Phylogeny of the Ichthyosauria (Amniota: Reptilia) with special reference to Triassic forms

grantor: University of TorontoThe interrelationships among Triassic ichthyosaurs have been poorly understood due to the lack of knowledge regarding their basic osteologies, especially that of the earliest forms from the Spathian (Lower Triassic). These earliest ichthyosaurs represent relatively new additions to our knowledge, most of them having been found during the last 25 years. Reexaminations of ' Utatsusaurus, Grippia, Chaohusaurus' (Early Triassic), 'Parvinatator ' (Early or Middle Triassic), and of 'Mixosaurus' (Middle Triassic), reveals many previously unrecognized features of the skull, dentition, and forefin. This new knowledge, together with preliminary examinations of other Triassic ichthyosaurs, enables a comprehensive comparison among Triassic ichthyosaurs, for the first time. A phylogenetic hypothesis for Triassic ichthyosaurs is proposed, based on cladistic analyses of osteological characters, again for the first time. The monophyly of the Order Ichthyosauria, which was previously never questioned although never explicitly stated, is established based on at least nine characters that unambiguously define the basal node of the Order. Early Triassic ichthyosaurs (including 'Parvinatator') form the stem group of the Ichthyosauria, while all others form a crown-group clade, the Euichthyosauria. This taxon is further divided into two large clades, the Shastasauriformes and Ichthyosauriformes. The former includes 'Cymbospondylus' and 'Shastasaurus ' and their most recent common ancestor, while the latter comprises the Mixosauroidea and Ichthyosauroidea, and their most recent common ancestor. ' Toretocnemus' and 'Californosaurus', which have long been considered as shastasaurids, are shown to be ichthyosauroids. ' Cymbospondylus' and 'Ichthyosaurus', as previously designated, appeared paraphyletic on the cladogram, therefore two new generic names are proposed to solve the problem. 'Omphalosaurus' is considered non-ichthyosaurian, because none of the ichthyosaurian synapomorphies are known for this genus, while it shows several sauropterygian features. The taxonomy of 'Pessopteryx ' should be reconsidered. Tectonic deformation has remarkably distorted the original morphologies of some of the specimens studied, namely all specimens of 'Parvinatator ' and some specimens of Utatsusaurus. A method is devised to retrodeform the images of these specimens, using a personal computer. This method is unique for incorporating matrix algebra, thus simplifying the calculation process, while enabling the statistical selection of best solution.Ph.D