal fossa (labelled “temp f ” in Fig. 2 D-F) and the damaged area of connection between the zygomatic and nuchal crests ( Fig. 2D ). The external auditory meatus is very deep and wide open ventrally. It is set very high, being located at least 55 mm above the postglenoid apophysis and glenoid fossa. Its closeness in height relative to the zygomatic crest of the squamosal indicates it was located higher than the orbits. This is displayed in our reconstruction of the skull ( Fig. 11 ). The external auditory meatus is narrow anteroposteriorly (anteroposterior length = 14 mm) but long transversally (transverse length = at least 64 mm). In lateral view it has a typical reversed U-shape. The broken medial side of the posterior wall of the postglenoid process shows the large (estimated diameter 4.6 mm) and long postglenoid canal in section ( Fig. 2E ). It opens as a postglenoid foramen at the mid height of the external auditory meatus. The medial position of the postglenoid foramen is shared with proboscideans ( Tassy 1981; Gheerbrant et al. 2005). The postglenoid canal extends dorsally up to the roof of the external auditory meatus. More dorsally and anteriorly there is a (slightly less) large canal within the bone which is more or less parallel to the postglenoid canal; it might be homologous to the canalis temporalis described in proboscideans ( Tassy, 1981) and to the posttemporal canal in other placentals. The posttemporal canal conveys diploetica magna vessels and, in mammals, is typically located between the lateral surface of the pars canalicularis of the petrosal and the overlying squamosal ( Wible 2008; Muizon et al. 2015; MacPhee et al. 2021). This canal is independent of the postglenoid canal, which suggests that Namatherium has indeed both an intracranial opening of the posttemporal canal and an external and ventral postglenoid foramen. Posterior to the external auditory meatus, the posttympanic process is large and stout. It shows on its posterior side a bony suture with the exoccipital ( Fig. 2E, F ). The nuchal (= lambdoid) crest is damaged; it is barely visible on the back of the right squamosal fragment bearing the external auditory meatus and the posttympanic process. In this area, the nuchal crest seems to comprise mostly the squamosal, rather than the occipital.

Occipitals

The occipitals are preserved as three large and thick fragments of bones ( Fig. 3 ): the basioccipital together with the occipital condyles, the exoccipitals preserving distinctly the dorsal margin of the foramen magnum, and a damaged fragment of the supraoccipitals. In the reconstruction assembling the isolated bones ( Fig. 11 ), the occipital side of the skull is canted anteriorly.

Posterior to the external auditory meatus, the right squamosal fragment displays a suture with the right exoccipital which is partly preserved. In this area, the exoccipital is a bony tuberosity slightly inflated ventrally ( Fig. 2C, E, F ); it is homologous in position to the paroccipital process seen in Arsinoitherium at the ventro-lateral corner of its occiput (e.g., Andrews 1906, pl. I: “p.p.”). It extends ventral to the postglenoid foramen level. However, the paroccipital process remains small in Namatherium , as in Arsinoitherium .

A damaged fragment of supraoccipital (maximal dimensions as preserved 52x 46 mm), which lacks cortical bone, is present in the new material of Namatherium blackcrowense herein described ( Fig. 3E, F ). It probably corresponds to the upper part of the exoccipital bone, and it preserves only cancellous bone and the internal cerebellar surface. The cerebellar surface of the supraoccipital is characterised by the presence of a double median and longitudinal bony ridge which separates two smooth and elongated concave fossae ( Fig. 3E ) probably corresponding to endocasts of the cerebellum (separated in two lateral parts, possibly by cast of median vermis).

Exoccipital

A large fragment of the lower part of the exoccipitals is broken away as a single thick bony plate lacking the condyles. Its surface is more or less smooth, with low relief. The suture of the exoccipital with the supraoccipital is poorly preserved, and the occipital suture is not distinguishable. The exoccipital bone is comprised of one median concave surface, flanked by two oblique lateral surfaces that are inclined anteriorly. There is a very smooth median ridge of bone (nuchal tuberosity; minimum length 35 mm) that extends vertically from above the foramen magnum ( Fig. 3C ), likely for attachment of the ligamentum nuchae. This bony ridge is only weakly pronounced and much smaller than in Arsinoitherium . The median concave surface of the occipital bone is bounded laterally at mid height by a symmetrical bony tuberosity on both right and left sides.

