title: Morphological and functional changes in the vertebral column with increasing aquatic adaptation in crocodylomorphs.
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.
authors.affiliation: Department of Anatomy , Howard University College of Medicine , Washington, DC 20059, USA ; Structure and Motion Lab, Department of Comparative Biomedical Sciences , The Royal Veterinary College , Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA, UK.
authors.forename: Julia L
authors.initials: JL
authors.lastname: Molnar
authors.affiliation: Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology , Harvard University , Cambridge, MA 02138, USA ; Structure and Motion Lab, Department of Comparative Biomedical Sciences , The Royal Veterinary College , Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA, UK.
authors.forename: Stephanie E
authors.initials: SE
authors.lastname: Pierce
authors.affiliation: Department of Geology and Geophysics , Yale University , New Haven, CT 06520, USA.
authors.forename: Bhart-Anjan S
authors.initials: BA
authors.lastname: Bhullar
authors.affiliation: Department of Anatomical Sciences , Stonybrook University , Stony Brook, NY 11794, USA.
authors.forename: Alan H
authors.initials: AH
authors.lastname: Turner
authors.affiliation: Structure and Motion Lab, Department of Comparative Biomedical Sciences , The Royal Veterinary College , Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA, UK.
authors.forename: John R
authors.initials: JR
authors.lastname: Hutchinson
keywords: aquatic adaptation; axial skeleton; crocodylomorph; range of motion; stiffness; vertebrae