Abstract
The lineage leading to modern Crocodylia has undergone dramatic evolutionary changes in morphology, ecology and locomotion over the past 200+ Myr. These functional innovations may be explained in part by morphological changes in the axial skeleton, which is an integral part of the vertebrate locomotor system. Our objective was to estimate changes in osteological range of motion (RoM) and intervertebral joint stiffness of thoracic and lumbar vertebrae with increasing aquatic adaptation in crocodylomorphs. Using three-dimensional virtual models and morphometrics, we compared the modern crocodile Crocodylus to five extinct crocodylomorphs: Terrestrisuchus, Protosuchus, Pelagosaurus, Steneosaurus and Metriorhynchus, which span the spectrum from terrestrial to fully aquatic. In Crocodylus, we also experimentally measured changes in trunk flexibility with sequential removal of osteoderms and soft tissues. Our results for the more aquatic species matched our predictions fairly well, but those for the more terrestrial early crocodylomorphs did not. A likely explanation for this lack of correspondence is the influence of other axial structures, particularly the rigid series of dorsal osteoderms in early crocodylomorphs. The most important structures for determining RoM and stiffness of the trunk in Crocodylus were different in dorsoventral versus mediolateral bending, suggesting that changes in osteoderm and rib morphology over crocodylomorph evolution would have affected movements in some directions more than others.
Highlights
Crocodiles, alligators and gharials are the only extant representatives of the clade Crocodylomorpha, whose members span the continuum from terrestrial to aquatic
The early crocodylomorph vertebrae are characterized by relatively long centra, short vertebral processes and medium-sized zygapophyseal joints, qualitatively similar to the ancestral archosaurian state (JR Molnar & JR Hutchinson 2015, personal observation)
While we only studied two early crocodylomorphs, the similar features of Terrestrisuchus and Protosuchus provide strong qualitative support that relatively greater intervertebral joint (IVJ) stiffness and smaller range of motion (RoM) were ancestral for the crocodylomorph lineage
Summary
Crocodiles, alligators and gharials are the only extant representatives of the clade Crocodylomorpha, whose members span the continuum from terrestrial to aquatic. Crocodylomorphs had more erect (as opposed to sprawling) limb postures and parasagittal gaits and were probably highly terrestrial, like many other archosaurs [1], whereas later crocodylomorphs diversified to inhabit both terrestrial and aquatic environments [2]. Modern crocodylians are semi-aquatic and use a range of postures and gaits between sprawling and erect; the generally more sprawling posture of modern crocodylians must have been re-acquired at some point in their evolution. One unusual attribute of the crocodylomorph lineage is the convergent evolution of erect postures and asymmetrical (bounding and galloping) gaits otherwise found only in mammals. Small (shorter than 3 m) crocodylids and gavialids (not alligatorids) have been observed to use asymmetrical gaits; architectural properties of the limb muscles may be a limiting factor [3]. Bounding and galloping abilities in modern crocodylians may have been inherited from their more terrestrial ancestors [1], or these abilities may have originated more recently [4]
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