A new stahleckeriid dicynodont record from the late Ladinian-?early Carnian levels of the Chañares Formation (Ischigualasto-Villa Unión Basin) of northwestern Argentina

Abstract

The kannemeyeriiforms of the latest Ladinian-?early Carnian Chañares Formation (Ischigualasto-Villa Unión Basin) exemplify the historical taxonomic controversies of several South Am erican dicynodonts. Three taxa were originally recognized based on skull anatomy: “Chanaria” platyceps and two species of Dinodontosaurus (genus otherwise known in the Pinheiros-Chiniquá Sequence, Brazil), but in a recent taxonomic revision they were all synonymized in a single valid species (i.e., Dinodontosaurus brevirostris). The only postcranial record includes some undescribed specimens referred to D. brevirostris, an indeterminate shoulder girdle (MCZ 3459), and an ulna recently referred to Stahleckeria sp. Here we describe a new, large dicynodont specimen (CRILAR-Pv 82) from the Tarjadia Assemblage Zone (AZ) of the lowermost levels of the Chañares Formation, consisting of a right scapula, left ilium, sternum, and a few indeterminate fragments. The tall, dorsally expanded scapular blade with anteroventrally inclined dorsal margin and the anterolaterally directed acromion process broadly match the stahleckeriid morphotype. CRILAR-Pv 82 shares with Stahleckeria potens a sharp, vertical scapular spine on the anteroventral margin of the scapular blade, markedly convex in anterior view. The “hook-shaped” curvature of the preacetabular iliac process also resembles Stahleckeria potens and Eubrachiosaurus browni. These features differ at least with the Brazilian specimens of Dinodontosaurus (D. tener). The available data shows that MCZ 3459 also fits the stahleckeriid morphology, becoming the first historical record of the clade for the unit. A rigorous comparison with D. brevirostris will await the examination of more complete specimens. The dicynodont postcrania described so far from the Chañares Formation represent the oldest record of Stahleckeriinae in the Ischigualasto-Villa Unión Basin, expanding back its Argentinian record very likely beyond the Ladinian-Carnian boundary.