MOTANI, R.: Using models of extant swimmers to infer aspects of locomotion in thunniform ichthyosaurs.

Studying the functional morphology of extinct vertebrates poses challenges because only fossil remains are available. Robust correlations among certain physical, physiological, or kinematic variables in extant vertebrates can help infer functional parameters of extinct animals. I present a case where such correlational approaches prove useful in estimating some swimming characteristics of extinct marine reptiles. Ichthyosaurs (250 to 90 million years ago) evolved a tuna-like body shape from a lizard-like body plan of their ancestors, and some well-preserved specimens of the genus Stenopterygius, for example, show the outlines of lunate caudal fins. I used two correlations observed among living swimming vertebrates to estimate optimal swimming speeds in Stenopterygius (0.5 to 2.5 m fork length). The first set of estimates was based on the scaling of caudal fin kinematics observed in thunniform, carangiform, and subcarangiform swimmers. The second set was derived from the commonly used energetic model of constantly swimming vertebrates. The first estimates showed an almost identical distribution to the second set, providing some confidence in the results obtained, and suggesting that Stenopterygius cruised more slowly than delphinids of similar body size. The two estimation methods were tested by applying them to extant swimming vertebrates, prior to the application to Stenopterygius. The methods successfully approximated the optimal speed values in the literature (based on speed vs. oxygen consumption data). These results hold the promise of testable quantifications of some general performance variables in extinct swimming vertebrates, and of more accurate characterization of their functional and ecological modes of life.