Anurans from the Lower Cretaceous Crato Formation of northeastern Brazil: implications for the early divergence of neobatrachians

The upper Aptian-lower Albian lacustrine limestones of the Crato Formation of northeastern Brazil have yielded one of Gondwana's most important Cretaceous fossil assemblages. This assemblage includes a few articulated anuran remains that have been previously referred to a single neobatrachian taxon, Arariphrynus placidoi [Leal, M.E.C., Brito, P.M., 2006. Anura do Cretáceo Inferior da Bacia do Araripe, Nordeste do Brasil. In: Gallo, V., Brito, P.M., Silva, H.M.A., Figueiredo, F.J. (Eds.), Paleontología de Vertebrados. Grandes Temas e Contribuçoes Científicas. Interciencia, Rio de Janeiro, pp. 145–152]. Herein we redescribe these specimens, which document two additional genera and species, Eurycephalella alcinae and Cratia gracilis, as well as a possible pipoid. Although the monophyly of neobatrachians can be considered a well-corroborated hypothesis, neobatrachian interrelationships are still far from being satisfactorily resolved. In order to address the high-level relationships of the taxa to which these specimens belong, we conducted a phylogenetic analysis of a matrix of 42 taxa, including extant representatives of most of the higher groups of neobatrachians as well as non-neobatrachians, and 75 mostly osteological characters in TNT 1.1 under implied weights with different values of the concavity constant (k). As in other analyses based on morphological data, within Neobatrachia we recovered a monophyletic Ranoides but hyloid taxa appear as stem-ranoids. Our analysis consistently place A. placidoi and E. alcinae in nested positions among hyloid taxa, although the topology of the tree varies slightly, whereas C. gracilis appears to be a stem neobatrachian or have a basal position within crown Neobatrachia. Recent studies based on molecular data have estimated divergence times for several anuran clades and proposed the main radiation of hyloid neobatrachians, excluding the australobatrachians, as a Late Cretaceous-Paleogene event. The taxonomically diverse anurans from the Crato Formation show that some hyloid lineages might have diverged already by the mid Cretaceous and that the early history of neobatrachians is as yet not documented in the fossil record.