A dicynodont therapsid in La Plata Museum´s collection: rediscovery of an historical specimen from the Middle-Late Triassic Puesto Viejo Group (Mendoza Province, Argentina).

Abstract

The Middle-Late Triassic Puesto Viejo Group (San Rafael, Mendoza Province,Argentina) comprises a sequence of volcaniclastic rocks, divided from base to top in the Quebrada de los Fósiles and the Río Seco de la Quebrada formations. Between 1962 and 1963, Emilio González Díaz found the first therapsid remains of the Group, including a dicynodont skull and elements of cynodonts, figured in 1966 and deposited in the Museo de La Plata. Later on, additional specimens were recovered from the same unit by José F. Bonaparte (deposited in the Instituto Miguel Lillo, Tucumán province), and Humberto Lagiglia (housed in the Museo de Historia Natural de San Rafael, Mendoza province). Over the years, several therapsid species were recognized in the Puesto Viejo Group, including the dicynodonts Kannemeyeria argentinensis and Vinceria vieja, and the cynodonts Pascualgnathus polanski, Cynognathus crateronotus, and Diademodon tetragonus. However, the fossils collected by González Díaz have been lost of sight and virtually ignored from most of the literature of Triassic fauna, and the exact composition, procedence and institutional location of them remained unclear. Here, we report the identification of one of these specimens, during a recent revision of the Vertebrate Paleontology Collection in La Plata Museum, thanks to the photograph published in 1966. The specimen (previously without collection number) is now catalogued as MLP-Pv 65-VI-18-3, and consists of a partial skull with both mandibular rami and part of the cervical region, heavily impregnated with a dark oxide coat. Although it is currently under preparation, some initial comparisons can be made. As Kannemeyeria argentinensis and Vinceria vieja, MLP-Pv 65-VI-18-3 is a dicynodont with ventrally directed caniniform process of the maxilla bearing a tusk, and squamosals strongly projected backward (beyond the level of the occiput), with a concave dorsal border in occipital view. Notably, the anterior palatal ridges are exposed in lateral view, as also seen in Sangusaurus parringtoni (Middle Triassic Manda Beds, Tanzania) and Ischigualastia jenseni (Late Triassic Ischigualasto Formation, Argentina), but not in Puesto Viejo´s dicynodonts. An exhaustive anatomical description and direct comparisons with the known dicynodont taxa are needed to test if this specimen represents a new morphotype.