First rhynchosaur record from the Chañares Formation (Ladinian?earliest Carnian, Ischigualasto-Villa Union Basin) of northwestern Argentina

M.D. EZCURRA ; M.J. TROTTEYN; J.B. DESOJO; L.E. FIORELLI; J.R.A. TABORDA; M.B. VON BACZKO; M. IBERLUCEA

La Rioja

Jornada; XXVII Jornadas Argentinas de Paleontología de Vertebrados, La Rioja, Argentina; 2013

The Triassic witnessed the origin of multiple reptile clades, such as the rhynchosaurs. Rhynchosaurs appear in the Early Triassic fossil record and flourished during the late Carnian as the dominant members of several worldwide assemblages. In Argentina, the rhynchosaur record is restricted to the late Carnian?earliest Norian Ischigualasto Formation. Recent fieldwork in the new locality of Brazo del Puma, in the lowermost levels of the Chañares Formation, yielded three rhynchosaur tooth-bearing bones (CRILARPV 461?463), collected five meters above the contact with the underlying Tarjados Formation. The most complete specimen is the posterior end of the alveolar region of a left dentary. The lateral border of the occlusal surface preserves the base of a coronoid process. The dentary possesses densely packed tooth rows on the lingual surface and medial half of the occlusal surface. The teeth are organized in multiple rows and possess a worn pattern from almost unworn posterolingual teeth to worn flat occlusal teeth (i.e., Zahnreihen), as occurs in Hyperodapedontidae. In addition, the dentary teeth are conical and anteroposteriorly compressed, resembling the condition observed in hyperodapedontines. The rhynchosaur remains reported here are the oldest collected in Argentina and among the oldest in South America, together with an unnamed form from Brazil. The new rhynchosaur specimens come from levels in which dicynodonts were numerically dominant, whereas cynodonts are considerably less abundant. Accordingly, the specimens reported here bolster faunistic differences within the Chañares Formation and add a new faunistic component to this already diverse vertebrate assemblage.