The foramen magnum opens dorsally entirely in the exoccipitals, as in Arsinoitherium . The preserved dorsal part of the foramen magnum is high and narrow (minimum height and width,respectively 12 mm and 27mm).It has a triangular outline (reversed V-shape) in contrast to the U-shape in Arsinoitherium . In the skull reconstruction,the foramen magnum faces posteriorly and its dorsal border arcs dorsally well above the condylar level. The occipital condyles are attached to the basioccipital fragment, instead of to the exoccipital one ( Fig. 3A, B ) but the sutures are not visible and these structures were most likely comprised of the exoccipitals, as usual in mammals. They are large, more widely separated and much less pedunculate and posteriorly prominent than in Arsinoitherium . The minimal intercondylar transverse length is 15 mm, which is 4 times smaller than in Arsinoitherium . The maximal intercondylar width, also known as the occipital condyle width (OCW, see Engelman 2022), is 92.8 mm. Most parts of the occipital condyles are located lower than the basioccipital. Caudally, the long axis of the condyles is oblique dorsolateral.They are narrow and shaped liked an ovoid cylinder (slightly more convex medially); this is different from

Arsinoitherium in which the condyles are subvertical, rounded, convex and dilated dorso-ventrally. In ventral view ( Fig. 3A, B ), the condyles have the shape of a right-angled triangle with the long oblique side oriented anteriorly. The articular surface of the condyles is extended dorsally and ventrally, and more so on the ventral side. In ventral view the articular surface of the occipital condyles is significantly shorter than in Arsinoitherium , being less expanded anteriorly.

Basioccipital

The ventral border of the foramen magnum (odontoid notch) consists of the basioccipital. It is wide (~ 35.5 mm) and deep (26 mm). In ventral view, the basioccipital is notched laterally on both sides by a wide and long otic cavity which lodged the petrosal. The large extent of the otic vacuity, at least on the medial side (the only preserved part neighbouring the petrosal), indicates that the petrosal was weakly connected with surrounding bones of the basicranium, similar to Arsinoitherium ( Court 1992b) . Posterolaterally in this vacuity, there is a smooth but distinct notch for the jugular foramen, which was open medially, as is visible on the right side of the fragment. More posteriorly, there is a large hypoglossal (condylar) foramen (l = 7.5 mm; w = 4.5 mm), in contrast to Arsinoitherium which lacks it. It has an oval outline with a longitudinal long axis and is composed dorsally of two openings and canals that are also visible on the dorsal surface of the basioccipital. On the lateral side, and better preserved on the right side, in front of the condyle, there is a basioccipital process for attachment to the posttympanic process of the squamosal. The ventral condyloid (hypoglossal) fossa between this basioccipital process and the condyles is not deep and is bounded posteriorly by an antero-medial crest of the condyles. In front of the condyles, the anterior portion of the basioccipital narrows rapidly, forming a rod-like bone as in Arsinoitherium . At its anteriomost part, it is slightly higher (28.5 mm) than wider (22.5 mm) but not as much as in Arsinoitherium . The dorsal surface of the basioccipital is more or less flat and it bears a well-developed median bony ridge in its anterior part. The ventral surface bears a median bony keel that fades out about 12 mm in front of the condyles and about five millimetres anterior to the posterior edge of the otic vacuity.

Petrosal ( Figs 4-9 )

Both the right and left petrosals are present in the specimen GSN BC 21’19. The petrosal was loosely attached to the occipital and did not contact the basioccipital (see the vacuity described above) due to its small size relative to the skull, as in most large mammals. This is probably the result of a negative allometric growth of this bone relative to the rest of the cranium in mammals (its size increases at a lesser rate than that of the overall cranium; see Billet et al. 2015). Since direct indication for the connection of the petrosal to preserved parts of the rest of the cranium are lacking (bone broken), the orientation of these bones in the following description is tentatively based on patterns commonly observed in mammals (e.g., fenestra vestibuli facing laterally). This orientation is further supported by the presence of a remnant of

the surface of connection with the exoccipital, which is best preserved and visible on the left petrosal and faces medially (to posteromedially) ( Fig. 5B )

Middle ear ( Figs 4 -7)

The petrosal of Namatherium is characterised by the presence of a large and flat medially extended epitympanic wing that is larger than the promontorium. It is likely that part of this wing is missing anteriorly, since the uneven anterior edge of the preserved portion is suggestive of a fracture. The preserved part of the epitympanic wing is plate-like, but there are two small longitudinal ridges extending across its surface from the lateral sides of the promontorium. The medialmost of these ridges corresponds to the mesial extension of the rostral tympanic process seen on the promontorium, and might be its homologue.

