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

Namatherium blackcrowensePickford, 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). TYPE LOCALITY AND AGE. — Black Crow, Sperrgebiet, Namibia. Lutetian in Pickford et al. (2008a). REFERRED MATERIAL (new material). — GSN BC 21’19, posterior part of skull (basicranium) including right squamosal (two fragments), basioccipital, exoccipital, supraoccipital and left and right petrosals. MEASUREMENTS. — See Tables 1-5. 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 occipita