Fusion of sacrals and anatomy in Champsosaurus (Diapsida, Choristodera)

Abstract

Sacral centra are occasionally fused with or without severe deformation in Champsosaurus (Diapsida, Choristodera). The sympatrical occurrence of fusion and non-fusion of sacra in adults through their evolution questions that sacral fusion represents the final form of a simple ontogenetic change or specific variation. Females are proposed to possess more robust limb bones than males because they are considered to have been more terrestrial due to the nesting behaviour on land. The coincidental occurrence of fusion of sacral centra without severe deformation and more robust limb bones in same individuals suggests that sacral fusion is a phenomenon occurring in females as a result of terrestrial adaptation for reproductive activities. Sacral fusion associated with severe deformation is considered a pathological condition although its etiology and factors remain undefined.

Keywords:

• Sacral fusion
• Champsosaurus
• sexual dimorphism
• ethology
• Late Cretaceous-Early Eocene
• North America

Acknowledgements

I wish to express my appreciation to J. R. Horner, H. D. Picton, M. T. Lavin, J. G. Schmitt, S. K. Gibson, and B. R. Erickson for supervising this project. I thank all the staff, students and volunteers of the Museum of the Rockies for their help, and F. Jackson and D. J. Varricchio for editing. I also would like to thank the American Museum of Natural History, Museum of Paleontology at University of California at Berkeley, Museum of the Rockies, Royal Ontario Museum, Royal Tyrrell Museum, and Science Museum of Minnesota for allowing me to study their collections. J. R. Horner, Barbara Lee, and the Museum of the Rockies provided financial aid for this study.