Re-examination of the oldest known frog from South America: New data prompt new evolutionary interpretations.

Abstract

Available paleontological evidence, although scarce, points to the early diversification of salientian lissamphibians in the Triassic and Early Jurassic. This study provides new key anatomical information on the earliest mostly articulated frog currently known, thereby improving our understanding of the early evolution of this lissamphibian lineage. Herein, available specimens of the Early Jurassic Vieraella herbstii from Patagonia, which consist of dorsal and ventral imprints of the incomplete, partially articulated skeleton of a single individual, are thoroughly redescribed. Although we comment on its known features, we focus on those that had been misinterpreted or overlooked previously. Among other features, we address the relative proportions of the skull regions and limbs, the morphology of the vomers, the peculiar articulation of the palatine flanges of premaxilla and maxilla that suggests the presence of well-developed cristae subnasales, and the presence of prepollex. The most surprising, significant findings are the presence of stapes, indicating the possibility of a complete tympanic middle ear, and of a short, tapering urostyle and postsacral vertebral elements. Based on available information, we present partial reconstructions of the pectoral girdle and the skeleton of Vieraella in the dorsal aspect. Discussion of the evolutionary significance of these features and perusal of the phylogenetic and functional analyses that included Vieraella in the taxon sampling highlight the necessity of thorough revision of the scorings and measurements, incorporating information presented herein. This reassessment will be relevant not only to clarify its relationships but also to provide sound insights into the early diversification of frogs.