Tympanic (ventral) view. The promontorium (pars cochlearis) is inflated, rounded and ovoid. The rostral tympanic process is present but not strongly pronounced. As in Arsinoitherium the surface of the promontorium is smooth. It lacks any osteological feature (e.g., sulcus) that would be indicative of a transpromontorial course of the internal carotid artery ( MacPhee & Forasiepi 2022). There is no medial flange of the promontorium.

The fenestra vestibuli is large and oval (stapedial ratio = 1.81; mean of right and left petrosals) in Namatherium . Arsinoitherium has a rounder fenestra vestibuli than Namatherium with a stapedial ratio of 1.6 ( Benoit et al. 2013b), in a condition hitherto considered more derived among eutherians (e.g., Court 1992b; Ekdale 2013; but see discussion in Ruf et al. 2016). The proximal base of the fenestra vestibuli, at the cochlear canal junction, is constricted with respect the outer part of fenestra vestibuli ( Fig. 9B , asterisk). In addition, it is oriented along an axis that is slightly oblique, i.e. it faces slightly more dorsally than the outer part of the fenestra vestibuli ( Fig. 9B , arrow). Posterior to the fenestra vestibuli and just lateral to the facial sulcus is a well-developed and deep depression, corresponding to the stapedial fossa. It has an elongated, oval or teardrop outline. The external aperture of the cochlear fossula which encloses the fenestra cochleae is subcircular and larger than the fenestra vestibuli. It is vertical and faces posteriorly, being barely visible in tympanic view; this shape and orientation of the cochlear fossula is shared with Prorastomus and early proboscideans, and it was considered by Court (1990) to be derived among placentals, although the condition of an external aperture of the cochlear fossula facing posteriorly is found in most placentals. The crista interfenestralis is broad between the fenestra vestibuli and cochlear fossula and continues posteriorly as a sharp ridge posteromedial to the stapedial fossa.The cochlear aqueduct (aquaeductus cochleae) opens more dorsally and medially than the external aperture of the cochlear fossula. The presence of a cochlear aqueduct is plesiomorphic with respect to Arsinoitherium in which it is merged with the fenestra cochleae, forming a single perilymphatic foramen. The cochlear aqueduct is located at a distance of about 8 mm from the external aperture of the cochlear fossula in Namatherium .

Lateral to the promontorium, the floor of the cavum supracochleare, which encloses the geniculate ganglion of the facial nerve ( Voit, 1909), is broken on both petrosals, leaving apparent the entire course of the facial sulcus. The latter is wide and runs from the primary facial foramen (the outline of which is not preserved due to breakage in this area) anteriorly to the stylomastoid notch posteriorly. The secondary facial foramen is barely distinct, due to the breakage of the cavum supracochleare, but it might have opened slightly anterior to the fenestra vestibuli at the level of a crest marking the posterior edge of the tensor tympani fossa. Although most of the extent of this fossa must have lain on the cavum supracochl eare, a slight depression extending anteroposteriorly on the promontorium just anterior to the fenestra vestibuli is suggestive of its presence. The facial sulcus is bordered laterally by a high crista parotica joining the base of the tegmen tympani anteriorly and that of the tympanohyal posteriorly. The tympanohyal is large relative to the entire petrosal, which recalls a condition seen in other large mammals (e.g., rhinos, astrapotheres; G.B. pers. obs.). The tegmen tympani is present but broken on both petrosals. Its base is preserved on the right petrosal and its large dimensions suggest that the entire process had at least a moderately inflated condition. A prominent depression flaring laterally in the ventralmost preserved part of the tegmen tympani further suggests that it may have been pierced by a broad canal. In the absence of any clear indication for a transpromontorial course of the internal carotid artery, this may be identified as a canal either for the ramus superior of the stapedial artery or for the lateral head vein (prootic sinus) (see MacPhee et al. 2021). As in Arsinoitherium there is no inflated caudal tympanic process, and the postpromontorial sinus posterior to the cochlear fossula is not clearly delimited, except laterally by the crista interfenestralis. The stylomastoid notch for the exit of the facial nerve (VII) is very large and extends posterolaterally as a long and unclosed ossified tube (hereafter called the stylomastoid tube) in direct continuity with the facial sulcus ( Fig. 4D ). The stylomastoid notch opens just posterolateral to the base of the tympanohyal, the ventral extremity of which is missing. The tympanohyal is preserved as a broad process and is compressed in an oblique anterolateral-posteromedial direction. There is no distinct fallopian aqueduct, a character known in the tethytheres ( Court 1992b), but the area of the hiatus Fallopii is broken on both right and left petrosals.

