Axestemys infernalis Joyce, Brinkman and Lyson 2019

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,