Sauropia

The holotype skull of S. macrorhinus is 9.5 millimetres (0.37 in) long. The skull is almost as wide as it is long, with a maximum width at the temporal region of 8.7 mm (0.34 in). As such, it is the smallest tetrapod skull known from the Middle Triassic of South America. Besides this very small size, there are no immediately apparent indicators of the specimen's ontogenetic status (age). Müller and colleagues (2026) suggested that, since all other procolophonoids have skulls at least 2.5 times longer, this strongly supports an immature status for the specimen. Like many fossils from the Dinodontosaurus AZ, much of the specimen is covered in mineral concretions, with the external bone surface poorly preserved. As such, much of the anatomy and details of CAPPA/UFSM 0510 are obscured. This is exacerbated by the specimen's tiny size.^[1]

The holotype skull of Sauropia exhibits a unique combination of anatomical traits seen in either owenettids or procolophonids (the two distinct lineages within Procolophonoidea). When seen from above, the snout of the specimen is broad and rounded. This region comprises a comparatively short and deep part of the skull. These proportions are only seen in some procolophonids, and not in owenettids, as the latter are characterized by a longer, flatter snout. However, it is possible that snout shape is ontogenetically variable (changes through age) in procolophonoids, but the effects of ontogeny on skull architecture in these taxa are not well understood. The external nares ('nostril hole' in the skull) are especially wide and tall. When seen from the side, the external naris is elongated, similar to owenettids and basal procolophonoids, but different from procolophonids, in which it is generally more circular. The internarial bar of the premaxilla (upward-projecting part between the external nares, forming the front border of the snout) is directed anterodorsally (forward and upward), an exclusively procolophonid feature. The orbitotemporal fenestra ('eye hole' in the skull) is similarly elongate, with the rear margin almost reaching the posterior part of the skull. In owenettids, the orbit is anteroposteriorly shorter (shorter from front to back). The greatly expanded form of this fenestra may be due to its immaturity, with it becoming proportionately smaller through development.^[1]

There are three tooth positions in the premaxilla, similar to some procolophonids and in contrast to owenettids, which have at least five. These teeth in Sauropia are straight, cylindrical in cross-section, and lacking cusps. The front tip of the premaxilla projects downward. The maxilla is incomplete, missing the rear portion. The number of maxillary teeth is unclear, as only one tooth is preserved in the right maxilla. The lower part of the maxilla that contacts the premaxilla (subnarial process) is relatively long, similar to owenettids and Coletta (one of the basalmost procolophonids), but distinct from other procolophonids. The bones of the skull roof are poorly preserved and difficult to distinguish, but it likely had particularly wide frontals, a distinctive feature not seen in other procolophonoids (possibly due to the individual's young age). The mandible is generally horseshoe-shaped. Its dentition is not visible.^[1]

To assess the relationships of Sauropia within the Procolophonoidea, Müller and colleagues included it in an updated version of the phylogenetic matrix of Mueller et al. (2023).^[2] This analysis placed Sauropia within the family Procolophonidae, in a large uresolved polytomy with several other early-diverging taxa, one of which, Oryporan, is also from the Triassic of Brazil.^[3] These results are displayed in the cladogram below. Müller et al. cautioned that this placement for Sauropia should be regarded as tentative, due to the likely early ontogenetic stage of the holotype specimen and presence of some owenettid traits.^[1]