Only a small portion of the petrosal bone is preserved lateral to the crista parotica on the right petrosal, between the tegmen tympani anteriorly and the tympanohyal posteriorly. The epitympanic recess occurs as a large but shallow depression that excavates the posterior aspect of the tegmen tympani more than the roof of the middle ear posterior to the tegmen tympani. The recess thus faces more posteriorly than laterally or ventrally. The fossa incudis is located more posteriorly, on the lateral aspect of the crista parotica, slightly posterior to the level of the fenestra vestibuli and anterior to a knob-like process of unknown homology at the base of the tympanohyal. The fossa incudis is more distinct on the left petrosal and occurs as an ovoid fossa elongated anteroposteriorly. On both petrosals, the unidentified knob-like process located at the anterior base of the tympanohyal (see Fig. 6C) on the lateral aspect of the crista parotica forms the anterior wall of an unidentified foramen. This foramen represents the outlet of a canal that runs within the base of the tympanohyal, parallel to the articular surface with the squamosal bone and to the stylomastoid tube (Fig. 7). This canal opens posterolaterally ( Figs. 4A , 6C. and 7, labelled “thyc”, “thyc f2”) and may have been connected to an internal canal within the squamosal bone. The nature and homology of this canal is unknown and to our knowledge has not been observed in other placentals.

The pars mastoidea is broken and incomplete. Its preserved part is highly pneumatised.

Cerebellar view. The fossa subarcuata is very shallow as in Arsinoitherium , extant tethytheres ( Benoit et al. 2013b), and large placental species in general ( Billet et al. 2015; Le Verger et al. 2024). There is a sharp ridge (eminantia arcuata) separating it from the internal auditory meatus. In contrast to Arsinoitherium , there is a fully distinct petromastoid canal in the fossa subarcuata (Fig. 6A). The petromastoid canal is doubled in Namatherium .

The morphology of the internal auditory meatus also resembles that of Arsinoitherium ( Court 1990) : it forms a deep fossa housing the two foramina acustica, which are separated by a thin but distinct crista transversa (= crista falciformis). The foramen acusticum inferius, better preserved on the right petrosal (Fig. 6A), is very deep and oval, being elongated in the ventromedial-dorsolateral axis. It is pierced in its deepest part by small foramina forming a sieve that corresponds to the tractus spiralis foraminosus for the passage of fascicles of the vestibulo-cochlear nerve (nerve VIII). More laterally and dorsally, there is a small but distinct circular foramen corresponding to the foramen singulare. This foramen, which contained a portion of the cranial nerve VIII connected to the posterior ampulla of the vestibule (nervus ampullaris posterior canal), is also distinct on the reconstructed 3D Model of the labyrinth (Fig. 7D-E). The foramen acusticum inferius expands medially through a short and wide groove that is divided by two small median ridges (Fig. 6A). Antero-laterally to the foramen acusticum inferius, a sharp crest corresponding to the crista transversa separates a small (much smaller than the foramen acusticum inferius) subcircular fossa corresponding to a part of the foramen acusticum superius ( Fig. 5A ); it is also pierced by small foramina corresponding to a cribriform tract for the passage of the superior branch of the vestibular nerve (connecting to the superior vestibular area). The anteroventral edge of the foramen acusticum superius is not preserved on the two petrosals due to breakage in this region (see above), which precludes observing the outline of the canal transmitting the facial nerve (VII) towards the cavum supracochleare. Arsinoitherium departs from Namatherium in the absence of crista transversa within the internal auditory meatus ( Court 1990).

