Rhomaleosaurid plesiosaurs, a group of Mesozoic aquaticreptiles, are known from a number of well preserved skeletons.The research outlined here seeks to update our understandingof pliosaur cranial anatomy, assess the status of the genusRhomaleosaurus, and test the monophyly of theRhomaleosauridae. Lower Jurassic pliosaurs may besubdivided by stage.

Hettangian fauna - Rhomaleosaurus megacephalus (Figs 1A,B, C and 3D, E), Eurycleidus arcuatus (Fig 3C), Macroplatatenuiceps, and Archaeonectrus rostratus. An additional taxon isrecognised by a unique symphysis morphology (Fig 3F). A newreconstruction of the skull of R. megacephalus is presented here(Fig 1). Casts of the holotype, the original of which was destroyedin the Bristol Museum during an air raid, are also providing vitalinformation.

Toarcian fauna - Rhomaleosaurus cramptoni (Figs 2A, B), R.zetlandicus (Fig 3B), R. propinquus, R. thorntoni (Fig 3A), R.longirostris, and R. victor (background image). The holotype ofthe genus (R. cramptoni) (Fig 2A) is currently under preparationand a thorough interpretation of the whole skull including thepalate will follow.

There is a considerable diversity in skull anatomy between theseforms (compare Fig 1B with 2B), and variation in symphysismorphology is equally great (Fig 3). Indeed, A review of pectoralgirdle anatomy also reveals striking variation between speciespertaining to Rhomaleosaurus. (Fig 4)

Cranial Anatomy and Systematics of Lower Jurassic Pliosaurs

Adam Stuart SmithSchool of Biology and Environmental Science, University College Dublin,

Belfield, Dublin 4, Ireland. <adam.smith@ucd.ie> <www.plesiosauria.com>

AcknowledgementsMany thanks to Sandra Chapman, Arthur Cruickshank, Gareth Dyke, Mark Evans, Richard Forrest, Leona Leonard, Nigel Monaghan, Camilla Nichol, Leslie Noé,Roger Osborne, Jon Radley, Remmert Schouten, Julia Sigwart, Paolo Viscardi, Matt Williams, Patrick Wyse-Jackson, who have all contributed valuably to this ongoingproject, and this poster. Portions of this research were made possible by a grant from Palaeontographical Society.

Preliminary morphometricanalyses based on skullproportions (Fig 5) and apreliminary cladistic analysis(Fig 6) show morphologicaldistinctions, notably betweenToarcian and Hettangian taxa

Above Fig 1.Rhomaleosaurusmegacephalus (NMINGF10194). A dorsal view ofSkull. B. Interpretation of A.C. Interpretation of palate,with annotations indicatingcharacters at variance withCruickshank (1994).

Left Fig 2. Rhomaleosauruscramptoni (NMING F8785).A. Dorsal view of skull. B.preliminary Interpretation ofA. with annotations indicatingmajor characters shared withother British Toarcianpliosaurs, but differing fromR. megacephalus.Characters shared with R.megacephalus: postfrontalridge and lack of nasals

Fig. 4. Reconstructed pectoral girdles of A. R. thorntoni (BMNH R4853)and B. one half of R. megacephalus (composite of left and right elementsof NMING F10194). The girdle of R. thorntoni is much larger, broader andless elongate than R. megacephalus, the latter also has a clear ‘V’ shapedposterior median coracoid embayment.

2

2.2

2.4

2.6

2.8

3

4 4.5 5 5.5 6 6.5 7

Length/ orbit length

Len

gth

/ p

rem

ax l

en

gth

R. megacephlalus (dublin)macrocephalus (cast) F8780R. propinquusR. zetlandicusR. cramptoni F8785Eurycleidus arcuatus F8749

Hettangian

Toarcian

ReferencesCruickshank, A. R. I. 1994. Cranial anatomy of the Lower Jurassicpliosaur Rhomaleosaurus megacephalus (Stutchbury)(Reptilia:Plesiosauria). Philosophical Transactions of the Royal Society of London,Series B, 343, 247-260.

Clear lateralpalatine vacuity

Vomers extend far posterior of internal nares

Jugal exposed and contacts suborbital fenestra

Largeectopterygoid

30CM

30CM

Short premaxillaryrostrum

Large dorso-medianforamen betweennares

Frontalsseparated on midline

Borders of Premaxillaerun parallel

Prefrontalcontactspostfrontal

30CM

ConclusionsR. propinquus, R. zetlandicus, R.thorntoni and R. cramptoni. are unitedvia cladistic analysis and may beregarded as Rhomaleosaurus sensustricto. Their interrelationships awaitthe ongoing preparation of R.cramptoni.

R. megacephalus and R. victor differsignificantly from Rhomaleosaurussensu stricto and from each other (Fig6). Lower Jurassic pliosaurs form asister relationship with thePliosauridae, however, a traditionalRhomaleosauridae is not supporteddue to the exclusion of R. victor. Thegenus name ‘Thaumatosaurus’ is oftenapplied to the holotype of ‘R’. victorand could be resurrected?

‘R’. megacephalus

Pliosauridae

Rhomaleosaurus sensu stricto

A traditionalmonophyletcRhomaleosauridaeincluding ‘R’. victoris not supported.

Fig. 3. Mandibular symphyses of various species currently pertaining tothe genus Rhomaleosaurus, and the species Eurycleidus arcuatus. Thewidth of each symphysis has been standardised to allow directcomparison between species (so not to scale). The top line represents theminimum symphysis length (R.thorntoni) and the bottom line representsthe maximum (WARWKS G10875). Areas shaded black help visualisevariation. Light grey shading = inferred missing bone.

20CM

Fig. 5. See text.

Fig. 6. MajorityRule consensustree (heuristicsearch, 31characters)

A

A B

BA

CB

A B C D E F