Repelinosaurus is an extinct genus of dicynodont from the Purple Claystone Formation of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. Its type and only known species is R. robustus. Repelinosaurus was originally described as the earliest known kannemeyeriiform dicynodont, supporting the idea of a more rapid radiation of the Triassic kannemeyeriiform dicynodonts during the Early Triassic following the Permian mass extinction. However, it may alternatively be more closely related to the Permian Dicynodon. The discovery of a potential early kannemeyeriiform in an understudied locality like Laos highlights the importance of such places in dicynodont research, which has been largely focused on historically important localities such as the Karoo Basin of South Africa.
Description Repelinosaurus was a medium-sized dicynodont (largest skull length of 19 centimetres (7.5 in)) currently known only from skulls missing lower jaws and the rest of the skeleton. However, it likely resembled other kannemeyeriiform dicynodonts, and so was probably a heavily built, stocky-limbed quadruped with a short tail and a large head with nearly toothless jaws and a tortoise-like beak, sporting a pair of prominent tusks.
Skull The skull of Repelinosaurus is relatively narrow for a dicynodont, and the snout in front of the eyes is especially short—among the shortest of any dicynodont—but proportionately wide, tapering slightly to a flat, squared-off beak. The bony nostrils are large, and occupy half of the length of the short snout. The upper surfaces of the snout are strongly rugose, particularly on the premaxilla, and this textured surface ends abruptly at the contact of the nasal and frontal bones. Like other Kannemeyeriiformes, the nasals sport bony bosses, however uniquely they form as a single central swelling on the snout while most other Kannemeyeriiformes have a distinct pair. This boss stops just above the edge of the nostrils, separated from the frontals by a notch, although the prefrontal bones also sport their own smaller, weakly developed boss. The rough texture of the snout implies it was covered with a layer of horny keratin, like the beak. The maxilla is robust, more so in one specimen than the other, in which the caniniform process housing the tusk is also more strongly developed. The caniniform process is vertical, and so the prominent tusks point directly downwards. The palatine bones are wide at the front and form rough, rugose pads on the roof of the mouth, also likely covered in keratinous horn. The frontals are wide, and so the eyes sit down on the side of the head and face outwards. The postorbital bars closing off the back of the orbits are short, and so the skull (and the temporal fenestra) appear relatively narrow from above for a dicynodont. The pineal foramen (the opening for the "third eye"), bordered by the preparietal in front and the elongated parietal bones behind, is oval and sits flat on the skull, and noticeably varies in size between the two known specimens (0.96 cm and 1.33 cm in length). The two skulls differ in other ways, some of which appear to be related ontogeny, such as the larger skull being more robust, having a more prominent caniniform process and better developed ornamentation with nasal bosses that extend further out to the sides of the snout. These differences appear to be due to changes in ontogeny or sexual dimorphism, as has been observed in some other dicynodonts, but this cannot be confirmed for Repelinosaurus.
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