On the anteromedial edge of the cerebellar surface just opposite to the promontorium, there is a sulcus running medially from the internal auditory meatus. It opens via a notch on the anteromedial edge of the promontorium (Fig. 6B, C, labelled “?”). The nature of this sulcus is unclear since its connections with neighbouring sulci and foramina are blurred by the extensive damage in that area. Although one might speculate that this represents the hiatus Fallopii, the latter could also have occupied a less medial position in line with the facial sulcus and cavum supracochleare, leaving no sulcus on the cerebellar aspect, as seen in many taxa (e.g., O’Leary 2010; Wible 2012, 2022; Benoit et al. 2013b; Billet et al. 2015; Muizon et al. 2015; Wible & Shelley 2020).

The vestibular aqueduct (aquaeductus vestibuli) opens as a large slit-like opening facing posteriorly and not visible in either dorsal or ventral views as in Arsinoitherium . The flattened morphology of the vestibular aqueduct was considered by Court (1990) to be derived among placentals, but this is a rather widespread feature, especially in large species. The opening of the vestibular aqueduct is far from that of the cochlear aqueduct. It is located about 22 mm postero-dorsal to the external aperture of the cochlear fossula. Their relative positions are also similar to what is known in Arsinoitherium (see Court 1990: fig. 2C).

Bony labyrinth ( Figs 8-10 )

Both the right and left bony labyrinths are well preserved in the specimen GSN BC 21’19.

Cochlea. The volume ratio of the cochlea of Namatherium with respect to the entire labyrinth is 44.4% which is greater by 148% than in Arsinoitherium (30% in Benoit et al. 2013b). The cochlear canal resembles that of Arsinoitherium in several respects. It is flat (planispiral), with a low aspect ratio of 0.5. The vestibulo-cochlear angle between the plan of the basal turn of cochlea and the axis of the crus commune is wide (value of its obtuse angle: 136.4-132.3°) and close to that of Arsinoitherium (141° inBenoit et al. 2013b), in contrast to a wider angle in basal eutherians (see Ekdale & Rowe 2011) and Ocepeia Gheerbrant & Sudre, 2001 (102° in Gheerbrant et al. 2020). In addition, the cochlea (basal turn) plane and LSC plane make a relatively wide angle (value of its acute angle: 52-53°) with respect to the plesiomorphic eutherian condition in which they are more parallel (e.g., Ekdale & Rowe 2011; Gheerbrant et al. 2020). The apex of the cochlear canal and the helicotrema remain relatively thick (not narrowed) and circular in section. The apical lacuna for the bony modiolus is large. The secondary bony lamina (lamina secundaria) is faint and short ( Fig.9 ), it does not extend beyond the first quarter of the basal turn of the cochlea (mean length = 7.4 mm; 29% of the cochlear canal length). It gradually rises from the ventral to dorsal side of the cochlear canal before disappearing ( Fig. 9 ). The presence of a long secondary bony lamina is a plesiomorphic eutherian trait ( Ekdale & Rowe 2011; Gheerbrant et al. 2020). The secondary bony lamina is present in Stylolophus major Gheerbrant, 2021 (although the cochlea is partially preserved in this species). The absence of the secondary bony lamina, also the condition in Arsinoitherium and elephantiforms, is related to low frequency hearing ( Court 1992a; Benoit et al. 2023). The primary bony lamina is present and narrow ( Fig. 10 ). The ganglion canals (spiral canal) are well defined ( Fig. 10 ). The cochlea of Namatherium differs from that of Arsinoitherium in a few, mostly plesiomorphic traits. It has less than two turns, with about 1.7 turns (620°), in contrast to Arsinoitherium (720°). The cochlear whorls are well separated from each other, whereas in Arsinoitherium they are more coalescent.

