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Correlated trends in the evolution of the plesiosaur locomotor system

¹ F. Robin O'Keefe. Department of Anatomy, NYCOM II, Room 326, New York College of Osteopathic Medicine, New York Institute of Technology, Old Westbury, New York 11568. frokeefe{at}nyit.edu
² Matthew Carrano. Department of Paleobiology, Post Office Box 37012, MRC 121, Smithsonian Institution, Washington, D.C. 20013-7012. Carrano.Matthew{at}nmnh.si.edu

This paper investigates trends in the evolution of body size and shape in the Plesiosauria, a diverse clade of Mesozoic marine reptiles. Using measures from well-preserved plesiosaur specimens, we document and interpret evolutionary patterns in relative head size, body size, and locomotor variables. Size increase is a significant trend in the clade as a whole, and in constituent clades. The trend in relative head size is of variance increase; observed head sizes are both smaller and larger than ancestral values. In the locomotor system, changes in propodial and girdle proportions appear concomitant with body size increase and are interpreted as allometric responses to the physical constraints of large body size. Other trends in the locomotor system are significantly correlated with both body size and relative head size. These locomotor trends evolved convergently in several clades of plesiosaurs, and may have had an ecomorphological basis, although data are lacking to constrain speculation on this point. The evolution of the locomotor system in plesiosaurs sheds new light on the response of aquatic tetrapods to the physical constraints of foraging at large body size.