Comparative morphology of presacral vertebrae in extant crocodylians: Taxonomic, functional and ecological implications

Abstract

Despite its systematic and functional relevance, the axial skeleton of crocodylians has received considerably less attention than the cranial and appendicular skeleton due to the assumed evolutionary conservativeness. The current study provides comprehensive comparisons of presacral vertebrae in extant crocodylians to demonstrate: (1) taxonomic variation, (2) size-dependent shape changes and (3) potential integration between vertebrae and skull functional modules. Our comparisons highlighted the uniqueness of the Indian gharial, Gavialis gangeticus, among extant crocodylians. The presacral vertebrae of G. gangeticus are characterized by reduced level of regionalization and increased intervertebral mobility in the neck (more narrowly placed zygapophyses and short vertebral processes), which would help lateral head sweeping under water for foraging. The scaling relationships of vertebral dimensions against the body size proxy indicate that dorsal vertebrae become stiffer with increasing body size: positive allometries were observed in areas and heights of inter-central joints, and lengths of vertebral processes (neural spines and transverse processes). These structural changes presumably serve to resist increasing compression loads and dorsiflexion bending moment on dorsal vertebrae of larger individuals during terrestrial locomotion. The analyses of correlation between vertebral dimensions and snout shape revealed that slender-snouted species tend to have more laterally mobile necks (specifically narrow zygapophyses), implying the potential integration of cranial and neck modules to optimize the foraging strategy.