Mosaic evolution of the skeleton in a clade of miniaturized turtles

Abstract

Extreme body size reduction (miniaturization) is associated with the mosaic (trait-specific) evolution of the skeleton in vertebrate animals, though the relevant ecological context wherein shifts in osseous tissue production occur is challenging to evaluate. To address this concern, we examined skeletal mass variation in the most species-rich clade of freshwater turtles, comprised of miniaturized lineages that acquired the capacity to survive extended periods of time on land. The hypothesis that increasingly terrestrial behaviour would favour a relatively heavier skeleton was supported. Structural reinforcement of the postcranial skeleton in semi-terrestrial species featured the relative enlargement of the ventral shell and the refinement of functionally novel articulations that aid in protection from terrestrial predators and the environment. Furthermore, the spatial distribution of skeletal mass was consistent with a mosaic evolution trend wherein many aquatic species forego the protective benefits of a structurally robust shell and instead invest on much heavier skulls. This altered trait correlation points to a potential reallocation of osseous tissue from the cranial to the postcranial skeleton, which is probably linked to adaptation to terrestrial habitats in miniaturized turtles. Our study provides impetus to further clarify how skeletal traits are modified in the mosaic evolution of extremely small animals.