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A taxonomic review of Ichthyosaurus from Somerset
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
Ichthyosaurus specimens from Somerset have not been examined in the light of current views of taxonomy and phylogeny. Presently, all Ichthyosaurus specimens from the Upper Triassic–Lower Jurassic of Somerset are referred to I. communis. Additional species or, perhaps, genera could exist among the Somerset specimens. In order to resolve the taxonomic questions, all Ichthyosaurus specimens from Somerset have been examined as part of this study. A total of 15 complete, or nearly complete, specimens were selected for detailed examination and study. Upon examination of the specimens, in order to separate any potentially distinct species present in the Somerset material, a character list of 87 distinct characters was compiled and each of the specimens was coded. A cladistic analysis was run and a total of 15 equally parsimonious trees were retained in the analysis. A total of 11 trees showed similar groupings of specimens. The results suggest that five species (regarded as: Species #1, #2, #3, #4 and #5) of Ichthyosaurus are present in the Somerset specimens. Three of the species are represented by single specimens (Species #1, #2 and #5) and two others (Species #3 and #4) are each represented by a cluster of specimens. Species #1 is the most primitive Ichthyosaurus species; it possesses autapomorphies of the skull and pelvic girdle morphology. Species #2 has some potential autapomorphies in the skull and tooth morphology; the latter may suggest that tooth morphology may potentially be a reliable character. Both Species #1 and #2 also share an independent combination of features with more primitive ichthyosaur taxa that are not found in any of the other species discovered in this study. Species #3 does not possess any autapomorphies, but it shares characters with more primitive taxa that are not found in Species #4. However, it possesses a combination of characters that may prove to be taxonomically useful for distinction of this species, but it is fairly poorly supported. Species #4 is the most derived species and possesses an autapomorphy in the skull and possibly in the humerus; it appears to be synonymous with I. communis, potentially suggesting its presence among the Somerset specimens. However, the study has uncovered wider implications and issues with this species. Species #5 is very poorly supported and is not considered valid. Coupled with the four currently valid species of Ichthyosaurus from Dorset (I. communis, I. breviceps, I. conybeari, and I. anningae described by Lomax and Massare (2015)), and the recognition of at least three further distinct species (with Species #4 possibly synonymous with I. communis) from Somerset, this study suggests that as many as seven species of Ichthyosaurus are known. The study provides a firm foundation for the much needed revision of the genus, Ichthyosaurus.