Comparative limb proportions reveal differential locomotor morphofunctions of alligatoroids and crocodyloids.

Masaya Iijima,Yoshitsugu Kobayashi,Tai Kubo

doi:10.1098/rsos.171774

Masaya Iijima, Yoshitsugu Kobayashi + Show 1 more

Open Access

https://doi.org/10.1098/rsos.171774

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Abstract

Although two major clades of crocodylians (Alligatoroidea and Crocodyloidea) were split during the Cretaceous period, relatively few morphological and functional differences between them have been known. In addition, interaction of multiple morphofunctional systems that differentiated their ecology has barely been assessed. In this study, we examined the limb proportions of crocodylians to infer the differences of locomotor functions between alligatoroids and crocodyloids, and tested the correlation of locomotor and feeding morphofunctions. Our analyses revealed crocodyloids including Gavialis have longer stylopodia (humerus and femur) than alligatoroids, indicating that two groups may differ in locomotor functions. Fossil evidence suggested that alligatoroids have retained short stylopodia since the early stage of their evolution. Furthermore, rostral shape, an indicator of trophic function, is correlated with limb proportions, where slender-snouted piscivorous taxa have relatively long stylopodia and short overall limbs. In combination, trophic and locomotor functions might differently delimit the ecological opportunity of alligatoroids and crocodyloids in the evolution of crocodylians.

Highlights

Crocodylia is a remarkably successful group of large semi-aquatic predators, thriving in the world’s tropics and subtropics
The fossil record shows that the two living major groups, Alligatoroidea and Crocodyloidea were split during the Late Cretaceous, and both have survived through several environmental crises afterwards, including the devastating K–Pg mass extinction event and the Plio-Pleistocene climatic deterioration [4,5]
These two crocodylian lineages are readily distinguished by the osmoregulatory organs found in the mouth

Summary

Introduction

Crocodylia is a remarkably successful group of large semi-aquatic predators, thriving in the world’s tropics and subtropics. [1,2,3]) were split during the Late Cretaceous, and both have survived through several environmental crises afterwards, including the devastating K–Pg mass extinction event and the Plio-Pleistocene climatic deterioration [4,5] These two crocodylian lineages are readily distinguished by the osmoregulatory organs found in the mouth. While all crocodyloids show keratinized buccal cavity and all crocodyloids except Gavialis gangeticus possess lingual salt glands, alligatoroids’ buccal cavity is non-keratinized, and their tongues lack lingual salt glands [6,7,8,9] This led Taplin et al [7] to suggest the marine ancestry in crocodyloids, which enabled them to expand their distribution through transoceanic dispersals. These structures are present on the head as well as every postcranial scale of crocodyloids, but only on the head of alligatoroids

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