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 therib 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 inreasetheir 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 otherpredator 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

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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 ossified, pachyostotic. Humerishort and lightly constructed. Teeth stoutand conical, of circular cross-section, and with prominent striae around entire crown.

Description Some features of the new specimen suggest that it may represent ajuvenile, or atleast an immature individual: notably these include the lack of fusion of neural arches to the centra, and its estimated overall length of three metres which make it comparativelya small individual (the pliosaurs Liopleurodon and Kronosaurus may have reached 12 m in length (Benton 1990). In addition, the elements of the pelvic girdle are rather simple in outline and resemble those of a juvenile Eurycleidus (Cruickshank 1994~).Some b of the ventral gastralia, however, are fused, and the humeri, Fig. 1. Puchycostasaurus downi gen. et sp. nov. from the Oxford while lightly built, are well-ossified and do not appear to be Clay Formation of Whittlesey, Cambridgeshire. PETMG R338. (a) juvenile. Although the skull requires further preparation, it Teeth from front of maxilla, scale bar is 20 mm. (b) Tooth crown appears to be rather lightly constructed. The following is a from ?rear of jaw, scale bar is 5 mm; labial view to left, lingual view preliminarydescription. Thereare five pairs of teeth to right.

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Table 1. A selection of osteologicalmeasurements of Pachycostasaurus dawni gen. et sp. nov. based on specimen PETMG R338

Length of(calculated) skull 625 mm Length of(calculated) neck 645 mm Length of trunk 1000 mm Length(estimated) of tail 740 mm T otal animal lengthanimal Total 3010 mm Lengthcovered by gastralbasket 630 mm Length of L. humerus 165 mm Lengthhumerus of R. 160 mm Width of L. humerusproximally 45 mm Width humerusof L. distally 80 mm Width ofhumerus R. proximally 40 mm a b Width humerusof R. distally 80 mm Maximum width of left pelvis 225 mm Ifs speculative reconstruction of the caudal region suggests a total length of about 740mm. Thusthe estimated overall length of Puchycosrasaurus dawni is approximately 3.1 m. All the ribs (cervicals, pectorals, dorsals and gastralia) C are very heavily ossified, and appear relatively larger than the corresponding bones of larger pliosaur genera such as Fig. 2. Pachycostasaurus dawni gen. et sp. nov. PETMG R338. Liopleurodon and Peloneustes (Andrews 1913)(Fig. 4). In Cervical vertebra, (a) anterior view, (b) lateral view, (c) ventral thin section they show marked thickening of the bone with view. Scale bar is 50 mm. Abbreviations: fr, rib facet: fs, subcentral corresponding reduction of inter-trabecular voids (pachyos- foramina; pozyg, post-zygapophysis: przyg, pre-zygapophysis. tosis) (Fig. 5). The dorsal ribs have deeply concave articulatory surfaces and are thickly ossified, with a slight constriction several millimetres fromthe articulation. The preserved;they appearto be from approximately halfway distal ends are bluntly terminated. There are twenty three along the tail. They are similar to the dorsal vertebrae in rows of ventral gastralia extending for a length of 630 mm. theirrobust structure. but otherwise are not unusual. A The gastralia are thickened centrally, tapering to slender points distally (Fig. 4). The humeri are each about160mm in length, 80mm across distally, and 45 mm across proximally. They appear

fs

C Fig. 3. Pachycostasaurus dawni gen. et sp. nov. PETMG R338. Anterior dorsal vertebra in (a) anterior view, (b) lateral view, (c) ventral view. Scale bar is 50 mm. For abbreviations, see Fig. 2. Fig. 4. Pachycostasaurus dawni gen. et sp. nov. PETMG R338. Additional abbreviations: npit, notochordal pit. Cross-hatching Generalized body cross-section to show two dorsal ribs and two indicates damaged areas. pairs of fused gastralia. Scale bar is 100 mm.

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Fig. 5. (a) Photomosaic of thin-section of thoracic rib of Puchycostasaurus dawni gen. et sp. nov. PETMG R338, showing pachyostotic structure of bone, X8 approximately. (i) zone of cancellous bone tissue, X25 approximately. (ii) zone of ‘folded’ trabeculae, X25 approximately. (iii) dense inner zone with little or no void space, X25 approximately. Contrast this with (b) Thin-section through dorsal rib of plesiosaur Cryproclidus euryrnerus LEICS G418.1956.8.a.21. X6 approximately, showing ‘normal’ bone histology with large area of highly cancellous bone in centre, with most dense compactain outer zone. (c) Thin-section of dorsal rib of Liopleurodon ferox PETMG R318showing normal histology, X3 approximately.

