The specimen, MNA V9953, comprises the major portion of the skull minus the mandible and preserves most of the marginal tooth rows (Figure 3). However, the surface bone is largely eroded away or poorly preserved and as a consequence no sutures can be detected. The skull roof was partially weathered prior to collection of the specimen. Although the bone is worn and poorly preserved, the general shape of the skull and the dentition can be discerned, providing the opportunity to make direct comparisons with known Late Triassic procolophonids.
As preserved it is a very low flat skull (Figure 3.3). This might be partially a result of distortion, although other Late Triassic procolophonids, such as Hypsognathus are also known to have dorsoventrally compressed skulls.
In dorsal view the skull is approximately triangular in shape (Figure 3.1). The prominent oval orbitotemporal openings are directed dorsally. These openings are greatly extended posteriorly, markedly reducing the distance between the posterior margin of this opening and the posterior margin of the skull roof. Indeed, it would appear that this distance is approximately equal to the narrowest point across the frontals. Although the posterior corner of the skull is missing on the right side, it is possible to partially reconstruct it as the mirror image of the left side (Figure 3.1). On this basis it is clear that the skull was broader than it was long. The frontals are constricted slightly toward the anterior margin of the orbits. A perfectly circular slightly raised area of matrix occurs along the midline of the skull roof between the anterior parietals. This area most likely represents the pineal foramen. The posterior margin of the skull has been eroded, and the braincase is missing.
On the left side the quadratojugal bears at least three prominent spines (Figure 3.1, 3.3), although additional spines may have broken off. The jugal extends down below the level of the maxillary tooth row and its ventral margin appears to slope posteroventrally (Figure 3.2, 3.3).
The snout is damaged, and it is difficult to distinguish between the premaxillary and maxillary dentition. The tooth row is inset from the lateral margin of the skull. Post-mortem distortion has pushed the tooth row of the left side anteriorly, making it appear that the right side has more teeth. There appear to be two premaxillary teeth (Figure 3.2), which although damaged, are clearly labio-lingually expanded with a simple ridged occlusal surface. Four maxillary teeth are preserved on either side (Figure 3.2). Although the more derived procolophonids tend to exhibit reduction in marginal tooth numbers, tooth count is not necessarily significant for phylogenetic analysis. Differences in tooth count are known to represent ontogenetic variation in other procolophonids (Gow 1977; Sues and Baird 1998).
The maxillary teeth are transversely broadened (Figure 3.2). Wear facets on each tooth have obscured some of the structural details. They appear to possess a labial and a lingual cusp connected by anterior and posterior transverse ridges that form the margin of a deep occlusal basin. The anterior margin of the basin is always lower than the posterior margin. The labial cusp always appears higher than the lingual cusp, although both of these features could be a result of tooth wear. The wear patterns are very similar to that seen in an un-named procolophonid from the fissure deposits at Cromhall Quarry, England (Fraser 1986). However, the maxillary teeth in the Cromhall form are not as transversely broadened. Moreover the occlusal basins are considerably deeper in the Abajo form and comparable to that of Hypsognathus. The Late Triassic Brazilian form Soturnia also possesses an occlusal depression, although this is manifested as a prominent anterior-posterior groove on the occlusal surface, not a basin per se (Cisneros and Schultz 2003). Scoloparia differs greatly in having maxillary teeth with several cuspules on a single transverse ridge with no trace of an occlusal basin (Sues and Baird 1998). In palatal view, a bone extending anterior from the midline probably represents the right vomer (Figure 3.2). It does not preserve any teeth, although this is equivocal, because it is so poorly preserved. Posterior to the tooth row but anterior of the quadrate are a thin bone on either side extending posteriorly and slightly towards the midline (Figure 3.2). They possibly represent the pterygoids, but are too poorly preserved to confirm this identification.