Vestibule. The ampullae are inflated, as in Arsinoitherium and Stylolophus . The recessus sphericus (spherical recess) of the saccule and the recessus ellipticus (elliptical recess) of the utricule are inflated and separated by a distinct groove on the vestibule. The ratio of the mean radius of curvature of the semicircular canals (SC) to the inner ear height (IEH) is 0.37 ( Table 4 ), which indicates that the semicircular canals are well developed (see comparative data for Ocepeia , xenarthrans and litopterns in Billet et al. 2013, 2015; Gheerbrant et al. 2020). The semicircular canals are more or less planar and form large arcs. With a mean thickness ratio (tr) of 3.55 (for both right and left labyrinths), they are similarly stocky as in Arsinoitherium (tr = 3.42 in Schmitt 2016), in contrast to the thinner semicircular canals of Stylolophus major (mean tr = 1.46) ( Fig. 8 ). The semicircular canals are more or less flattened with an oval cross-section.This is another remarkable trait shared with Arsinoitherium , especially for the anterior semicircular canal (ASC) and posterior semicircular canal (PSC) ( Benoit et al. 2013b). It is distinct from Stylolophus and the generalised paenungulate condition of a more rounded SC cross-section ( Gheerbrant et al. 2021). The planes of the semicircular canals make right angles with small variation (mean angle 86.4° for both right and left inner ears; angle variance index Log 90var = 1.25), which are thus less acute than in Arsinoitherium (mean angle 79°). The ASC and PSC are of similar size, the former being slightly larger and higher ( Table 3 ). As in Arsinoitherium , the shape of the arc of the ASC is subcircular and that of the lateral semicircular canal (LSC) is oval (see ratios W/H in Table 3 ). The PSC is subcircular in contrast to that of Arsinoitherium ( Benoit et al. 2013b) , but similar to that of Stylolophus . The LSC is the smallest of the semicircular canals as in Arsinoitherium , although it is relatively larger and especially more elongated (more oval).

The crus commune reaches 76.7% of the ASC height, as in Arsinoitherium , but less than in Stylolophus (>85% of the ASC height; Gheerbrant et al. 2021) ( Fig. 8 ). It has a thickness ratio (tr) of 24.53 (mean of the right and left labyrinths), which indicates a stockier crus commune than in Stylolophus major (tr = 13.7 in Benoit et al. 2023) and also in Arsinoitherium (tr = 14.55 in Schmitt 2016). Namatherium lacks a secondary common crus, like Arsinoitherium . However, the morphology of the posterior branch of the LSC is in an intermediate state between Arsinoitherium and Stylolophus : it is partially coalescent with the posterior ampulla but less fused than in Stylolophus . In lateral view, the crus commune is slightly inclined posteriorly with respect to the LSC, in a similar degree to Arsinoitherium . The fenestra vestibuli opens well ventral to the lateral ampulla, and in an orthogonal plane with respect to the cochlear fenestra. The fenestra vestibuli is oval and large, and only slightly smaller than the fenestra cochleae.

The cochlear aqueduct (aquaeductus cochleae) is long (8.35 mm in the right petrosal), which implies a major size difference between the inner ear and the petrosal bone that contains it (Fig. 7) ( Billet et al. 2015). It extends medially and posteriorly, in a plane parallel to that of the LSC, and it flares gradually. Its section is anteroposteriorly - mediolaterally compressed. The cochlear aqueduct is partly coalescent at its entry on the cochlear canal with the fenestra cochleae and with the proximal part of the cochlear fossula ( Figs 8 ; 9 ). This is reminiscent of the morphology seen in Prorastomus ( Benoit et al. 2013a) . The specialised distinctive condition of a single perilymphatic foramen seen in Arsinoitherium is only known in modern tethytherians such as extant sirenians and proboscideans, where it evolved as homoplasies ( Court & Jaeger 1991; Court 1994; Savage et al. 1994; Gheerbrant et al. 2005; Benoit et al. 2013a, b; Gheerbrant et al. 2021). The vestibular aqueduct (aquaeductus vestibuli, endolymphatic canal) is much thinner than in Arsinoitherium , but its general shape is similar. It originates from a bulged pyramid-like area of the vestibule, ventromedial to the crus commune, that is quite distinctive. It is significantly longer (15.7 mm in the right petrosal) than the cochlear aqueduct. It extends as a very thin canal parallel to the crus commune, and it enlarges above the crus commune into a fan-like shape. It extends dorsally well beyond the dorsal tip of the common crus. There is a thin but distinct canal for the nervus ampullaris posterior (Figs 7; 8, labelled “cfsg”) linking the posterior ampulla to the cochlear nerve via the foramen singulare. This canal leading to the foramen singulare is tightly appressed to the vestibule. Ventral to the anterior ampulla, in between the utricule and saccule, there is a small bulge (more distinct on the left inner ear) that might correspond to the foramen for the superior vestibule area.

