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A Pliosaur (Reptilia, Sauropterygia) Exhibiting Pachyostosis from the Middle Jurassic of England

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A Pliosaur (Reptilia, Sauropterygia) Exhibiting Pachyostosis from the Middle Jurassic of England Journal of rhe Geological Society, London, Vol. 153, 1996, pp. 873-879, 5 figs, 1 table. Printed in Northern Ireland A pliosaur (Reptilia, Sauropterygia) exhibiting pachyostosis from the Middle Jurassic of England A.R. I. CRUICKSHANK', D. M.MARTILL' & L. F. NOE3 I Department of Geology, The University, University Road, Leicester LE1 7RH & Earth Science Section, Leicestershire Museums Service, The Rowans, College Street, Leicester LE2 OJJ, UK 2 Department of-Geology, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK "Division of Biology, School of Environmental and Applied Sciences, University of Derby, Kedleston Road, Derby DE22 IGB, UK Abstract: A new andunusual pliosauridfrom thePeterborough Member, Oxford Clay Formation (Callovian, Jurassic) of Cambridgeshire, UK, exhibits a type of pachyostosis and bone thickening of the rib cage and vertebrae. Pachycostasaurusdawni gen. et sp.nov. was mostprobably a benthic feeding carnivore with a generalist diet that included fish and invertebrates, and was a slow swimming underwater flyer with ventrally placed ballasting of the skeleton to resist roll. The apparent rarity of this pliosaur may indicate that it is allochthonous in the Oxford Clay biota. Froma palaeoecological viewpoint, the findis significantfor two reasons. Firstly, as an air breathing animal that is thought to have fed on benthos or nektobenthos, Pachycosrasaurus may have been important in transferring resources from the benthic food web to the surface food web. In a more general sense, thenew find provides further insights into the possible trophic structure of aquatic animal communities during deposition of the Oxford Clay. Keywords: England, Jurassic, Sauropterygia, plesiosauria. Plesiosaurs were one of the dominantmarine predator lags, often rich in vertebrates.A combination of storm groups for much of the Mesozoic, along with ichthyosaurs in condensation and soupya substrate hasresulted in a the Early Jurassic and marine crocodilians, mosasaurs and relatively high concentration of exceptionally well preserved large sharks during the mid-Jurassic and Cretaceous. They skeletons at some levels (Brown & Cruickshank 1995; Dawn are divided into several families of which the Pliosauridae 1991; Martill 1985). The ecology of this seahas recently occupied the fast pursuit and ambush predator niches of the been re-examined by Martill et al. (1994), who interpreted very extensive paralicseas of those times (Andrews the food web as being bipartite, having separate surface 1910-13; Brown 1981; Tarlo 1960;Taylor 1987, 1992~). water and benthic components. Skeletalmorphology of the giant pliosaurids (e.g. The new specimen is an almost complete skeleton of a Pliosaurus,Liopleurodon, Peloneustes, Simolestes) appears small pliosaur, collected in July 1994. The original animal to have beenaimed at reducing their overallskeletal had an estimated length of 3.0m, and the skeleton mineralization (within the bones), perhaps to inrease their comprises most of the skull and some of the mandible, 37 buoyancy andenhance theirunderwater mobility. This vertebrae (13 cervicals, pectorals,3 18 dorsalsand 3 skeletalreduction includes thinning of the pelvic and caudals), 25 ribs (4 left cervicals, 19 right dorsals, 2 left pectoral girdle elements to broad but thin sheets of bone, dorsals), the left pelvis, a right ilium, both humeri and three and increased intra-trabecular space within the corpus of the phalanges from a rear paddle. The posterior portion of the vertebrae and ribs. We report here a new pliosaurid having skull had been damagedduring quarrying. The remaining an unusual variation from the norm in which elements of the portion of the skull shows post-mortem collapse, similar to skeletonare thickened and comprise dense, pachyostotic that described for a Cryptoclidus by Brown & Cruickshank bone. Pachyostosis in plesiosaurs was reported early this (1995), the right side of the skull being folded over the left. century (Nopcsa 1923), while more recently Hampe (1992) What may be tooth marksfrom some other predator are reported pachyostosis in Kronosaurusboyacensis Hampe, impressed into the left maxilla. from the Lower Cretaceous of Colombia. The new The skull was prepared using air abrasives (Texas specimen,from thePeterborough Member of the Oxford Airsonics Inc. JetsonicModel BW'7; SS White sodium Clay at Whittlesey, Cambridgeshire, is a unique addition to bicarbonate#4 powder). The postcranial skeleton was an already diverse biota, with palaeoecological implications prepared using conventional mechanical methods. Dilute (Martill et al. 1994). Paraloid B72 in acetone was used to seal prepared