THE EARLIEST RECORD OF FROGS FROM SOUTH AMERICA AND THE FORMATION OF THE ANURAN UROSTYLE

BÁEZ, A. M.; NICOLI, L.

Congreso; Reunión Anual de Comunicaciones de la Asociación Paleontolgógica Argentina 2023; 2023

Many remarkable features conform the anuran bauplan, which was conserved with relatively few modifications in the time-span of nearly two hundred-million years since the earliest known records in the Lower Jurassic. Among these features, which turn anurans into one of the most derived groups of extant vertebrates, is the urostyle. This unique structure is formed by several postsacral (caudal) vertebrae and an ossified hypochord, according to studies of its development in different groups of living anurans. The urostyle is clearly already present in the possibly Pliensbachian Prosalirus bitis (MNA V 8725, MCZ 324A) from the Kayenta Formation of Arizona, USA, although it remains poorly understood. Our re-examination of the earliest known frog from South America, the nearly coetaneous Vieraella herbstii (PVL 2188 and MLP 64-VII-15-1) from the Roca Blanca Formation of Santa Cruz (Lower Jurassic), Patagonia, represented by a single individual, revealed the presence of a short, cone-shaped urostyle. Previously, this element had been misinterpreted as an elongated proximal tarsal. Apart from the likely presence of a first complete vertebra succeding the sacrum, the neural arch pedicels of a postsacral element at the anterior-most end of the urostyle of Vieraella extend backwards and are partially synostotically fused to the ventral component of this structure. Herein we discuss these characters and possible evolutionary developmental changes in the formation of this key anuran feature.