Hilary F Ketchum

PhD Thesis

Christ's College, Cambridge

David Norman, Angela Milner and Leslie Noè

PhD

280

Plesiosauria was a diverse clade of predatory reptiles secondarily adapted to life in water, that played an important role in Mesozoic marine ecosystems. Plesiosaur fossils have been found on every continent, including Antarctica, and are known from the uppermost Triassic (Rhaetian) through to the uppermost Cretaceous (Maastrichtian). The Oxford Clay is an important source of exceptionally well-preserved marine vertebrate fossils from the Middle Jurassic, from which five genera of pliosaurid plesiosaurs and three genera of plesiosauroid plesiosaurs are currently known in the literature. Here a thorough re-description of the anatomy of a long-snouted (longirostrine) pliosaurid taxon from the Oxford Clay, Peloneustes philarchus, is presented, coupled with new reconstructions of the cranium, based on material held in institutions in the UK, Germany and Switzerland. A revised taxonomic diagnosis of P. philarchus is offered, which includes three new autapomorphies and 17 character states from the cranial and postcranial skeleton that can be used in combination to distinguish the species from other pliosaurid plesiosaurs. Plesiosaur diversity within the Oxford Clay is greater than previously thought, and two new taxa of longirostrine pliosaurids, Gen. et sp. nov. A and Gen. et sp. nov. B are described. A new hypothesis of the phylogeny of Plesiosauria is presented that incorporates 64 taxa scored for 175 new and critically re-examined morphological characters, the majority of which are based on personal examination of relevant specimens, making it the largest and most comprehensive cladistic analysis of Plesiosauria to date. The strict component consensus tree of the 18 most parsimonious trees recovered by the parsimony analysis is well resolved, and removal of only three �wildcard� taxa by reduced consensus methods results in a fully resolved reduced consensus tree; however, bootstrap proportions for the majority of ingroup relationships are low. In agreement with some previous analyses, the present analysis does not support the traditional hypothesis of a simple dichotomy between taxa with long necks and small heads (�plesiosaurs�) and those with short necks and large heads (�pliosaurs�), which had dominated pre-cladistic taxonomy for over 100 years. Instead, the large headed, short-necked clade, Polycotylidae, is more closely related to Plesiosaurus dolichodeirus than to Pliosaurus brachydeirus. However, the recovered topology differs in many important respects from those generated by previous analyses, in particular in the relationships of the more basal taxa, which necessitates re-definition of a number of clades to produce a more stable phylogenetic taxonomy.

English