Therapsid dicynodonts (Amniota, Synapsida) from Argentina.

DOMNANOVICH, NADIA; MARSICANO, CLAUDIA

Bristol

Congreso; 69th Annual Meeting Society of Vertebrate Paleontology; 2009

Therapsid dicynodonts have been an important component of the Early Mesozoic terrestrial communities in southern South America. In the last years, several authors have briefly discussed the taxonomic status of the Argentinean taxa but only recently a comprehensive revision of all materials was performed and the validity of the several taxa discussed. In Argentina, dicynodonts have been collected from several localities and stratigraphic levels spanning most of the Triassic. The basalmost Triassic Quebrada de los Fósiles Formation (southern Mendoza) contains specimens here consider as kannemeyeriiforms indet., previously assigned to the Indian Rechnisaurus, as well as a specimen of the shansiodontid Vinceria. Until now, shansiodontids were restricted to Middle Triassic levels of Gondwana and Laurasia thus this new record rule out the shansiodontids as useful stratigraphic indicators as was previously proposed. From the same basin, the overlying Río Seco de la Quebrada Formation (lower Middle Triassic) includes Vinceria and Kannemeyeria (K. argentinensis) a taxon also known from southern Africa. From northern Mendoza (Cuyana Basin, Potrerillos area) the Anisian Cerro de las Cabras Formation has yielded Vinceria and fragments of a kannemeyeriid indet. Several dicynodont specimens are known from the Ischigualasto-Villa Unión Basin located between La Rioja and San Juan. Thus, the Middle Triassic Chañares Formation records the presence of Dinodontosaurus platyceps and D. brevirostris. Nevertheless, in the present revision the taxon Jachaleria is not recognized in this unit as was previously proposed and the specimen is reassigned to its original assignation, D. platygnathus. The Late Triassic levels of the basin include large specimens of Ischigualastia jenseni (Ischigualasto Formation) and Jachaleria colorata Bonaparte (Los Colorados Formation). Therefore, dicynodonts constituted a quite diverse group during the Triassic in this part of Gondwana and since they first appareance in the earliest Triassic of Argentina they were already diversified and represented by both small (shansiodontids) and large (kannemeyeriids) forms. Also as a result of the present revision, closest affinities of the Triassic Argentinian dicynodonts are found with those taxa of Southern Africa and Brazil.