Untangling the morphological contradiction: First ontogenetic description of the post-hatching skeleton of the direct-developing frog Brachycephalus garbeanus Miranda-Ribeiro, 1920 (Amphibia: Anura: Brachycephalidae) with comments on the genus miniaturization

Abstract

The study of ontogenetic series facilitates understanding developmental and evolutionary patterns and processes amongst vertebrate lineages, being widely explored in this taxon over the last decades. However, developmental data for osseous elements of anurans are mostly focused on pre-hatching stages of lineages with a larval stage, with very little data on post-hatching phenotypic transformations, especially in direct-developing anurans. Previous osteological studies of the miniaturized genus Brachycephalus have considered the taxa as a morphological contradiction, given the existence of both paedomorphic and peramorphic traits on the skull and other post-cranial elements. However, none of them have provided a full post-hatching developmental variation of the skeleton for any species of the genus, and thus the observation of such heterochronic changes has remained underexplored. This study aims to provide a detailed developmental description of the osteology of skull and post-cranium osteology of Brachycephalus garbeanus based on an ontogenetic series of 16 post-hatched individuals. Our results show that several skull and post-cranial elements are not fully developed after hatching, with complete bone ossification and fusion occurring at later developmental stages. The combination of our data with previous studies also reinforces the occurrence of delayed skeletal development — i.e., ossification — in Brachycephalus when compared with other brachycephaloids. We also discuss the emergence of morphological novelties — as the skull and axial plates — and structural simplification — as digit reduction/loss — as possible consequences of the extreme miniaturization of the genus.