surfaces, The Oxford Clay of the Peterborough district has been while H.M.G. restoration adhesive (Paraloid in acetone with worked for brick making for many years (Leeds1956), microbead filler) was used to repair breaks. yielding a wealth of vertebrate fossils (Martill & Hudson The new specimen is unusual in several respects. The 1991). The Oxford Clay was deposited in a shallow (10-50 m features which first alerted us to its unique character were deep) epeiric sea, with sedimentation interrupted by the relatively delicate construction of the skull (most infrequent tempests which generated numerous thin shell pliosaurs have massive skulls) with a relatively short 873 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/873/4889211/gsjgs.153.6.0873.pdf by guest on 26 September 2021 874 R. A. I. CRUICKSHANK ET AL. mandibular symphysis (5 pairs of teeth), and the massive inserted in narrow,a elongate mandibular symphysis, and pachyostotic nature of the ribs and lumbar vertebrae. features which preclude this individual from being congeneric with the contemporaryPeterborough Member pliosaurs Liopleurodon (6 tooth pairs), Simolestes (5 tooth Systematic palaeontology pairs in a spatulate symphysis) and PeloneusteslPliosaurus (up to 12 tooth pairs). Despite the posterior portion of the Institutionalabbreviations used: LEICS, Earth Sciences Section, Leicestershire Museums, Arts and Records Services, The Rowans, skull having beendamaged, the rami of the lower jaw College Street, Leicester LE2 OJJ, UK; PETMG, Peterborough City extend posteriorly forsome distance, indicating a skull Museum and Art Gallery, Priestgate. Peterborough, PE1 1LF. UK. length of approximately 625mm. The teeth arestout, Subclass Sauropterygia Owen, 1860 conical, striated all round,and circular in cross-section Order Plesiosauria de Blainville, 1835 (Fig. 1). Thereappear to be 25 teeth in each row. The Family Pliosauridae Seeley, 1874 anterior teeth are somewhat longer than the posterior teeth, Genus Pachycostasaurus gen. nov. as in Rhomaleosaurusmegacephalus (Cruickshank 19946), while the smaller teeth are straight and not recurved as in Typespecies. Pachycostasaurus dawni gen. et sp. nov. Pliosaurus brachyspondylus (Taylor & Cruickshank 1993). (monotypic genus). The cervical vertebrae have short centra with normally Derivation of name.Pachus, greek, thick; costa, latin, rib; situated rib articulations (Fig. 2). The neural arches are sauros, greek, lizard high, surrounding an enlargedneural canal. The zyga- Diagnosis. As for type and only species. pophyses are situated high above the centrum, and widely Species Pachycostasaurus dawni gen. et sp. nov. spaced with theirarticular faces almost horizontal. The Derivation of name. After AlanDawn, discoverer of this zygapophyses of the cervical vertebrae are more robust than new animal. those of the dorsal vertebrae. The cervical vertebrae appear Holotype. Only known specimen, PETMG R338. not to be heavily ossified. The neck region extends for Stratum typicum. Peterborough Member of the Oxford Clay approximately 645 mm. The pectoral and dorsal vertebrae Formation(Callovian, Middle Jurassic, Hudson & Martill are heavily ossified (Fig. 3), but the notochordal pits and the 1994), Bed 10/11. Jason Zone, JasonSubzone. Monster subcentral foramina are weakly developed. The anterior and Beds of Martill (1985). posterior articulatory faces of the centra are tabular and lack Locumtypicum. The King’s Dyke Clay Pit at Whittlesey, rugosities on their margins. The neuralarches of the nearPeterborough, Cambridgeshire,UK [NGRTL 248 pectoral vertebrae have deepened transverse processes with 9761. convex endsto accommodatethe strongly concave Diagnosis. Pliosaurid plesiosaur with five tooth pairs on the articulatoryfacets of the ribs. Thereare small accessory mandibular symphysis. Headabout 20% of overall body articular surfaces medial to the pre- and post-zygapophyses length. Cervical vertebralcount at least 13, with cervical which themselves are not prominent, though the articulating centrashortened and with enlargedneural canal, but not surfaces are almost flat and horizontal. The midlines of the heavily ossified except in the region of theneural spine. neural spines are excavated, and the dorsal surface of the Cervical ribs much shortened and thickened; double headed. neuralspines have broad open grooves running antero- Pectoral centra triangular in outline. Dorsal vertebral centra posteriorly. As reconstructed, the trunk region is approxim- heavily ossified, with indistinct subcentral foramina and flat ately 1 m long (Table 1). Only three caudal vertebrae are anteriorand posterior faces. Anteriorand posterior zygapophyses reduced, neural spines thickened, with midline anterior andposterior excavations. Dorsal ribs thickened and pachyostotic, with concave heads,round cross-section and ‘sausage-like’ profile. Gastralia heavily
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