Paludicola 9(3):144-158 October 2013-10-03 © Rochester Institute of Vertebrate Paleontology 144 MORPHOLOGY OF THE CAUDAL VERTEBRAE IN RHOMALEOSAURUS ZETLANDICUS AND A REVIEW OF THE EVIDENCE FOR A TAIL FIN IN PLESIOSAURIA Adam S. Smith Nottingham Natural History Museum, Wollaton Hall, Nottingham, NG8 2AE, UK; adam.smith@nottinghamcity.gov.uk ABSTRACT The holotype of the large plesiosaurian Rhomaleosaurus zetlandicus from the Toarcian (Lower Jurassic) of England comprises an almost complete skeleton, including the caudal vertebral series. The osteology of the tail is described and two morphological characteristics are interpreted as evidence for a caudal fin in Rhomaleosaurus: 1. A distinct node consisting of two relatively anteroposteriorly shortened vertebrae; and 2. Laterally compressed terminal caudal centra. This inference is based on osteological correlates derived from other marine reptile groups that possessed a vertically oriented bilobed dermal tail fin in life (ichthyosaurs, thalattosuchian crocodylomorphs, and mosasaurs). This corroborates evidence from other plesiosaurian taxa and suggests that a caudal fin may have been widespread among plesiosaurians, with implications for locomotion and behavior. INTRODUCTION Plesiosaurians are an iconic group of extinct marine reptiles with an extensive stratigraphic range spanning approximately 135 million years from the early Jurassic to the late Cretaceous (Ketchum and Benson, 2010). Plesiosaurians belong the diapsid group Sauropterygia, and were adapted for aquatic life, with a sturdy trunk region and four large wing-like limbs, used to propel themselves through water (Robinson, 1975). This paraxial or appendicular locomotory style represents an unusual deviation from the general convergent trend towards axial (oscillatory) locomotion in the majority of derived secondarily aquatic reptiles (Robinson, 1975; Massare, 1988; Storrs, 1993; Motani et al., 1996; Motani, 2005; Lindgren et al., 2010, 2013). Plesiosaurians were entirely carnivorous and comprised an important component of oceanic ecosystems during the Mesozoic Era (Massare, 1987). They exhibit a variety of body plans ranging between two extremes: short-necked large-headed ‘pliosauromorphs’ and long-necked small-headed ‘plesiosauromorphs’ (O’Keefe, 2002; O’Keefe and Carrano, 2005). Recent studies have demonstrated that plesiosaurian body proportions were evolutionarily plastic and that these two extreme morphotypes arose multiple times independently in several clades (Bakker, 1993; O’Keefe, 2002; O’Keefe and Carrano, 2005; Benson and Druckenmiller 2013; Benson et al., 2013). Rhomaleosaurids are a clade of Early–Middle Jurassic plesiosaurians, morphologically intermediate between the plesiosauromorph and pliosauromorph extremes (Cruickshank 1994; Smith and Dyke, 2008). Although the cranial anatomy of rhomaleosaurids has received considerable research attention and is generally well understood (Cruickshank 1994, 1996; Gasparini, 1997; Sato and Wu, 2008; Smith and Dyke, 2008; Smith and Vincent 2010; Vincent and Benson, 2012), the postcranial anatomy of these plesiosaurians is more poorly known. This paper provides a description of the caudal vertebrae in the rhomaleosaurid Rhomaleosaurus zetlandicus (Phillips, in Anon, 1854) to improve knowledge of the caudal anatomy in this taxon, and plesiosaurians in general. Institutional Abbreviations—NHMUK, The Natural History Museum, London, UK; NMING, National Museum of Ireland (Natural History Division), Dublin, Ireland; MB, Naturkundemuseum Berlin, Germany; USNM, Smithsonian Institution, Washington, D.C; WM, Whitby Museum, UK; YORYM, Yorkshire Museum, UK. MATERIALS AND METHODS The holotype of the large-headed plesiosaurian Rhomaleosaurus zetlandicus (YORYM G503) from the Lower Jurassic (Toarcian) of the Yorkshire Coast, UK, comprises an almost complete skeleton. The skull was described and figured by Taylor (1992a) and the postcranial skeleton was briefly described with an emphasis on its taphonomy and preservation (Taylor, 1992b). The caudal series is complete (with the possible exception of one or two terminal vertebrae) and forms the basis of the present study. The tail of a referred specimen of Rhomaleosaurus zetlandicus is incomplete (WM851.S, previously ‘Rhomaleosaurus