Coping with the extremes: comparative osteology of the tepui-associated toad Oreophrynella and its bearing on the evolution of osteological novelties in the genus

The only study of the osteology of the toad genus Oreophrynella dates back to 1971 and was based on a single species. Here, we use high-resolution X-ray microcomputed tomography to analyse the osteology of all nine described Oreophrynella species, which are compared with representatives of other bufonid lineages. Oreophrynella is unique among bufonids in having opposable digits. Osteological synapomorphies confirmed for the genus are as follows: presence of parietal fontanelles and exposed frontoparietal fontanelle, absence of quadratojugal, five presacral vertebrae, distally enlarged terminal phalanges and urostyle greatly expanded into flanges. Ancestral character reconstruction indicates that arboreal habits in some Oreophrynella species are likely to have evolved after the evolution of opposable digits. Opposable digits, in combination with an extension of the interdigital integument and the relative length/orientation of the digits, are likely to be adaptations to facilitate life on rocky tepui summits and an exaptation to arboreality. Cranial simplification in Oreophrynella, in the form of cranial fontanelles and absence of the quadratojugal, is possibly driven by a reduction of developmental costs, increase in flexibility and reduction of body weight. Cranial simplification combined with the shortening of the vertebral column and the shift towards a partly firmisternal girdle might be adaptations to the peculiar tumbling behaviour displayed by Oreophrynella.

Deconstructing cartilage shape and size into contributions from embryogenesis, metamorphosis, and tadpole and frog growth

Understanding skeletal diversification involves knowing not only how skeletal rudiments are shaped embryonically, but also how skeletal shape changes throughout life. The pharyngeal arch (PA) skeleton of metamorphosing amphibians persists largely as cartilage and undergoes two phases of development (embryogenesis and metamorphosis) and two phases of growth (larval and post-metamorphic). Though embryogenesis and metamorphosis produce species-specific features of PA cartilage shape, the extents to which shape and size change during growth and metamorphosis remain unaddressed. This study uses allometric equations and thin-plate spline, relative warp and elliptic Fourier analyses to describe shape and size trajectories for the ventral PA cartilages of the frog Xenopus laevis in tadpole and frog growth and metamorphosis. Cartilage sizes scale negatively with body size in both growth phases and cartilage shapes scale isometrically or close to it. This implies that most species-specific aspects of cartilage shape arise in embryogenesis and metamorphosis. Contributions from growth are limited to minor changes in lower jaw (LJ) curvature that produce relative gape narrowing and widening in tadpoles and frogs, respectively, and most cartilages becoming relatively thinner. Metamorphosis involves previously unreported decreases in cartilage size as well as changes in cartilage shape. The LJ becomes slightly longer, narrower and more curved, and the adult ceratohyal emerges from deep within the resorbing tadpole ceratohyal. This contrast in shape and size changes suggests a fundamental difference in the underlying cellular pathways. The observation that variation in PA cartilage shape decreases with tadpole growth supports the hypothesis that isometric growth is required for the metamorphic remodeling of PA cartilages. It also supports the existence of shape-regulating mechanisms that are specific to PA cartilages and that resist local adaptation and phenotypic plasticity.