Glyptosauridae

Field: Earth and Planetary Sciences (Physical Sciences)

Glyptosauridae indet. (Figs 7, 8) Material examined: IRSNB R486, IRSNB R487,two premaxillae; IRSNB R 488, right nasal; IRSNB R 489, supraocular; IRSNB R 490, R 491, R 492 and R 493, osteoderms; IRSNB R 494, one dorsal vertebra. Locality and horizon: Dormaal, Flemish Brabant, eastern Belgium, Dormaal Member, Tienen Formation, Landen Group, earliest Eocene (MP 7). Description Premaxilla: Two premaxillae are preserved. This bone is a triradiate, T-shaped element. The larger specimen IRSNB R 486 represents a ventral portion of the premaxilla with nine tooth positions—although the first left one is partly damaged (two teeth are still preserved, their crowns are, however, weathered; Fig. 7A–E). The anterior face of this premaxilla is smooth and broadly arched. It is pierced by a pair of anterior premaxillary foramina (ethmoidal foramina). They form the openings of the canals. These canals are not closed inside the bone, and their dorsal portions are open. This open area is large on the right side, whereas there is a tendency of its closing (or, in fact, most likely initial opening) on the left side, although a notch is still present. Thus, the connection posteriorly with the posterior premaxillary foramen and a canal running ventrally are exposed on both sides in dorsal view. Lateral to that, the wedge-shaped facet for the maxilla is present on the dorsolateral surface of the maxillary process of the premaxilla (Fig. 7C, D). The maxillary processes are slightly reduced, although well defined. They form posterolaterally running triangles in anterior view (Fig. 7A). In this view, the ventral margin of the bone is straight. In posterior view, the supradental shelf is well posteriorly expanded, having a slightly wavy appearance (Fig. 7B). Its median portion extends ventrally into a bilobed incisive process. A short median fissure is present only in the posterior section between both incisive processes (Fig. 7E). Dorsally, a vomerine process is preserved (Fig. 7B). It is triangular in shape in dorsal view, gradually narrowing posteriorly. Only the base of the broad nasal process is preserved. The smaller specimen IRSNB R 487 represents an element with probably nine tooth positions—note that the left corner is broken off and the internal surface is partly weathered (two worn teeth are still attached to the left side). Its anterior face is smooth and arched (although less than in IRSNB R 486). It is pierced by a pair of anterior premaxillary foramina, but these are completely enclosed in the bone. Note, however, that the bony tissue of the dorsal side of the left foramen is distinctly narrowed. Close to the posterior opening, there are some additional small foramina located medially on each side (the same is true for the premaxilla IRSNB R 486; Fig. 7J). The vomerine process is well preserved, identical as on the larger specimen, IRSNB R 486. In both, a pit is located on the dorsal side and one additional smaller one is located anterior to the vomerine process. In IRSNB R 487, the nasal process is preserved, although its posterodorsal end is broken off. It is laterally broad, running distally with more-or-less the same width along the preserved portion. The external surface of the process is flat and an osteoderm is attached to it in the dorsal section. It is partly preserved and the tuberculate sculpture is still visible. On the internal surface, there is a low ridge running across the midline of the nasal process, being sharper in the dorsal region, giving to the process section a triangular shape. Remarks A premaxilla has been indeed described in the North American taxon Gaultia silvaticus, although being highly worn (Smith 2009: fig. 18A) and resembling the premaxillae here described from Dormaal (Fig. 7A–K). However, the nasal process of the Gaultia silvaticus premaxilla is broad basally and tapers distally (see: Smith 2009). This is not a similar feature to the Dormaal specimens, which have the width almost equal along the preserved portion of the nasal process. Moreover, the tooth crowns of both premaxillae are more-or-less worn. Thus, the allocation of the premaxillae IRSNB R 486 and IRSNB R 487, even to the genus Gaultia, cannot be supported without doubts. Nasal: The right nasal IRSNB R 488 is preserved (Fig. 7L–O). It is an elongated plate-like bone. It extends anteriorly into a broad premaxillary process—its anterior end is, however, broken off. The median suture of the nasal is straight, whereas the lateral portion slightly expands ventrally into an anterolateral process. The process is flexed ventrolaterally. The bone thus slightly widens anteriorly (expect of the premaxillary process). The dorsal surface of this nasal is covered with two sculptured shields separated by a slightly anterolaterally directed sulcus. The anterior sculptured shield is much smaller than the posterior one. The former one is anteroposteriorly elongated, slightly widens posteriorly and its