Peggy Vincent

Roger B J Benson

Academic paper

Journal of Vertebrate Paleontology

32

5

1049-1063

ABSTRACT�The plesiosaur specimen NHMUK OR49202, from the Lias Group of Lyme Regis, consists of a complete skull, palate, and mandible, with eight associated cervical vertebrae, including the atlas-axis complex. This juvenile plesiosaur specimen was originally referred to �Plesiosaurus� macrocephalus. However, reexamination indicates that it does not belong to Plesiosaurus and comparison with the type specimen of �Plesiosaurus� macrocephalus (NHMUK OR1336) suggests that it is taxonomically distinct. Therefore, NHMUK OR49202 is made the holotype of a new genus and species: Anningasaura lymense. The specimen possesses plesiomorphic characters, including premaxillae that do not separate the frontals on the midline, narrow cranioquadrate passages, lack of a constricting groove around the occipital condyle, and several autapomorphies not observed in other plesiosaurian taxa: posteromedial processes of the premaxillae (or possible anterior portion of the frontal) forming a dorsoventrally thick, mediolaterally expanded platform; supplementary foramen penetrating the parietal sagittal crest; absence of a pterygoid-vomerine contact; absence of a contact between the pterygoids in palatal aspect; cultriform process of the parasphenoid wider mediolaterally than the combined posterior interpterygoid vacuities; and two closely spaced foramina in the lateral surface of the exoccipital. Anningasaura lymense is a plesiomorphic Early Jurassic taxon, and provides anatomical data that clarify the early evolutionary history of Plesiosauria. INTRODUCTION Plesiosaurians are predatory marine reptiles ranging from the Late Triassic (∼210 million years ago [Ma]; Walker and Geissman, 2009) to latest Cretaceous (∼65 Ma) (Bardet, 1992; Storrs, 1994; Gasparini et al., 2003; Benson et al., 2010; Sennikov and Arkhangelsky, 2010; Vincent et al., 2011). They are uniquely modified for marine life (Storrs, 1993), possessing a short, stiff trunk, four large propulsive flippers, and plastic body proportions, including taxa with very long necks and small heads (�plesiosauromorph�), taxa with short necks and large heads (�pliosauromorph�), and a spectrum of intermediate body plans (O�Keefe, 2001; O�Keefe and Carrano, 2005). Basal sauropterygians, representatives of the wider clade including Plesiosauria, document the origins of the plesiosaurian body plan (e.g., Sues, 1987; Sander et al., 1997; Sato et al., 2010). They are abundant in Triassic shallow marine deposits, especially in Europe, China, and the Middle East (Rieppel, 2000). However, plesiosaurians are first known from abundant remains in the Lower Jurassic marine strata of Europe, especially the Toarcian of Germany (e.g., Großmann, 2007) and several British units, including outcrops of the Lias Group (Cope et al., 1980) on the Dorset coast, which are mainly Sinemurian in age (e.g., Benton and Spencer, 1995). Plesiosaurians of this age have a long history of discovery (e.g., De la Beche and Conybeare, 1821; Owen, 1881; Sollas, 1881; Andrews, 1896) and are known from many complete or near-complete skeletons (Benton and Spencer, 1995; Storrs, 1997; Milner and Walsh, 2010). However, few of these classically known fossils have been described in detail. Consequently, our understanding of the early evolution of Plesiosauria is *Corresponding author. incomplete, and characterized by high levels of phylogenetic uncertainty (compare the contrasting phylogenetic topologies in: O�Keefe, 2001; Druckenmiller and Russell, 2008a; Smith and Dyke, 2008; Ketchum and Benson, 2010; Kear and Barrett, 2011; Benson et al., 2012). Restudy of Lower Jurassic plesiosaurians

English