Pentasaurus is an extinct genus of dicynodont of the family Stahleckeriidae, closely related to the well known Placerias. It was found in the Lower Elliot Formation of South Africa, dated to the Norian of the Late Triassic period. The genus contains the type and only species, Pentasaurus goggai. Pentasaurus is named after the ichnogenus Pentasauropus, fossil footprints that were originally described from the lower Elliot Formation in 1970 decades before the body fossils of Pentasaurus itself were recognised. Pentasauropus footprints were likely made by dicynodonts, and in South Africa Pentasaurus itself was the likely trackmaker. The name reflects the fact that a large dicynodont was predicted to have existed in the lower Elliot Formation before any body fossils were recognised, and so Pentasaurus was named after its probable footprints. This is a reversal of the more typical occurrence where fossil footprints are named after their presumed trackmakers. The name of the species honours its collector Alfred Brown, nicknamed "Gogga", which means "bug" in Afrikaans. Pentasaurus is also unique for being one of the only known dicynodonts to have coexisted with large-bodied sauropodomorph dinosaurs. Prior to its discovery, large sauropodomorphs and dicynodonts were thought to have never ecologically coexisted, with dicynodonts either going extinct prior to large sauropodomorphs evolving or being outcompeted and driven to extinction by the herbivorous dinosaurs. Pentasaurus challenges these assumptions, suggesting that dicynodonts and large sauropodomorphs could be contemporaneous, and may instead have been separated through habitat and dietary preferences. Furthermore, Pentasaurus expands the range of stahleckeriid dicynodonts in the Late Triassic into South Africa, refuting ideas that dicynodonts were geographically restricted by the Late Triassic.
Description The fossil remains of Pentasaurus are fragmented and very incomplete, so its overall morphology is poorly understood. However, it was similarly sized to other large Late Triassic dicynodonts, such as the closely related Placerias, and likely resembled them in appearance.
Skull and mandible
The only known bones from the skull of Pentasaurus is a portion from the roof of the skull, specifically the front end of the intertemporal region consisting of both frontal bones, the left postorbital and the preparietal (a bone unique to dicynodonts and some other therapsids). There are two depressed regions of the skull piece, one along the bottom left side and another cut into the back end, interpreted as representing a site for jaw muscle attachments at the edge of the temporal fenestra and for the pineal foramen, respectively. Otherwise, the skull is notably rough and strongly textured, particularly on a pair of mound-like rugosities at the back edge of the frontals. Frontal rugosities are absent in other kannemeyeriiforms and may be unique to Pentasaurus, although the bones around the eyes of Placerias are similarly rugose. Only the very front of the mandible of Pentasaurus is known, mostly consisting of the fused mandibular symphysis (including the splenial) and part of the left dentary and angular, although the tip of the beak is missing. The mandible is robust, and like other dicynodonts was fused into a toothless beak. The beak was relatively short in Pentasaurus, with a steep front edge drawn into a mid-line ridge like those of other placeriines, although it is relatively shorter and lacks a pair of grooves typically found on either side of the ridge in other stahleckeriids. The beak is proportionately broader than that of Placerias, and its edges are smoothly rounded and curve towards the tip, unlike the sharp edged, flattened surfaces on the beaks of Placerias or Stahleckeria. A mid-line groove on the top surface of the beak, present in other kannemeyeriiforms, is unusually shallow in Pentasaurus, even more so than the notably shallow groove in Placerias. The lateral dentary shelf, a characteristic ridge on the jaws of dicynodonts for attaching jaw muscles, is unusual in Pentasaurus. It is remarkably robust and prominent, and the shelf sits uniquely far forward on the jaw compared to all other dicynodonts. Further unlike other dicynodonts, the shelf does not extend back to meet the mandibular fenestra either, instead curving up at its end from an otherwise horizontal position. Features such as the shape of the beak and lateral dentary shelf likely influenced the feeding style of dicynodonts, although the significance of the unusual morphology in Pentasaurus is currently unknown.
Postcranial skeleton The postcranial skeleton of Pentasaurus is mostly represented by the appendicular skeleton, including parts of the limbs, shoulders and pelvis, with only a single poorly preserved cervical vertebra from the neck representing the axial skeleton.
The shoulder is only known from the glenoid region of the fused scapulocoracoid and is typical for stahleckeriid dicynodonts, including a relatively large glenoid opening for the shoulder joint directed back and out to the side, indicating a sprawled forelimb posture. The forelimb itself is only known from the end of the humerus and the proximal portion of the ulna. The humerus is largely typical of kannemeyeriiform dicynodonts in shape, however, the distal end is unusually large and thickened compared to other stahleckeriids. The capitulum (part of the elbow) in the only known specimen is unossified and would have been cartilaginous, suggesting the specimen may be immature compared to similarly sized specimens of Placerias. The known portion of ulna is badly worn, but the roundness of its tip suggests that Pentasaurus had a separated olecranon process of the elbow, similar to Placerias and Ischigualastia but unlike some specimens of Stahleckeria. An unidentified long bone may represent a radius. The pelvis is known from a partial left pubis and ischium and is typical of kannemeyeriiforms, including notable 'twisting' of the pubic shaft. The only known hindlimb element is the proximal end of a right tibia, and likewise it is typical of kannemeyeriiform dicynodonts. Although similar in construction to sauropodomorph dinosaurs, the internal structure visible at the broken end shows extensive trabecular bone, characteristic of Triassic dicynodonts.
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