Figures

FIG. 2. — Right squamosal of Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008, referred specimen GSN BC 21’19 (photographs): A, B, part of the zygomatic process in posterior and ventral views, C-F, part of the external auditory meatus (postglenoid process and posttympanic process) in posterior,anterior,medial and lateral views.Abbreviations:bon tub, bony tuberosity of exoccipital; can temp, canalis temporalis;eam, external auditory meatus; EOC, exoccipital: gf, glenoid fossa; pgc, postglenoid canal; pgf, postglenoid foramen; pgp, postglenoid process; pty, posttympanic process; SQ, squamosal; sut, suture; temp f, temporal fossa (scale of the squamosal); zyg cr (br), zygomatic crest of the squamosal (broken); zyg pr, zygomatic process. Orientation axes: dors, dorsal; med, medial; lat, lateral; post, posterior. Photos by P. Loubry (CR2P, MNHN). Scale bar: 20 mm.

FIG. 3. — Occipital of Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008, referred specimen GSN BC 21’19 (photographs): A, B, basioccipital with attached occipital condyles in dorsal and ventral views; C, D, supraoccipital, lower part, in posterior and anterior (internal) views; E, F, supraoccipital, damaged upper part, in posterior and anterior (internal) views. Abbreviations: Boc pr, basioccipital process for attachment of the posttympanic process of squamosal; bony tub, bony tuberosity; cb ec, cerebellum endocasts; fm, foramen magnum; hf, hypoglossal foramen; jf, jugular foramen; Nuch tub, nuchal tuberosity;Oc, occipital condyle;od n, odontoid notch of the foramen magnum;ot cav, otic cavity (place of the petrosal);vent k, ventral keel.Photos by P. Loubry (CR2P, MNHN). Scale bar: 20 mm.

FIG. 4. — Right petrosal of Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008, referred specimen GSN BC 21’19 (photographs): A, lateral view; B, posteromedial view; C, anterior view; D, posterior view. Abbreviations for petrosal morphology: acf, external aperture of the cochlear fossula; ca, cochlear aqueduct; ci, crista interfenestralis;cp, crista parotica;ea, eminantia arcuata;exo. con. surf. (broken), connecting surface of the right petrosal bone with the exoccipital; ew, epitympanic wing; fs, facial sulcus; fsa, fossa subarcuata; fsg, foramen singular; fv, fenestra vestibuli; iam, internal auditory meatus; pmc, petromastoid canal; pr, promontorium; rtp, rostral tympanic process; smn, stylomastoid notch; thy, tympanohyal; thyc f2, tympanohyal canal, posterolateral foramen; tt, tegmen tympani; ttf, tensor tympanic fossa; va, vestibular aqueduct. Orientation axes: ant, anterior; dors, dorsal; med, medial; lat, lateral; post, posterior; ventr, ventral. Photos by P. Loubry (CR2P, MNHN). Scale bars: 10 mm.

FIG. 5. — Left petrosal of Namatherium blackcrowense Pickford,Senut,Morales, Mein & Sanchez, 2008, referred specimen GSN BC 21’19: A, medial view (photograph); B, posteromedial view (3D digital model reconstructed from micro-CT scan).Abbreviations for petrosal morphology:ca, cochlear aqueduct; ea, eminantia arcuata; ew, epitympanic wing; exo. con. surf. (broken), connecting surface of the left petrosal bone with the exoccipital; fsa, fossa subarcuata; iam, internal auditory meatus;pm, mastoid part of the petrosal; pmc, petromastoid canal; pr, promontorium; smn, stylomastoid notch; va, vestibular aqueduct. Orientation axes: med, medial;lat, lateral;post, posterior;ventr, ventral.Photo by P.Loubry (CR2P,MNHN);3D Model reconstructed by N.Poulet (CR2P,MNHN).Scale bars:10 mm.

