A NEW ICHTHYOSAUR FROM THE MIDDLE JURASSIC OF GERMANY PROVIDES INSIGHTS INTO THE ASSOCIATION BETWEEN LIMB FORM AND FUNCTION

MAXWELL, E. ; FERNANDEZ, M. S.; LARSSON, H.; SCHOCH, R.

Congreso; Society of Vertebrate Paleontology 74th Annual Meeting; 2014

Loss of proximodistal regionalization in the forelimb of ichthyosaurs is associated with the evolution of flippers. The diaphysis of the distal limb elements is gradually reduced via delayed perichondral ossification to a small series of notches on the anterior edges of the bones in the most anterior digit, and is lost completely in the youngest clade, Ophthalmosauridae. This trend has led to the argument that the retention of an ossified diaphysis is a primitive characteristic. A new ichthyosaur from the Middle Jurassic of Germany provides insight into possible functional underpinnings of this trait. The new taxon is based on a single large individual comprising the skull, partial vertebral column, pectoral girdle, forelimbs, and proximal hind limb. Parsimony analysis places it near the base of the Neoichthyosauria based on characteristics of the braincase and temporal region, suggesting a close affinity to the Early Jurassic genus Suevoleviathan. However, Suevoleviathan and the new taxon differ in flipper structure. Suevoleviathan has a broad, palmate forelimb, and diaphyseal ossification is absent in the distal limb. In the new taxon, the flipper is narrower than in Suevoleviathan and the diaphyses are retained on the anterior edges of the bones of the most anterior digit. We tested the association between flipper shape and diaphyseal ossification in parvipelvian ichthyosaurs by examining the relationship between flipper aspect ratio, as measured by the ratio between maximum length and chord of the limb skeleton, and the number of elements with ossified diaphyses in 19 neoichthyosaurian ichthyosaurs using independent contrasts. We recovered a significant relationship between the two variables, with long, narrow flippers having a higher number of elements retaining ossified diaphyses than shorter, broader flippers. Thus, flipper aspect ratio, a highly adaptive trait relating to locomotion and maneuverability, may directly influence diaphyseal ossification independent of phylogenetic relatedness. We suggest that pressure along the leading edge of the limb in taxa with high aspect ratio flippers induces diaphyseal ossification. These findings contradict the historical view of diaphyseal reduction over evolutionary time being a progressive process without functional correlates, and suggests that a locomotor shift favoring short, broad flippers is responsible for the loss of diaphyseal ossification in ophthalmosaurids and may be related to their taxonomic dominance in the Late Jurassic and Cretaceous.