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A revision of Ichthyosaurus (Reptilia, Ichthyosauria)

Lomax, Dean

[Thesis]. Manchester, UK: The University of Manchester; 2019.

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Abstract

The first ichthyosaur to be scientifically recognized was the common Early Jurassic form, Ichthyosaurus. Ichthyosaur specimens collected during the 19th century were almost exclusively referred to that genus, resulting in numerous species assigned. Most recent work prior to this study considered four valid species: Ichthyosaurus communis De la Beche and Conybeare, 1821, I. breviceps Owen, 1881, I. conybeari Lydekker, 1888 and I. anningae Lomax and Massare, 2015, although a fifth, I. intermedius Conybeare, 1822, was recognised by some workers. The type species (I. communis) has for a substantially long time been considered highly variable and common. Practically every Ichthyosaurus specimen that could not be identified as one of the other species was regarded as I. communis, essentially making it a wastebasket taxon. The genus and species therein have received little study since the 1970s. The recent description of I. anningae, coupled with other studies undertaken by the author, has provided a foundation for a revision of the genus. This study recognises two new species of Ichthyosaurus (I. larkini and I. somersetensis), and confirms the synonymy of I. communis and I. intermedius. Thus, six species of Ichthyosaurus are recognised as part of this work, each of which can be reliably distinguished on the basis of skull and humerus morphologies. In addition, a new specimen described herein represents the largest example of the genus and provides new information on the size range of the genus and species; it is also one of only a handful of pregnant specimens known from the UK. Furthermore, based on specific skull and postcranial characters defined in this study, the first neonate I. communis is recognised and formally described, which will assist in future studies on ichthyosaur ontogeny. This body of work also examines the morphological variation of the hindfin of Ichthyosaurus, a part of the skeleton that is often overlooked in ichthyosaur taxonomy. With a smaller sample size it would have appeared that the hindfin was taxonomically useful, but instead the large sample shows a continuum of variation across species. The examination of Ichthyosaurus provided a basis for the assessment of the contemporaneous Early Jurassic ichthyosaur Protoichthyosaurus Appleby, 1979, a genus that was previously synonymised with the former. Based on the unique forefin structure and features of the skull, Protoichthyosaurus is here considered distinct from Ichthyosaurus. A new species is also formally described, P. applebyi, along with the description of a large, three-dimensionally preserved skull and postcranial skeleton that was CT-scanned. This research confirms the presence of two Early Jurassic ichthyosaur genera with a wide forefin and anterior digital bifurcations. A revised diagnosis of both taxa is presented herein.