Ruthenosaurus is an extinct genus of caseid synapsids that lived in what is now southern France during the Early Permian (late Artinskian) about 285 million years ago. It is known from the holotype MNHN.F.MCL-1 an articulated partial postcranial skeleton. It was collected by D. Sigogneau-Russell and D. Russell in 1970 in the upper part of the M2 Member, Grès Rouge Group, in the Rodez Basin, near the village of Valady (département of Aveyron), in Occitanie Region. It was first named by Robert R. Reisz, Hillary C. Maddin, Jörg Fröbisch and Jocelyn Falconnet in 2011, and the type species is Ruthenosaurus russellorum.[1]
Ruthenosaurus is named after a Gallic regional tribe, Ruthenie (in Medieval Latin), that have also given Ruthénois, the name of the inhabitants of the town of Rodez; and the Ancient Greek sauros, "lizard".[1] The type species is named in honour of Drs Denise Sigogneau-Russell and Donald E. Russell, the original collectors of the holotype.[1]
The holotype of Ruthenosaurus was discovered in the summer 1970 by the paleontologists Denise Sigogneau-Russell and Donald Eugene Russell, during a prospecting survey carried out in Permian red sandstones outcropping in badlands on the western flank of the Cayla Hill near the commune of Valady, northwest of Rodez.[2] An eroded vertebra picked up on the western slope of the hill led the scientists to explore the surrounding canyons, where they discovered a large articulated skeleton still in place in the sediments but damaged by erosion.[2] The skull, neck, most of the limbs, and the tail were missing, probably destroyed by erosion. The known material includes ribs and vertebrae of 18 presacrals, three sacrals, and 12 anterior caudals; incomplete scapulocoracoids and interclavicule; right humerus badly crushed and damaged; left humerus in two pieces, shaft damaged, but proximal and distal heads well preserved; complete left ulna and nearly complete radius; complete right femur, complete right tibia, and proximal portion of right fibula; and the complete right pelvis overlain by vertebral column. On the southeastern flank of the same hill, but in older strata, the same team discovered the anterior part of a skeleton (including the skull) belonging to a smaller animal. This specimen was first assigned to a new species of the genus Casea, Casea rutena.[2] But it is now regarded as a distinct genus named Euromycter, with the new combination Euromycter rutenus.[1] The larger skeleton, found stratigraphycally 120 meters above the Euromycter level, remained largely unprepared until 2003. The few overlapping elements with Euromycter suggested that it belongs to a different taxon named Ruthenosaurus russellorum in 2011.[1]
Ruthenosaurus is diagnosed by several autapomorphies including dorsal vertebrae with anteriorly tilting neural spines and a diamond-shaped outline in transverse section; a first sacral rib with robust distal head, twice that of the second sacral rib; and a short iliac blade with prominent posterior process. It can be distinguished from Euromycter, from older deposits of the same locality, by the shape of the distal part of the humerus, including an ectepicondylar notch rather than a fully enclosed foramen, the specific shape of the ulna, and the overall robustness of the specimen.[1]
The lack of fusion of the neural arches with their respective vertebral centra and incomplete ossification of the ends of the limb elements, including the absence of an ossified olecranon on the ulna, show clearly that this specimen represents a juvenile individual. However, it is distinctly larger than the fully mature specimen of Euromycter, suggesting a very large size for adult Ruthenosaurus.[1]