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small for this animal,but are otherwisequite typical of connected to the sternum (Domning & de Buffrenil 1991), pliosaurs. A single scapula is preserved. The pelvic elements andnoother ventral elements are presentforthe are not well co-ossified. concentration of skeletal weight. Stability is generated by placing weight as low as possible in the body, but is not desirable in sirenians. By contrast, in Pachycostasaurus both Occurrence and functional significance of pachyostotic gastralia and limb girdles were present in a pachyostosis ventral position, so the animal must have beenstable, Three types of ‘pachyostosis’ of vertebrate skeletons can be probably to prevent roll during slow subaqueous flight. recognized; bones can become largerand more massive; Wiffen et d. (1995) described a type of pachyostosis in they may become more dense due to the infilling of spaces juvenile plesiosaurs from theCretaceous of New Zealand. within the bone by new mineralized tissue: or they may have Theyexamined examples of vertebrae, ribs, humeriand a higher mineral content. It appears, aspreliminarya phalanges, andfound pachyostotic bone only in those conclusion atleast, that in the new animal the ribs have elements they considered to be from juvenile individuals. become both larger, and have less void space internally. They concluded that all Cretaceous sauropterygia must have Sirenia(dugongs and manatees) arethe only extant undergone an unusual ontogenetic histological transforma- fauna with marked skeletal pachyostosis, showing charac- tion, which involved a lightening of the skeleton with teristically swollen, solid ribs (Domning & de Buffrenil increasing age. Such a transformation is unknown in any 1991). In dugongs, pachyostosis extends tothe skull and extantaquatic animal. Although we consider the new mandible posterior to the rostrum, the pectoral girdle, the specimen described here to be a young individual, it differs humeri and the neural arches of the thoracic vertebrae. This significantly from described pliosaurs fromthe English pattern is reflected in Puchycostasaurus, with additional Jurassic in several ways that we feel preclude it from being a pachyostosis of the gastralia andneural arches of the young Liopleurodon, Pelorreustes, Stnlolestes or Pliosaurus. cervical vertebrae. However, no very young pliosaurs have previously been The heavy skeleton of the extant sirenians acts as ballast described from the English Jurassic, andtherefore the for benthicfeeding and part of hydrostatica control degree of ontogoneticvariation of skeletalelements is at mechanism (Domning & de Buffrenil 1991). The addition of present unknown. ballast increasesvolume, surface areaand cross-sectional area of the body, creatingadditional drag (Taylor 1981, 1993). As bone is a relatively inefficient form of ballasting compared to ingested stones, pachyostosis leads to a Tooth form, function and feeding disproportionate increase in drag. As drag becomes greater with speed, pachyostosis therefore appears to be restricted Massare (1987) proposed two measures of tooth size and to slow moving benthic feeders (Taylor 1993, 1994). Massare various features of tooth morphology to distinguish prey (1988) interpreted all the known pliosaurids as relatively preference in extinct marine reptiles, using modern rapid swimmers and pursuit predators, but this is unlikely to analogues. Pachycostasaurus was clearly a predator, with be the case for Pachycostasaurus. sharp,robust, conical teeth, circular in cross-section and Ballast may serve several functions other than buoyancy heavily striated, but lacking prominent carinae. The relative control,however, including hydrostaticcontrol so that the tooth size and tooth shape fall within Massare’s (1987) ‘cut’, animal can rise and sink with minimum muscular effort, ‘pierce 11’ and ‘general’ guilds, indicating a diet ranging from maintenance of horizontal posture (by counteracting head or soft teuthoids, such as belemnites, to bony fish, and perhaps tail heaviness), generating stability mainly by counteracting some of the larger reptiles. This trophic range overlaps with roll, and subduing prey by increasing body mass (Darby & that of other pliosaurids and marine crocodilians from the Ojakangas 1980; Domning & de Buffrenil 1991: Taylor 1981, Peterborough Member (Massare 1987; Martill et al. 1994). 1993). It may also have actedas a phosphate reservoir. The suite of features (listed above) displayed by the Pachyostosis in Pachycostasuurus was presumably used in teeth of Pachycostasuurus does not fit directly into any of hydrostatic control,to regulate depthand horizontal the categories suggested by Massare (1987). The absence of posture, and may have been beneficial for benthic feeding. carinae and lack of robustness of the tooth crowns indicates Although it is possible that pachyostisis could be a result of Pachycostasaurus was not a high order predator of the ‘cut’ a pathological disorder, we rule this out in the case of the guild, and the striations on the teeth appear too heavy for specimendescribed as the distribution is localized, regular the ‘pierce 11’ or ‘general’ guilds. Massare (1987) indicates and very functional in appearance. the latter two guilds may not be significantly different: the Pliosaurids are interpreted as underwater flyers (Taylor teeth of Pachycostasaurus are most typical of a combination 1981, 1993), and it is likely that Pachycostasaurus was no of ‘general’ and ‘pierce 11’ guilds. Puchycostasaurus can be exception.This mode of locomotion has beencorrelated considered a generalist feeder, probably able to take many with thepresence of gastroliths(Taylor 1993), but as of the 45 generaconsidered benthic or nektobenthic by pachyostosis developed, the needfor gastric stones may Martill et al. (1994), especially the soft bodied forms. have been reduced or eliminated altogether. Martill (1992) However, mobility in Puchycostasuurus might indicate that it reported the first gastroliths from an Oxford Clay pliosaur, took more active prey. The benthic community of the including pebbles as well as a finer fraction of grit and sand. Peterborough Member sea floor was rich in arthropods and Taylor (1993) reported that benthic feeders often acciden- molluscs, notably ammonitesand belemnites (Martill & tally ingest sediment that is superficially gastrolith like. No Hudson 1991), although other groups, such as echinoids gastrolithswere found associated with the specimen (Smith 1995) were rare. We speculate that Pachycostusaurus described here. fed on nektobenthic arthropods, cephalopods, or possibly In most modern sirenians only three or four ribs are on the assumed nektobenthic, heavily scaled ganoid fishes

