Repelinosaurus

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.[1] 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.[2]

Both specimens of Repelinosaurus were discovered in the Purple Claystone Formation of the Luang Prabang Basin in northern Laos. This sedimentary unit mostly consists of purple silty-claystones mixed with layers of conglomerates and sandstone, as well as volcaniclastic sediments. Estimated dates for the age of the formation have ranged from the Late Permian to the Late Triassic or even the earliest Jurassic period. More recently, radiometric dating using U—Pb geochronology from detrital zircon has yielded a maximum age for deposition of 251.0 ± 1.4 Ma.[3] However, the mixing and reworking of the sediments implies that the actual depositional age of the formation is probably younger than this, likely placing it in the Early Triassic.[1]

However, the reliability of this date was contested by Jun Liu in 2020, who argued that based on biostratigraphy the Purple Claystone Formation should instead be regarded as Late Permian in age, comparing Counillonia to dicynodonts found in the 255-253 million year old Daptocephalus Assemblage Zone of South Africa. Furthermore, Liu argued that the conditions of the Permian mass extinction in equatorial regions between the palaeolatitudes where Laos was situated (such as high temperatures over 40°C) would have been inhospitable for dicynodonts, and concluded that Counillonia instead likely pre-dates the extinction event for these reasons.[4]

The first dicynodont remains to be discovered in the Purple Claystone Formation was a single, poorly preserved partial skull discussed by French geologist Jean-Baptiste-Henri Counillon in 1896.[5] This skull was described in 1923 by another French geologist, Joseph Répelin, who named it as a new species of Dicynodon, "Dicynodon incisivum".[6] The incomplete and damaged nature of the skull made identification difficult, and it has been variously attributed to Dicynodon and Lystrosaurus due to a supposed resemblance to the latter. The specimen has since been lost, and the poor quality of the remaining illustrations of the skull are unsuitable for supporting the validity of the species, and "D. incisivum" has since been considered a nomen dubium.[1][7][8] As such, its relationships to other Purple Claystone dicynodonts like Repelinosaurus remain unknown.[7]

More dicynodont remains were recovered by a Franco-Laotian expedition between 1993 and 2003 lead by palaeontologist Philippe Taquet. Three skulls in particular were studied and briefly described in 2009 and were assigned to Dicynodon, tentatively as a new species, although this relationship was not tested and remained uncertain.[7] In 2019, the three skulls were more described in full detail and were recognised as representing two distinct new taxa. Two of these skulls, specimens LPB 1993-2 and LPB 1993-9, were assigned to the new genus Repelinosaurus. The third skull was assigned to another new genus, Counillonia. The specimens were temporarily stored, prepared and studied at the Muséum National d'Histoire Naturelle in Paris, and is permanently housed at the Savannakhet Dinosaur Museum in Laos.[1]

The larger of the two specimens, LPB 1993-2, was made the holotype of Repelinosaurus. It is a partial skull missing portions from the left back side including the postorbital bar, the zygomatic arch, the quadratojugals, quadrate bones and part of the squamosal, as well as the external portion of the tusks and the stapes. The palate is also somewhat weathered in this specimen. The referred specimen LPB 1993-9 is smaller than the holotype (15.72 centimetres (6.19 in) long) and also more complete, however it has been subjected to taphonomic distortion during fossilisation that distorts some features. The skull has been laterally compressed, particularly distorting the shape of the zygomatic arches, obscuring details of the palate, twisting the tusks so that they curl inward, and altering the symmetry of the skull in general (although the left orbit appears to have maintained its shape). LBP 1993-9 is also missing the left quadratojugal and stapes, and the quadrates, while the right stapes and epipterygoids are poorly preserved. The genus was named in honour of the geologist Joseph Répelin who described the first remains of Laotian dicynodonts from the Purple Clay Formation ("D. incisivum"), with the latinised Greek suffix saurus ("lizard"). The species name is from the Latin robustus, referring to the robust construction of the skull in this species.[1]

Preliminary studies of both LPB 1993-2 and LPB 1993-9 found them to be closely comparable to Dicynodon based on comparative anatomy.[7] A phylogenetic analysis was later performed when Repelinosaurus was officially described, utilising the dataset of Angielcyzk & Kammerer (2017),[9] where Repelinosaurus was found to be the basal-most member of Kannemeyeriiformes. Both specimens were included individually to test and re-affirm that they belonged to the same taxon. Repelinosaurus shares three autapomorphies (derived traits) with other Kannemeyeriiformes: the absence of a postfrontal bone, the ventral keels on the anterior pterygoid rami do not converge, and there is no intertuberal ridge on the basioccipital of the braincase. Compared to all other Kannemeyeriiformes, Repelinosaurus can be distinguished by its strongly reduced, short snout, as well as by only having a single median boss and not a pair as with other Kannemeyeriiformes. It also differs in lacking the characteristic temporal crest of many Kannemeyeriiformes, due to its dorsal-facing postorbital bars.[1]

A simplified cladogram, an excerpt from the full analysis by Olivier and colleagues focused on the relationships of Kannemeyeriiformes, is shown below:

However, an analysis performed in 2020 by Jun Liu found Repelinosaurus to not be a kannemeyeriiform at all, but instead as the sister taxon to the contemporary Laotian dicynodont Counillonia within the "Dicynodon"-grade of dicynodontoids. This sister relationship is identified by three synapomorphies; a relatively wide median pterygoid plate, distinct contributions of the exoccipital and basioccipital to the occipital condyle at the back of the skull, and the lateral edge of paroccipital process drawn into sharp, posteriorly directed process that is distinctly offset from the surface of the occipital plate. Furthermore, Liu identified a 'core-Dicynodon' clade containing the two Laotian taxa, the Chinese Taoheodon, the Russian genera Delectosaurus and Vivaxosaurus, and Dicynodon itself.[4]

A simplified excerpt of the cladogram produced by Liu (2020) is shown below:

In the Purple Clay Formation, Repelinosaurus is currently only known to have co-existed with the "Dicynodon"-grade dicynodont Counillonia and the semi-aquatic chroniosuchian tetrapod Laosuchus.[10] The only direct evidence of plants in the formation are preserved root traces in palaeosols, but a locality underlying the Purple Claystone Formation and above late Changhsingian (Late Permian) deposits preserves a rich and diverse palaeoflora. The sediments preserved indicate that the Purple Clay Formation was deposited in a braided river environment that gradually transitioned to an alluvial plain with ponds.[11] The region was volcanically active, as evidenced by the volcaniclastic rocks mixed in with the sediments of the formation. This appears to be associated with a volcanic arc that was formed as the then isolated Indochina Block where Laos was located approached the rest of the supercontinent Pangaea.[3]

Life restoration of the contemporary chroniosuchian Laosuchus.