A new fossil species of Calyptocephalella (Anura: Australobatrachia) from the Miocene of northern Patagonia: Novel evidence of the broad past diversity of the genus

Abstract

Calyptocephalella is nowadays only represented by the emblematic Chilean Helmeted Water Toad C. gayi, restricted to water bodies of Central Chile. Numerous fossil remains from latest Cretaceous to Miocene sedimentary rocks at the east of Los Andes have been attributed to this genus and several extinct Calyptocephalella species were erected. The relationships of these taxa, including the monophyly of the genus and the validity of some species, are however still unresolved. Here, we described a fossil anuran from the late Miocene of the northern Argentine Patagonia, which we attributed to a new extinct species of Calyptocephalella after analyzing the osteology and shared characters of all presumed species of the genus. These characters, whose co-occurrence seems to be exclusive of Calyptocephalella, include ornamented skull, extensive nasal processes contacting maxillae, broad nasal-frontoparietal contact, occipital artery enclosed by bone, maxilla contributing to the orbital margin and contacting squamosal, squamosal zygomatic and otic rami forming a continuous plate, scapula with a well-developed anterior lamina, and high iliac dorsal crest. The new species is differentiable in the proportion of the maxillary partes, the presence of a triangular maxillary postorbital process, a narrow maxillary contribution to the orbital margin, and an oblique frontoparietal-squamosal suture. We also analyzed the putative morphological and ecological diversity of Calyptocephalella along the geological record, from which this fossil is nowadays the youngest and northernmost record east of Los Andes. We concluded that Calyptocephalella would be a successful lineage that inhabited Patagonia since, probably, the latest Cretaceous and during most of the Cenozoic, whose diversification might be related with the environmental changes that took place in southern South America during the these times, and from which C. gayi is only a vestigial representative.