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Lepidotus (three species in thePeterborough Member), transfering resources fromthe benthicfood web tothe Mesturus and Heterostrophus. surface food web.

Mode of life and role in the food web Note on rarity' Sirenians are fully aquatic,inhabiting coastal shallows, The discovery of a new pliosaur in thePeterborough estuariesand rivers (Harrison & King 1965: Macdonald Member of theOxford Clay is certainly surprise,a 1984), and feeding on sea grasses and frequently resting on considering that this important marine reptile lagerstatte has the bottom(Taylor 1993). Sea cows rise tobreathe every been studied and heavily collected for more than 100 years 40-70 seconds when feeding, and every 2-3 minutes while (Martill & Hudson 1991). That Pachycostusuurus has resting (Macdonald 1984), but can remain underwater for up remained undetected for so long requires explanation: could to 16 minutes (Harrison & King 1965; Hildebrand 1988). it be an allochthonous fossil? Two sources foran Although usually inhabiting shallow waters only a few allochthonous Pachycostasaurus are considered possible. metres deep, dugongs have been known to feed at depths of Firstly, Pachycostasaurus may have been coastal, estuarine 23 m (Domning & de BuffrCnil 1991). Pachycostasaurus may or even have inhabited fresh water. Martill (1988) noted that have had a similar life style,but living sireniansalthough anumber of clearly allochthonous (because they were pachyostotic, are herbivorous,and do not swimby terrestrial)dinosaurs have been recovered fromthe subaqueous flight. The overlap of Pachycostasauras with the Peterborough Member. They are represented by articulated feedingstrategies of marine crocodiles, andnon- material, including an almost completeskeleton. He pachyostotic pliosaurids, both commonelements of the speculated that these could have drifted down rivers, a PeterboroughMember biota (Martill & Hudson 1991)is notion supported by the abundance of wood and the close notable. proximity (in the Dutch sector of the North Sea) of coeval Martill etal. (1994) suggested thatthe co-existence of non-marine facies. The implication is that Pachycostusuurus numerousmarine carnivores in the PeterbroughMember could also have drifted into the basin from a river. However, sea (four genera of pliosaurs, three genera of long-necked morecoastal and quasi-marine sediments of the Midlands plesiosaurs, one or two ichthyosaur genera, two genera of Platform have yet to yield any plesiosaurian bones. crocodilians and several large predatory fish) could be Alternatively, but perhaps less likely, as a deep diving attributed to niche partitioning on the basis of locomotory reptile, Pachycostasaurus may have occupied waters deeper ability, trophic habits and water depth. Some pliosaurids and than those represented by the Peterborough Member, and perhapsthe marine crocodilian Metriorhynchus were twist have drifted into the basin from further offshore. feeders (Taylor 1987, 1992b; Taylor & Cruickshank 1993). If Pachycostasaurus had adopted a similar strategy, it would One of the most observant collectors from the brick pits in recent have been in -competition with the large pliosaurids in times is A.Dawn of Stamford,who has discovered the skeletons deeper watersand with crocodilians in shallow coastal or of manyfine marine reptiles. We specially thank A. Dawnfor estuarine waters. Adaptations found in pliosaurids and his unceasingefforts. We also thank G. Chancellor, J. Martin, crocodilians include ballasted bodies for subduing prey and M. Barker, D. T. J. Smith, the director of LMARS, M. Munt, R. a skull resistant to strongtorsional forces (Taylor 19926; Pulley and J. Davidson for assistance and facilities. The University Taylor & Cruickshank 1993; Cruickshank 19946). of Derby is thanked for financially supporting L.F.N. Preparation of Pachycostusaurus has a lightly constructedskull, a