Morphology and orientation of the ichthyosaurian femur

Abstract

Orienting ichthyosaur femora, especially in the most derived clade, Ophthalmosauridae, is notoriously problematic (Andrews, 1910; Kirton, 1983; McGowan and Motani, 2003). Structural detachment of the pelvic girdle from the vertebral column early in ichthyosaur evolution (Motani et al., 1998) means that the hind limb is rarely preserved in articulation with other skeletal elements; indeed, in ophthalmosaurids associated hind fins are only reported in Aegirosaurus leptospondylus, Caypullisaurus bonapartei, Ophthalmosaurus icenicus, and Sveltonectes insolitus (Andrews, 1910; Bardet and Fern´andez, 2000; Fern´andez, 2007a; Fischer et al., 2011b). Femora frequently occur in isolation, but even when articulated preservation of the hind paddles precludes unambiguous orientation. There is usually a great deal of morphological and taphonomic variation in the proximal region of the femur (E.M., pers. observ.), which has limited the systematic utility of this element as well as the hind limb more generally. In one of the most widely cited phylogenetic analyses of the Ichthyosauria, 26 characters pertain to the forelimb, whereas only six are drawn from the entire hind limb (Motani, 1999a). The proximal-distal axis of the ichthyosaurian femur is easily identified due to prominent distal facets for articulation with the tibia and fibula and the tendency for the distal end of the femur to be dorsoventrally flattened relative to the proximal end. However, the anterior-posterior and dorsal-ventral axes are much more difficult to differentiate, and are the primary source of controversy (Andrews, 1910; Kirton, 1983; McGowan and Motani, 2003).