FIG. 8. — Comparison of the bony labyrinth of Namatherium blackcrowense Pickford, Senut, Morales & Sanchez, 2008 (A-D, left petrosal referred specimen GSN BC 21’19) with that of Arsinoitherium zitteli Beadnell, 1902 (E-H) and Stylolophus major Gheerbrant, 2021 (I-L): A, E, I, lateral views; B, F, J, anterior views; C, G, K, posterior views; D, H, L, dorsal views. E-H, from Benoit et al. (2013b); I-L from Gheerbrant et al. (2021). 3D digital models reconstructed from micro-CT scan.Abbreviations for labyrinth morphology:aa, anterior ampulla;apc, apical turn of cochlea;asc, anterior semicircular canal;av, vestibular aqueduct;btc, basal turn of cochlea; cc, crus commune; ca, cochlear aqueduct; cfsg, canal of the foramen singulare (for nervus ampullaris posterior); co, cochlear canal; er, elliptical recess; fc, fenestra cochleae; fsg, foramen singulare; fv, fenestra vestibuli; h, helicotrema; la, lateral ampulla; lsc, lateral semicircular canal; pa, posterior ampulla; psc, posterior semicircular canal; sbl, secondary bony lamina; sr, spherical recess; tr, tractus foraminus; va, vestibular aqueduct. Orientation axes: ant, anterior;dors, dorsal; med, medial; lat, lateral;post, posterior;ventr, ventral.A-D, 3D Model reconstructed by N. Poulet (CR2P, MNHN). Scale bars:4 mm.

FIG. 9. — Detail of the bony labyrinth of Namatherium blackcrowense Pickford, Senut,Morales & Sanchez,2008 (referred specimen GSN BC 21’19,left petrosal). 3D digital model reconstructed from micro-CT scan: A, view showing the short extension of the bony labyrinth,and the canal for the nervus ampullaris posterior (opening in the foramen singulare); B, view showing the peculiar morphology of the fenestra vestibuli, which is constricted at its opening on the cochlear canal (asterisk at the neck of the constriction of the fenestra) and which presents a change in its orientation (angled arrow depicting the broken axis of the fenestra opening); A and B are two subposterior views with slightly different orientations. Abbreviations: asc, anterior semicircular canal; ca, cochlear aqueduct; cc, crus commune; cna, canal for nervus ampullaris posterior; co, cochlear canal;fc, fenestra cochleae; fv, fenestra vestibuli; la, lateral ampulla;lsc, lateral semicircular canal; pa, posterior ampulla; psc, posterior semicircular canal; sbl, secondary bony lamina;va, vestibular aqueduct.3D Model reconstructed by N. Poulet (CR2P, MNHN). Scale bar: 5 mm.

FIG. 10. — CT Scan sections of the petrosal cochlear area of Namatherium blackcrowense Pickford, Senut, Morales & Sanchez, 2008, referred specimen GSN BC 21’19: A, B, left petrosal; C, right petrosal; D, 3D digital model of the right petrosal showing the right labyrinth by transparency and the position of CT sections A and B (tympanic view); E, same as D, for the left petrosal, CT scan section C. Abbreviations for labyrinth morphology: cc, cochlear canal; cgc, cochlear ganglion canal; co, cochlea; iam, internal auditory meatus; pbl, primary bony lamina; sbl, secondary bony lamina; sp for, tractus spiralis foraminosus; st, scala tympani; sv, scala vestibuli; tf for, tractus foraminus. 3D Model reconstructed by N. Poulet (CR2P, MNHN). Scale bars: 5 mm.

FIG. 11. — Composite reconstruction of the skull of Namatherium blackcrowense Pickford, Senut, Morales & Sanchez, 2008 based of the new specimen GSN BC 21’19 and the holotype GSN BC 13’08:A-E, ventral (A), dorsal (B), lateral (C), anterior (D) and posterior (E) views. The reconstruction of the posterior and basicranial parts of the skull was made by assemblage of the 3D digital models of the broken bones of GSN BC 21’19 that were obtained from surface-scanning. The reconstruction of the anterior and median parts of the skull is based on a restored model of the holotype GSN BC 13'08 where distorted and missing parts were corrected and completed as far as possible by mirror-imaging of the best preserved bones. See also Appendix 3. Abbreviations: boc, basioccipital; boc pr, basioccipital process for attachment of the posttympanic process of squamosal; eam, external auditory meatus; eoc (bony tub), exoccipital bony tuberosity;fm, foramen magnum;pet, petrosal; oc, occipital condyle;pgp, postglenoid process; ptp, posttympanic process; so, supraoccipital;sq, squamosal;temp f, temporal fossa (scale of the squamosal);zyg/nuch cr, zygomatic and nuchal crest;zyg p, zygomatic process.3D Model reconstructed by F. Goussard (CR2P,MNHN). Scale bar: 10 cm.