The origin of crocodilian locomotion

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.

A new species of Baurusuchus (Crocodyliformes, Mesoeucrocodylia) from the Upper Cretaceous of Brazil, with the first complete postcranial skeleton described for the family Baurusuchidae

“…The fourth trocanther is positioned posteroventrally to this structure, corresponding to an Archosauria synapomorphy (Benton & Clark, 1988). The fourth trocanther is placed more posteriorly in B. albertoi than in living crocodiles (Parrish, 1987). A longitudinal crest is present laterally to the fourth trocanther.…”

“…As in Pol (2005), vertebral count was made considering the atlas as the first vertebra. Following Parrish (1987;see also Pol, 2005), positional terms used for the description of the appendicular skeleton refer to a vertical orientation of limb elements. For the sake of clarity in comparisons, the same terminology was used to describe the limbs of extant crocodiles.…”

“…Living forms also possess a fifth cartilaginous phalanx in the fourth digit (Müller & Alberch, 1990) that cannot be inferred in B. albertoi. The fifth digit lost the phalanx in living crocodiles (Brinkman, 1980), and also in Protosuchus, Orthosuchus, and Hallopus (Parrish, 1987). The proximal phalanges are longer than the same phalanges of the manus.…”

“…This torsion orients this portion of the bone in an anteroposterior direction. The distal end is strongly compressed laterally due the diagenetic process, preventing any observation of a fibular condyle, typical of erect forms (Parrish, 1987). There are two distal condyles, with a longitudinal concavity between them (intercondylar fossa).…”

“…According to Parrish (1987), a supra-acetabular crest is present only in taxa that retain an erect position (e.g., Protosuchus, Notosuchus, Mahajangasuchus), by providing a different point of attachment to the muscles Mm. ilio femoralis and ilio tibialis.…”