Structure and evolution of the embryonic cartilaginous skull of crocodilians.

The evolution of skull diversity in Crocodylia is rather well documented, but the developmental foundation of their cranial architecture remains poorly understood. Here, we present the first three-dimensional reconstruction of the embryonic cartilaginous skull of Caiman crocodilus based on histological sections. We provide a comprehensive anatomical description and morphometric analysis of the chondrocranium of this species, integrating linear measurements and comparative anatomical data to assess interspecific variation and evaluate whether closely related taxa exhibit greater similarity in chondrocranial morphology. We identified both qualitative and quantitative differences between major crocodilian clades, which may reflect diverse ecological demands. Within Crocodylidae, orbitotemporal proportions strongly influence chondrocranial morphology, likely reflecting adaptations related to bite force and visual acuity, linked to prey type and diverse aquatic habits. Within Alligatoridae, by contrast, the emphasis on nasal capsule proportions suggests a reliance on olfactory and respiratory functions, consistent with their occupation of densely vegetated environments and more restricted geographic ranges. Additionally, we identified a set of embryonic diagnostic features located in the splanchnocranium of Caiman and in the neurocranium of Alligatoridae. These findings shed new light on the developmental basis of cranial diversity in Crocodylia and emphasize the evolutionary significance of chondrocranial traits in shaping macroevolutionary patterns.