The origin of crocodilian locomotion

TLDR

Archosaur fossils show that erect gait adaptations are widespread and likely ancestral, while modern crocodilians’ sprawling gait is a secondary aquatic adaptation. Morphological and ecological evidence indicates that early crocodilians and their relatives possessed erect posture and terrestrial habits, and that contemporary crocodiles retain both sprawling and erect step cycles.

Abstract

The morphology of the tarsi, hindlimbs, and pelves of the earliest crocodilians and their nearest relatives, Hallopus and the “sphenosuchians,” indicates that these animals had adaptations for erect posture. The widespread distribution of apparently homologous adaptations for erect gait among the archosaurs with crocodile-normal tarsi suggests that those structures are plesiomorphic for this group, which comprises the Aetosauria, “rauisuchians,” “sphenosuchians,” Hallopus , and the Crocodylia. Adaptations for erect posture are seen most clearly in the structure of the proximal tarsus (astragalus and calcaneum). An important implication of this argument is that the most primitive crocodylomorphs, comprising the “protosuchian” crocodiles, the “sphenosuchians,” and Hallopus , had an erect stance and gait. The sprawling stance and associated gait used by modern crocodilians during swimming and upon entering the water can be viewed as secondary adaptations to an aquatic existence. The environments of deposition and faunal associations of “sphenosuchians” and “protosuchian” crocodiles are consistent with primarily terrestrial habits. Living crocodilians have two types of step cycles, sprawling and erect; the sprawling pattern is overprinted onto the inferred ancestral “high-walk,” and onto the gallop sometimes used by juvenile crocodilians.