posteriormost end forms a narrow expansion. The sculpture consists of small, rounded, discrete tubercles. The ventral surface of the nasals is finely weathered. It is not flat, but due to ventrolateral flection, there is a longitudinal shallow depression. The posterior end bears a frontal articulation. Remarks In glyptosaurine taxa, hexagonal osteoderms cover the entire skull (Sullivan 1979). This does not appear to be the case on the right nasal IRSNB R 488 (Fig. 7L–O) where osteoderms are more irregular in shape. The division of osteodermal cover (the pattern of osteoderm shapes) might indicate a member of ‘Melanosaurinae’, but it is also possible that it belongs to Gaultia. Supraocular: The supraocular IRSNB R 489 is anteroposteriorly elongated (Fig. 7P–S). In cross-section it narrows laterally. Its lateral margin is only slightly rounded, almost straight in dorsal view. The medial portion is trapezoidal. The whole external surface is sculptured by numerous small, discrete tubercles radiating from the ossification centre. Remarks The sculpture of this specimen is typical for glyptosaurids in general. However, the tubercles are smaller and more densely distributed in this supraocular than in the frontal and parietal of Gaultia. Therefore, we refer it to Glyptosauridae indet.. Osteoderms: Several tuberculated osteoderms are described. Most are isolated and rectangular/rhombic in shape. The tubercles are prominent. A keel appears to be absent or is only weakly developed as a tiny line in the middle of some osteoderms (e.g. IRSNB R 490; Fig. 8A). In one osteoderm, IRSNB R 491 (Fig. 8E, F), in contrast, there is a small depression in the central region. Osteoderms have a prominent and smooth overlap surface running on nearly one-third of the anteroposterior length. It is slightly higher than the posterior sculptured portion, from which it is separated by a transverse groove. The sculptured portion is covered by discrete tubercles of various sizes. The internal surface is smooth and pierced by several foramina located along the central region and, in some cases, also in the anterior region. One osteoderm (IRSNB R 492; Fig. 8I–K) is of irregular or roughly rounded shape. It is also slightly smaller than the other osteoderms. The overlap surface of this osteoderm is large (compared to sculptured surface) and is a somewhat reversed V-shape. There is also one specimen (IRSNB R 493) that represents two fused osteoderms, although traces of fusion are still visible (Fig. 8L–O). In anterior view, its lateral sides are bent, so it has a slightly rounded appearance (Fig. 8O). Remarks Taxonomic assignment of isolated osteoderms below Glyptosauridae is not possible without doubts (Rage 1978, Sullivan 1979, 2019, Estes 1983, Gauthier et al. 2012, Čerňanský et al. 2023b). Although differences in general shape exist among these osteoderms, their positions on the body probably play a more major role in this case. Dorsal vertebra: One large dorsal vertebra, IRSNB R 494, is available (Fig. 8P–U). In ventral view, the centrum is significantly anteriorly widened. The maximum anteroposterior length of the centrum is 8 mm, whereas its maximum width is 9.6 mm. In lateral view, the neural spine is robust and inclined posteriorly but its dorsal margin is damaged. It starts to rise dorsally in the first anterior third of the anteroposterior length of the neural arch. In anterior and posterior views, the neural canal is large and slightly oval in shape (higher than wide). In dorsal and ventral views, the pre- and postzygapophyses are large and extend more laterally. There is a slight interzygapophyseal constriction, but overall, the vertebra has a distinctly wide appearance. The neural arch has no anterior notch, thus only a dorsal rim of the cotyle is visible in dorsal view. Both cotyle and condyle are strongly dorsoventrally depressed. Note, however, that the surface of the condyle is damaged and the spongy bone is exposed. In lateral view, the cotyle is inclined anterodorsally–posteroventrally. For this reason, its ventral rim is located more posteriorly than the dorsal one and a small dorsal portion of the cotyle is clearly visible in ventral view. The anocotylar foramina (sensu Georgalis et al. 2021) appear to be present—at least the right one is visible (CT-scan reveals that it continues further inside the vertebra as a canal). The centrum is almost triangular in ventral view. Its anterior portion expands laterally into synapophyses (their extremities are, however, damaged on both sides). The subcentral ridges are more-or-less straight in ventral view, except near the condyle, where a small concavity is present. A distinct and wide depression is present on the ventral surface of the centrum, being located slightly anterior from the mid-section. Remarks This dorsal vertebra has the anterior centrum markedly widened compared to other glyptosaurines, similar to that observed for the North American Melanosaurus maximus Gilmore, 1928, and, to a lesser degree, Paraplacosauriops