Ichthyosaurs of the British Middle & and Upper Jurassic & and the Evolution of Ichthyosaurs

Abstract

Ichthyosaurs were a long-lived clade of Mesozoic marine reptiles with an excellent fossil record and a history of 200 years of study. However, this historical baggage means that several aspects of ichthyosaur palaeobiology are in need of revision.
The ichthyosaur material of the British Middle and Upper Jurassic is reassessed and at least four taxa are found to be present: Ophthalmosaurus icenicus, Brachypterygius extremus, Nannopterygius enthekiodon, and a newly identified indeterminate ophthalmosaurid. The available material is described and compared with a focus on the Ophthalmosauridae. Additional remains from the early Middle Jurassic of Scotland and the Kimmeridgian–Tithonian of England suggest that this diversity may be an underestimate.
A new phylogenetic matrix is compiled with 289 characters from recent phylogenetic analyses coded for 106 valid ichthyosaur species. Analysis of this dataset used maximum parsimony and likelihood criteria, and implied weighting of characters. Initial results were poorly resolved, but improved by a priori and a posteriori removal of unstable taxa. The topology follows previous hypotheses of ichthyosaur phylogeny, but several genera are not monophyletic, or taxa have different affinities to those found previously. Uncertainty in ichthyosaur phylogenetics suggests a need for revision of the characters used.
Ichthyosaur morphology through the Mesozoic is investigated using the above cladistic dataset alongside analysis of rates of change in continuous morphological characters. Ichthyosaurs rapidly increase in morphological diversity between the Early and Middle Triassic, retaining this disparity through the Mesozoic. A significant shift in morphospace occupation occurs between the Late Triassic and Early Jurassic bins with the evolution of Parvipelvia and Neoichthyosauria. The Middle Jurassic–Late Cretaceous bins show no significant difference in morphospace occupation. Increases in evolutionary rates occur towards the base of ichthyosaur phylogeny and on the stem towards Parvipelvia. This Late Triassic shift has previously been interpreted as a bottleneck.

Date of Award	27 Sept 2016
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael J Benton (Supervisor) & Emily J Rayfield (Supervisor)