The earliest records of pipimorph frogs from South America (Aptian, Crato Formaton, Brazil): a critical evaluation

Abstract

The Aptian limestones of the Crato Formation from the Araripe Basin of northeastern Brazil have yielded well-preserved fossils that provide an open window into a mid-Cretaceous Gondwanan biota, including the oldest known neobatrachian frogs. Herein we review the osteology of the pipimorph xenoanurans from these beds, which stand as the geologically oldest members of this anuran branch from South America. We present a redescription of the only known specimen of the recently named Cratopipa novaolindensis and a revised diagnosis based on a distinct combination of features that sets it aside from all other known pipimorph taxa. In addition, we describe another specimen from the same unit that, despite its partial poor preservation, shows features that document an additional probable new pipimorph taxon and to which we also refer another previously depicted specimen, but refrain from proposing a formal diagnosis because the available material is too incomplete. The probable new taxon differs from Cratopipa in possessing of a broader braincase, a shallow frontoparietal lacking dermal sculpturing and crests, hindlimbs with proportionally longer proximal tarsals, and ilia with round cross-sectioned shafts. This latter fossil occurrence also shows that the unique tympanosquamosal complex of crown pipids, a feature not present in putative basal pipimorph taxa, had already appeared by the mid-Cretaceous. This study reveals previously unknown morphological and taxonomic diversity among the earliest pipimorphs in South America, suggesting a complex, still largely unrecorded evolutionary history. It also highlights the need for a well-supported phylogenetic hypothesis to resolve their phylogenetic relationships.