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Cranial Anatomy, Taxonomic Implications

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Cranial Anatomy, Taxonomic Implications [Palaeontology, Vol. 55, Part 4, 2012, pp. 743–773] CRANIAL ANATOMY, TAXONOMIC IMPLICATIONS AND PALAEOPATHOLOGY OF AN UPPER JURASSIC PLIOSAUR (REPTILIA: SAUROPTERYGIA) FROM WESTBURY, WILTSHIRE, UK by JUDYTH SASSOON1, LESLIE F. NOE` 2 and MICHAEL J. BENTON1* 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK; e-mails: [email protected], [email protected] 2Geociencias, departamento de Fisica, Universidad de los Andes, Bogota´ DC, Colombia; e-mail: [email protected] *Corresponding author. Typescript received 5 December 2010; accepted in revised form 6 April 2011 Abstract: Complete skulls of giant marine reptiles of the genera. The two Westbury Pliosaurus specimens share many Late Jurassic are rare, and so the discovery of the 1.8-m- features, including the form of the teeth, but marked differ- long skull of a pliosaur from the Kimmeridge Clay Forma- ences in the snout and parietal crest suggest sexual dimor- tion (Kimmeridgian) of Westbury, Wiltshire, UK, is an phism; the present specimen is probably female. The large important find. The specimen shows most of the cranial size of the animal, the extent of sutural fusion and the and mandibular anatomy, as well as a series of pathological pathologies suggest this is an ageing individual. An erosive conditions. It was previously referred to Pliosaurus brachy- arthrotic condition of the articular glenoids led to pro- spondylus, but it can be referred reliably only to the genus longed jaw misalignment, generating a suite of associated Pliosaurus, because species within the genus are currently in bone and dental pathologies. Further damage to the jaw need of review. The new specimen, together with another joint may have been the cause of death. from the same locality, also referred to P. brachyspondylus, will be crucial in that systematic revision, and it is likely Key words: Plesiosauria, Pliosauroidea, Pliosauridae, Plio- that the genus Pliosaurus will be found to include several saurus, Kimmeridge Clay, Upper Jurassic, palaeopathology. P liosaurus is an enigmatic, advanced sauroptery- one of the first to suggest that the short pliosauromorph gian, first described almost two centuries ago (Owen neck might have evolved several times, an idea also dis- 1841), but which remains poorly understood. Pliosaurus cussed by other authors (White 1940; Bakker 1993; Car- is the basis of higher taxonomic groupings (Superfamily penter 1997). Pliosauromorph polyphyly was subsequently Pliosauroidea; Family Pliosauridae), and yet its holotype recovered in some phylogenetic analyses (Bardet and Go- lacks a modern redescription. This study describes a defroit 1998; Druckenmiller 1999; O’Keefe 2001, 2004), remarkable new specimen, a putative member of the but pliosauromorphs formed a clade in others (Druc- genus Pliosaurus (BRSMG Cd6172) recovered from the kenmiller and Russell 2008; Smith and Dyke 2008). For Kimmeridge Clay at the Lafarge cement works, Wilt- example, O’Keefe (2001, 2004) found that Pliosauroidea shire, compares it with a previously described specimen contained only Pliosauridae and Rhomaleosauridae, from the same locality (BRSMG Cc332) referred to whereas another short-necked clade, Polycotylidae, was P. brachyspondylus, and analyses and interprets a series included in Plesiosauroidea. Druckenmiller and Russell of pathologies in the specimen. (2008), on the other hand, recovered the clade Pliosau- The history of taxonomic relationships within the Plesi- roidea as traditionally defined and a new group, osauria and especially the Pliosauroidea is long and com- Leptocleidia, and they found that O’Keefe’s (2004) plicated (reviewed by Tarlo 1960; Brown 1981; Noe` 2001; Rhomaleosauridae was not a clade, but rather Rhomaleo- Ketchum and Benson 2010). Since the time of Owen saurus megacephalus, R. victor and Macroplata tenuiceps (1841), the position of pliosauromorphs, plesiosaurs with formed three successive basal taxa within Pliosauroidea. large canine teeth, large heads and short necks, within Finally, Ketchum and Benson (2010) found that the ‘plio- Plesiosauria, has been problematic. Williston (1907) was sauromorph’ taxa Leptocleididae and Polycotylidae were ª The Palaeontological Association doi: 10.1111/j.1475-4983.2012.01151.x 743 744 PALAEONTOLOGY, VOLUME 55 sister taxa within Plesiosauroidea, while Rhomaleosauri- and Cruickshank 1993) was discovered in the old quarry on dae was a clade within Pliosauroidea, as a sister group to 2 July 1980, 1 m below the Crussoliceras limestone horizon, Pliosauridae. in eudoxus biozone E5. More details of the locality, stratig- These contrasting hypotheses demonstrate deep uncer- raphy, discovery and excavation of the Westbury pliosaurs