Evolution, Diversity, and Development of the Craniocervical System in Turtles with Special Reference to Jaw Musculature

Abstract

Turtles are one of the most enigmatic groups of vertebrates with their highly modified “body plan” and, as such, attracted the attention of researchers for a long time. Aside from the unusual turtle shell, the skull in this group shows great changes in comparison to that of other amniote groups. Because the skull has been considered one of most important body regions when analyzing the phylogenetic relationships of amniotes, the distinct turtle skull morphology is one of the key features in defining their position among reptiles. Here, we review the current knowledge of the turtle head, summarizing the general morphology of the skull and neck as well as the different anatomical modifications characteristic of the main lineages of extant and extinct turtles. We explore the main questions that have been raised while studying those issues, for instance, the origin and diversity of the temporal emarginations (dermal bone reductions), the different neck retraction mechanisms and their influence on the shape of the skull, and the anatomy and development of the jaw adductor musculature and its relation to some characteristic features of the turtle skull, such as akinesis and the divergent trochlear mechanisms in cryptodires and pleurodires. Based on 3D reconstructions, we propose a hypothetical model for ancestral states and gross morphology of the jaw adductor musculature in Proganochelys quenstedti (the earliest turtle with a complete shell), an important step toward the understanding of the evolution of those muscles in turtles. Finally, we suggest that more integrative approaches that consider anatomical, developmental, and paleontological data and that employ modern techniques in morphological and functional anatomic analyses (such as μCT scanning and finite element analysis) have a greater potential to answer the still numerous open questions about the evolution of the turtle head.