Chapter 1 – Introduction and objectives

1.1. General Introduction Plesiosaurs are a group of secondarily aquatic diapsid reptiles belonging to the clade Sauropterygia (Carroll, 1988). Diapsids (subclass Diapsida) are a monophyletic group supported by both morphological and molecular data (Lee, 2001) including lizards, snakes, crocodiles and birds, and represent a major sub-group of the amniote vertebrates, i.e. vertebrate organisms with the ability to lay eggs on land (Benton, 1997). The Sauropterygia, as a subgroup of this lineage, includes plesiosaurs, nothosaurs, pachypleurosaurs, and placodonts, taxa in which there is a single pair of temporal fenestrae at the rear of the skull, as opposed to two pairs in most other diapsids (Figure 1.1): this modified diapsid condition is known as euryapsid and is shared with ichthyosaurs. The Euryapsida is variously considered a monophyletic taxon within diapsids (e.g. Caldwell, 1996), or as a recognised (Benton, 1997) or unrecognised (Müller, 2003) polyphyletic assemblage. Plesiosaurs were predominantly marine organisms, although their fossils are not uncommon in brackish, or even freshwater deposits (Cruickshank, 1997; Sato et al. 2002). In terms of their general palaeobiology, all plesiosaurs were carnivorous, ranging in ecological strategy from piscivores and cephalopod catchers (Martill, 1992; Sato and Tanabe, 1998), to super-predators capable of subduing other large marine reptiles (Massare, 1987; Thulborn and Turner, 1993). There is also inferred and direct evidence that plesiosaurs may have occasionally ingested shelled benthic organisms (McHenry et al. 2005). Plesiosaurs vary in size from around 1.5m long (e.g. Thalassiodracon from the Lower Lias of the UK, 205 million years ago [Ma] [Figure 3.12]) to 15m long giants (e.g. Pliosaurus from the Middle Jurassic Oxford Clay Formation of the UK, 149-159 Ma) (e.g. Buchy et al. 2003). Isolated remains, including a 3m long mandible Chapter