Structural Adaptations of Early Archaeocete Long Bones

Abstract

Fossil remains recovered during the past decade have provided the first glimpse of the appendicular skeleton of early cetaceans (Gingerich et al., 1990, 1993, 1994, 1995; Hulbert and Petkewich, 1991; Aleshire, 1993; Madar and Thewissen, 1994; Hulbert, 1994, this volume; Thewissen et al., 1994, 1996). When coupled with archaeocete craniofacial, dental, and axial remains, a much clearer picture is emerging of the morphological transitions that occurred during cetacean evolution. The ancestry of modern cetaceans is linked at present to the terrestrial mesonychian condylarths (Van Valen, 1966; Prothero et al., 1988; Thewissen, 1994). Members of this group possess postcranial features linked to cursoriality, though emphasizing endurance rather than speed (Szalay and Gould, 1966; Zhou et al., 1992; O’Leary and Rose, 1995). Given this ancestry, early archaeocete postcranial skeletons should document the series of structural modifications that occurred in a move from complete terrestrial competence, through an amphibious or semiaquatic stage, to the type of highly specialized aquatic locomotion that characterizes modern cetaceans (Thewissen et al., 1996; Thewissen and Fish, 1997; Buchholtz, this volume).