MOTANI, Ryosuke, Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
The shape of the vertebral column provides important information for analyzing the locomotion of aquatic vertebrates that swim by undulating the body axis. Measurements from the vertebral columns of various ichthyosaurs show three basic patterns in the caudal region. The first pattern, known for basal ichthyosaurs and shastasaurs, is similar to that seen in lizards: the diameter of the column decreases gradually toward the tip of the tail. The second pattern is seen in tuna-shaped ichthyosaurs: the diameter decreases rapidly just before the caudal fin, then the decrease suddenly slows in the caudal fin. In addition, the centra, which are wider than high in front of the caudal fin, abruptly become higher than wide inside the caudal fin. This point of change probably marks the second joint of the double-joint system typical of thunniform cruisers. The third pattern is only known in mixosaurs: the column increases height in the middle of the tail where there is anticlination of the neural spines. The neural spines are unusually high throughout. Such a high, flat tail is suitable for acceleration rather than for cruising.
Both mixosaurs and tuna-shaped ichthyosaurs are adapted for swimming, yet in very different manners. Functionally, type 1 falls between types 2 and 3, so thunniform and mixosaur-type swimming probably evolved separately from type 1, or something similar. This functional hypothesis can be tested by phylogeny. The most recent phylogenetic hypothesis places mixosaurs and the clade containing tuna-shaped ichthyosaurs as sister groups, with shastasaurs and basal ichthyosaurs as the successive outgroups. Therefore, to support the functional hypothesis, it would be sufficient if basal members of either mixosaurs or the clade with tuna-shaped ichthyosaurs had type 1 tail morphology. No mixosaur is known to have had such a tail, whereas Toretocnemus californicus, a basal member of the latter clade, seems to have a modified type 1 tail. This species, however, is poorly known, and further investigation is required to complete the discussion.