tainty concerning the interrelationships of major clades are given by Taylor and Cruickshank (1993), Taylor et al. within Plesiosauria. The pliosaurian body plan includes (1995), Grange et al. (1996) and Sassoon et al. (2010). not only a large head and short neck, but also relatively long coracoids and ischia, and low-aspect-ratio limbs Methods. Preparation of the material followed conven- (O’Keefe 2002), so multiple origins of the plio- tional manual mechanical techniques. Loose limestone sauromorph bauplan would imply striking convergences and shell-bed matrix was removed using a scalpel, while (O’Keefe and Carrano 2005). A full cladistic analysis of more stubborn encrustations were removed with a com- Pliosauroidea is in preparation (M. Go´mez-Pe´rez and L. pressed air–driven HD-type pneumatic engraving chisel F. Noe`, pers. comm. 2011). In this study, we contribute (Ken Mannion Fossils, Barton, Lincolnshire) or with a to the systematic revision of Pliosauroidea, as well as to stream of aluminium oxide abrasive powder (Powder No. understanding of pliosaurian palaeobiology, by: (1) pro- 1, 29 microns; Production Equipment Sales, Uckfield, East viding a description of the cranium and mandible of an Sussex, UK) from an Airbrasive jet tool (model AJ–1; Upper Jurassic pliosaur (BRSMG Cd6172) from Westbury Texas Airsonics Inc., Corpus Christi, TX, USA). Following in Wiltshire; (2) comparing BRSMG Cd6172 with a previ- initial preparation work, the skull and other bony ele- ously described pliosaur, BRSMG Cc332 (Taylor and Cru- ments were consolidated and reassembled. A solution of ickshank 1993), from the same locality; (3) questioning Paraloid B72 dissolved in acetone was used for both sur- the original criteria used to classify BRSMG Cc332 as face consolidation (10 per cent w.b.v. Paraloid in acetone) Pliosaurus brachyspondylus; and (4) describing and inter- and as an adhesive (60 per cent w.b.v. Paraloid in ace- preting a set of pathologies in BRSMG Cd6172, not previ- tone). Further details of the preparation and conservation ously recognised in a pliosauroid. of BRSMG Cd6172 are outlined by Sassoon et al. (2010). Cranial reconstructions of BRSMG Cd6172 are pre- sented in three standard views: dorsal, ventral and left lat- MATERIALS AND METHODS eral (Fig. 2A–C). As the mandible is substantially complete, only interpretive drawings in dorsal and ventral Material. BRSMG Cd6172 was found partially disarticu- views are presented. Diagrammatic reconstructions of cra- lated in a calcareous clay matrix, with a thick to thin nium and mandible were produced by scaling photo- indurated calcitic layer on bone surfaces. Both dorsal and graphs and referring to measured proportions, while ventral surfaces were also variably encrusted with inverte- making allowance for crushing. The positions and orien- brate epibionts. On preparation, the mineralised bone tations of the teeth were inferred from the size and orien- surfaces were pale or dark brown and variably crushed. tation of the functional alveoli. The cranium and mandible were preserved as complete X-radiographs of pathological elements were obtained elements, while postcranial elements were mostly using a Dragon mobile DR (digital radiography) system, extracted from nodules. Preparation of the material has with a 4-kW generator and a Canon CXDI-50G 400 speed continued since 1994, and the following elements have portable DR detector. Specimens were placed on the been exposed: cranium, mandible, ribs (seven large ele- detector, at 720 mm from the X-ray generator, and pho- ments and numerous fragments), vertebrae (at least 17 tographed. complete) some with associated neural processes, propo- dials, epipodials, paddle and phalangeal elements, gastra- Institutional abbreviations. BHN, Muse´e de Boulogne-sur-Mer lia, part of the right scapula, part of the left coracoid, (collections held now at Muse´um d’Histoire Naturelle, Lille); part of the left scapula and possible pelvic elements. BRSMG, Geology Collections, Bristol City Museum and Art Gal- lery; CAMSM, Sedgwick Museum of Earth Sciences; MNHN, ´ Locality and horizon. BRSMG Cd6172 was found in the Museum National d’Histoire Naturelle de Paris; NHMUK, Palae- ontology Department, Natural History Museum, London; OX- Kimmeridge Clay of the new quarry at the Lafarge cement FUM, Oxford University Museum of Natural History; PETMG, works, Westbury, Wiltshire, UK (NGR ST 8817 5267), on Peterborough City Museum and Art Gallery. 12 May 1994 and collected by a large team in the subsequent summer. The stratigraphic section at Westbury (Fig. 1; Anatomical abbreviations used in figures and tables. ?, probable Birkelund et al. 1983) shows that BRSMG Cd6172 was or questionable element; –, alveolus absent; a, angular; acr, found 7 m below the Crussoliceras limestone horizon
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