Significant new pipimorph remains (Amphibia, Anura) from the Cenomanian Candeleros Formation of Northwestern Patagonia, Argentina

BÁEZ, ANA MARÍA; MUZZOPAPPA, PAULA

VIRTUAL

Congreso; Annual Meeting of the Society of Vertebrate Paleontology; 2020

Society of Vertebrate Paleontology

In northwestern Patagonia, the Neuquen Group, which constitute part of the sedimentary filling of the Neuquen Basin, has yielded diverse vertebrate fossils that shed light on their evolution throughout most of the Late Cretaceous. In particular, the lowermost unit of this group, the Candeleros Formation, has furnished a large assortment of vertebrate remains including the holotype of the pipimorph anuran Avitabatrachus uliana Báez, Trueb and Calvo, 2000. Herein we describe anuran material recently collected near the town of El Chocón from outcrops of the Candeleros Formation exposed on the western coast of the Ramos Mexia Reservoir, Neuquen province. This material (Museo Municipal Ernesto Bachmann, Villa El Chocón-PV 259) consists of several incomplete, partially articulated postcranial elements exposed in ventral view, which clearly belong to a post-metamorphic individual. Seven discrete opisthocoelous presacral vertebrae, sacral vertebra, and anterior portion of the urostyle comprise the preserved axial skeleton. The centrum of the anteriormost discernable vertebra is missing but it bears short transverse processes. Long transverse processes that might include coalesced ribs occur associated with the succeeding two vertebrae, suggesting that they are presacrals III and IV. Vertebra IX bears distally expanded diapophyses that reach the iliac shafts. Expanded webs of bone denoting the presence of postsacral vertebrae occur on the anterior portion of the urostyle; the most anterior of these webs (vertebra X) are directed forward to join the posterior margin of the diapophyses on the preceding vertebra (IX). Although the anterior end of the ossified hypochord reaches the hind part of vertebra IX, it is not completely fused to the latter nor to the coccygeal neural arches indicating that this specimen might not be a fully grown individual. This evidence calls into question the previous interpretation of Avitabatrachus uliana as having a fused sacrum and urostyle. Instead, the partial transverse processes on the isolated urostyle of the holotype might belong to a postsacral vertebra, the last presacral centrum and sacral vertebra probably missing. This evidence confirms that the sacrum in pipoids results from different ontogenetic pathways and that the monovertebral sacrum formed by vertebra IX probably is a plesiomorphic condition that among pipimorphs evolved into a more complex structure incorporating either the last presacrals or postsacral vertebrae.