| Summary: | In this thesis, I describe work to resolve issues of bone identifications that have been outstanding since Smith Woodward's initial description in 1889, to assess the taxonomic validity of material assigned to the hypodigm of Leedsichthys and the interrelationships of the members of Family Pachycormidae. In addition I look at the palaeoecology of this animal on the basis of its size and growth and its locomotion capabilities and its likely feeding abilities and behaviour. Chapter 2 includes a review of the history of work on Leedsichthys, with particular reference to the discoveries made in the Peterborough district. In chapter 3, archival photographs and papers are used to establish the distinction between the type material, the tail specimen, and the gill basket specimen. In chapter 4, occurrences of Leedsichthys outwith the Peterborough district are considered, including the announcement of a new locality extending the range of the taxon into the Kimmeridgian. Some identifications of previously misidentified bones are made, specifically the hypobranchial and dorsal fin-rays. Feeding trace fossils are interpreted in the context of Leedsichthys. In chapter 5, a new Callovian pachycormid is described from the Oxford Clay of Peterborough district, and used in a reworking of Lambers' 1992 phylogenetic analysis of the interrelationships of the Pachycormidae. The Pachycormiformes are redefined on the basis of derived characters. In chapter 6, the value of gill rakers as a source of taxonomic characters is considered, with specific reference to their use in Lambers' 1992 character set, and the validity of Leedsichthys notocetes as a distinct species. In chapter 7, specimens are analysed using growth marks and scaling, in order to establish estimates of length-at-age for Leedsichthys. In chapter 8, the bone identifications of Smith Woodward (1889b) are revised, and further bone morphologies identified from within the hypodigm of the genus. In chapter 9, the size estimates derived in chapter 7 are used to inform interpretation of Leedsichthys palaeoecology, focussing primarily on locomotion and feeding. In the conclusions, an up-to-date reconstruction is presented.
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