A revision of Temnodontosaurus crassimanus (Reptilia: Ichthyosauria) from the Lower Jurassic (Toarcian) of Whitby, Yorkshire, UK

Abstract

Temnodontosaurus is known from the Lower Jurassic (Hettangian -€“ Toarcian) of England, Germany, France and Belgium and is the largest Jurassic ichthyosaur known from complete remains, with the total body length of the largest Temnodontosaurus exceeding 12 m. Six different species are presently identified: Temnodontosaurus platyodon (Conybeare, 1822), T. trigonodon (Theodori, 1843), T. crassimanus (Blake, 1876), T. eurycephalus McGowan 1974a, T. acutirostris (Owen, 1840), and T. azerguensis Martin et al. 2012. In the UK, Lower Jurassic ichthyosaurs have been collected extensively from many localities, primarily in Lyme Regis and Charmouth, Dorset; Street and Strawberry Bank, Somerset; and Whitby, North Yorkshire. The Yorkshire coast is of historic interest in being one of the earliest British localities to be exploited for fossil marine reptiles, and has produced many well-preserved specimens of plesiosaurs, ichthyosaurs and marine crocodilians. During the 19th century, the Yorkshire coast was extensively quarried for the manufacture of alum, which yielded many specimens of marine reptiles during its operation, including a large, partially-complete, three-dimensionally preserved specimen (YORYM 497) - the holotype of Temnodontosaurus crassimanus. While the holotype still remains on display at the Yorkshire Museum, it has remained largely understudied and the validity of the species has long been questioned. Through re-examination of YORYM 497, the study highlighted several morphological features of the postcranial skeleton and determined that T. crassimanus differs sufficiently from other species of Temnodontosaurus, specifically T. trigonodon, in possessing several distinct characteristics. These defining characters include a large, robust humerus that is proximodistally longer than the scapula (scapula length vs humerus length ratio: ~ 0.68); forefins which are significantly longer but less than twice the length of the hindfin (forefin length vs hindfin length ratio: ~ 1.51); and notching of the anterior facet of at least five leading edge elements (digit II) of both the fore- and hindfins. Several additional specimens, previously tentatively assigned to T. crassimanus, were located and examined, but none could confidently be assigned to the species. Furthermore, the species was incorporated into a phylogenetic analysis, and results confirm the temnodontosaurid affinities and close relationship with T. trigonodon. To understand further the relationship of T. crassimanus within Ichthyopterygia, a thorough taxonomic revision of the genus Temnodontosaurus and the associated species is greatly required, but this is beyond the scope of the present study. Additionally, this study further highlights the need for a complete re-examination of all the Yorkshire Lias ichthyosaur material, in order to understand better the variety and abundance of marine reptiles from the coastline during the Toarcian. This study has helped to lay the foundations for this future research to be undertaken.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	John Nudds (Supervisor)