short, the specimen was made possible by a grant from the Curry Fund. rather weak jaw symphysis, and ventral ballast for stability The London Brick Company PLC is thanked for allowing the new which would have resisted roll. Thus it is doubtful if specimen to be excavated and made available for the wider public Pachycostasaurus was a twist feeder. good, with special thanks going to D. Edwards and J. Howson. We Alternatively,marine turtles have very slow swimming especially thank M. A. Taylor for his review of the manuscript. predatory habits, andare underwater flyers without gastroliths(Taylor 1993), so this may be a ratherbetter analogy for Pachycostasaurus thanthesirenian or References pliosaurid/crocodilian models. Marinechelonians typically ANDREWS.C.W. 1910-1913. A descriptive catalogue of the marine reptiles of feed on sessile or slow-moving invertebrates in contrast to the O.rford Clay. Volume I (1910), volume 2 (1913). British Museum themore mobile prey favoured by most marine tetrapods (Natural History), London. BENTON,M.J. 1990. The reign of rhe reptiles, Kingfisher Books. London. (Taylor 1993). Turtles have yet to be reported from the BROWN.D.S. 1981. The English Upper Jurassic Plesiosauroidea (Reptilia) and PeterboroughMember, although they are common elsew- a review of the phylogeny and classification of the Plesiosauria. Bulletin here in Jurassicseas, and the presence of a pliosaurid of the British Museum (Natural History), Geology Series. 35, 253-347. analogue of a turtle in Pachycostasaurus may be an example - & CRUICKSHANK,A.R.I. 1995. The skull of the Callovian plesiosaur Cryprocliduseurymerus. andthe sauropterygian cheek. Palaeontology. of competitive exclusion. 37,941 -953. Martill et al. (1994) indicated the presence of a two phase CRUICKSHANK,A.R.I. 1994~.A juvenile plesiosaur (Plesiosauria:Reptilia) food web for the Peterborough Member sea with an upper, from theLower Lias (LowerJurassic) of Lyme Regis, England: a surface dominatedcomponent, and lower,a benthid pliosauroid-plesiosauroid intermediate? Zoological Journal of the Linnean Society. 112, 151-17X. nektobenthic component. Limited cross over between these - 19946. Cranial anatomy of the Lower Jurassic pliosaur Rhomalewaurus two systems is inferred by the mobility of the larger marine megacaphalus (Stutchbury) (Reptilia: Plesiosauria). Philosophical Trans- reptiles, andperhaps by daily migrational patterns of the actions of the Royal Society of London. B343, 247-260. cephalopods and some fishes (by analogy with modern seas). DARBY,D. G. & OJAKANGAS,R.W. 19x0. Gastroliths from anUpper Cretaceous plesiosaur. Journal of Paleontology. 54, 54-556. As an airbreathing animal, Pachycostasuurus must have DAWN, A. 1991. A rare pliosaur from theOxford Clay. GeologyToday. 7, journeyedrepeatedly tothe surface tobreathe despite 7-8. feedingat depth, andthus may have been important in DOMNING,D.P. & DE BUFFKENIL.V. 1991. Hydrostasis in the Slrenia:

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Received 2 January 19%; revised typescript accepted 12 July 1996. Scientific editing by Stewart Molyneux.

Note addedin prooE Wenote the record(Bartholomai 1966), without further commentfor the present, of very similar plesiosaurian vertebrae and neural arches recovered from fresh-water (cross-bedded fluvial sandstones) deposits from near Mount Morgan, Queensland, of Lower Jurassic age. We also note that Woodward, in his introduction to Andrews (1913), pointed out that the younger members of pliosaurian genera seemed to possess more heavily built skeletons, but without introducing the idea of pachyostosis. Clearly there is a growing need for a study of this feature in the Plesiosauria in general.

Additional reference BARTHOLOMAI.A. 1966. The discovery of plesiosaurian remains in freshwater sediments in Queensland. Australian Journal of Science, 28,437.

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