A review of British Upper Jurassic ichthyosaurs

Abstract

The introduction to the thesis presents a synopsis of British Jurassic stratigraphy, and a brief account of the occurrence of British Upper Jurassic ichthyosaur remains which highlights the importance of the Leeds Collection of ichthyosaurs. A historical review of the publications concerning British Upper Jurassic ichthyosaurs (members of the order Ichthyopterygia) is presented. The British Upper Jurassic ichthyosaur taxa are reviewed. Of the five genera and fourteen species erected, only four generic and four specific names are found to be valid. The rejected names are listed with reasons for their rejection. After a listing and discussion of the synonymy of each valid species, a diagnosis and list of referable material is presented, then each species is described in detail. New reconstructions of the skull in dorsal and lateral views, the palate, the lower jaw and the complete skeleton of Ophthalmosaurus icenicus are presented. A reinterpretation of the forepaddle of O. icenicus proposes that previous interpretations have presented the forepaddle laterally inverted. The discovery that a bone of uncertain homology, designated element B, is present in the temporal region of the skull of O. icenicus, has important implications in the problem of the phylogeny and affinities of the Ichthyopterygia, and this is discussed. The presence of element B in the skull, which has been denied by previous authors, leads to the proposal of two alternative hypotheses concerning the relationships of the Ichthyopterygia to other reptiles. The preferred hypothesis is one that states that element B is a neomorph, and that the Ichthyopterygia are diapsid derivatives. A critique of previous schemes of classification of the Ichthyopterygia is presented, and a new classification is proposed. The validity of the division of the Ichthyopterygia into two groups, the latipinnates and longipinnates, is questioned. Finally, a discussion of functional aspects of the anatomy of O. icenicus is presented.