New remains from the Cenomanian Candeleros Formation, Neuquén Basin (Patagonia, Argentina) provide insights into the formation of the sacro-urostylic complex in early pipimorph frogs (Amphibia, Anura)
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An incomplete anuran skeleton collected from outcrops of the Cenomanian Candeleros Formation on the western shore of the Ramos Mexia Reservoir, Neuquén Province, Patagonia, is described herein. It consists of several partially articulated postcranial bones that are exposed in ventral view and belong to a post-metamorphic frog that is probably an adult. The axial column consists of eight opisthocoelous presacral vertebrae; fusion of the first two anterior vertebrae cannot be determined. Most of the sacrum is formed by the expanded diapophyses of Vertebra IX. However, Vertebra X also has distally expanded diapophyses that fuse to those of Vertebra IX. Traces of a suture between the urostyle and sacrum are evident. The morphology of the sacro-urostylic region casts doubts on the previous interpretation of the fused condition of sacrum and urostyle in the holotype of Avitabatrachus uliana from the same stratigraphic unit. The material described here contributes new information about the formation of the sacrum and suggests that different ontogenetic pathways are involved in formation of different sacral morphologies. The sacrum formed only by Vertebra IX probably is plesiomorphic in pipimorphs. It is hypothesized that the more complex structure described here evolved from it and incorporates postsacral vertebrae as well as synostotic fusion with the urostyle. Complete synostotic fusion of sacrum and urostyle, when present, seems to have been an event that occurred relatively late in the ontogeny of early pipimorphs. This fusion might have shifted to earlier developmental stages relative to the ancestor independently on different branches of the pipimorph tree.