The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: Implications for plesiosaur phylogenetics

Abstract

The predatory pliosaurs were among the largest creatures ever to inhabit the oceans, some reaching gigantic proportions greater than 15 m in length. Fossils of this subclade of plesiosaurs are known from sediments all over the world, ranging in age from the Hettangian (approximately 198 Myr) to the Turonian (approximately 92 Myr). However, due to a lack of detailed studies and because only incomplete specimens are usually reported, pliosaur evolution remains poorly understood. In this paper, we describe the three dimensionally preserved skull of the giant Jurassic pliosaur Rhomaleosaurus cramptoni. The first phylogenetic analysis dedicated to in-group relationships of pliosaurs allows us to hypothesise a number of well-supported lineages that correlate with marine biogeography and the palaeoecology of these reptiles. Rhomaleosaurids comprised a short-lived and early diverging lineage within pliosaurs, whose open-water top-predator niche was filled by other pliosaur taxa by the mid-late Jurassic.

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SHORT COMMUNICATION
The skull of the giant predatory pliosaur Rhomaleosaurus
cramptoni: implications for plesiosaur phylogenetics
Adam S. Smith &Gareth J. Dyke
Received: 6 February 2008 /Revised: 2 May 2008 /Accepted: 2 May 2008 / Published online: 4 June 2008
#Springer-Verlag 2008
Abstract The predatory pliosaurs were among the largest
creatures ever to inhabit the oceans, some reaching gigantic
proportions greater than 15 m in length. Fossils of this
subclade of plesiosaurs are known from sediments all over
the world, ranging in age from the Hettangian (approxi-
mately 198 Myr) to the Turonian (approximately 92 Myr).
However, due to a lack of detailed studies and because only
incomplete specimens are usually reported, pliosaur evolu-
tion remains poorly understood. In this paper, we describe
the three dimensionally preserved skull of the giant Jurassic
pliosaur Rhomaleosaurus cramptoni. The first phylogenetic
analysis dedicated to in-group relationships of pliosaurs
allows us to hypothesise a number of well-supported
lineages that correlate with marine biogeography and the
palaeoecology of these reptiles. Rhomaleosaurids com-
prised a short-lived and early diverging lineage within
pliosaurs, whose open-water top-predator niche was filled
by other pliosaur taxa by the mid-late Jurassic.
Keywords Reptiles .Plesiosaurs .Phylogenetics .
Anatomy .Cladistics
Introduction
Plesiosaurs are one of the most familiar groups of Mesozoic
marine reptiles. Carnivorous, secondarily aquatic and
predominantly marine, these animals ranged in length from
less than 2 m up to more than 17 m (Tarlo 1959; Buchy et
al. 2003; Noè et al. 2004). However, the evolutionary
history and palaeobiology of plesiosaurs remains poorly
understood: Recent phylogenetic analyses (O’Keefe 2001;
Druckenmiller 2006) have revealed some consensus but
debate remains, especially regarding relationships of one
early diverging branch, pliosaurs.
Plesiosaurs have long been classified into groups of
uncertain monophyly (e.g. Seeley 1892;Persson1963;Brown
1981;O’Keefe 2001; Druckenmiller 2006). Traditional
arrangements recognise two morphotypes—large-headed
and short-necked ‘pliosauromorphs’and small-headed and
long-necked ‘plesiosauromorphs’—but encompass other
forms that have defied robust phylogenetic placement
(O’Keefe 2001). Rhomaleosaurus is a large plesiosaur that
shares some characteristics with both ‘pliosauromorph’and
‘plesiosauromorph’lineages. Because Rhomaleosaurus had
a long neck compared to other pliosaurs and a gigantic
head (Figs. 1a,b and 2), resolving its position within
plesiosaur phylogeny is important if we are to better
understand and document the evolution of feeding and
locomotor adaptations amongst these reptiles (O’Keefe
2002).
Here, we describe the three-dimensional skull of the
holotype specimen of Rhomaleosaurus cramptoni,oneof
the largest and best preserved ‘pliosauromorph’plesio-
saurs known (Fig. 2). The original description of this
taxon is short and outdated (Carte and Bailey 1863); re-
description is warranted because this skull was prepared
in 2006. We use newly revealed anatomical data to
Naturwissenschaften (2008) 95:975–980
DOI 10.1007/s00114-008-0402-z
Communicated by G. Mayr
Electronic supplementary material The online version of this article
(doi:10.1007/s00114-008-0402-z) contains supplementary material,
which is available to authorized users.
A. S. Smith :G. J. Dyke
School of Biology and Environmental Science,
University College Dublin,
Belfield,
Dublin 4, Republic of Ireland
Present address:
A. S. Smith (*)
Department of Geology, Trinity College Dublin,
Dublin 2, Republic of Ireland
e-mail: assmith@tcd.ie

present a novel phylogenetic hypothesis for pliosaur
relationships.
Material and methods
NMING F8785 (National Museum of Ireland, Natural
History), a complete articulated adult specimen (Fig. 2)
including the skull (Fig. 1a,b), was unearthed in 1848 in an
alum quarry at Kettleness, near Whitby, on the Yorkshire
coast of the UK (late Upper Lias, Toarcian; approximately
178 Myr) and was displayed as a centrepiece at the 1853
meeting of the British Association in Dublin. Stored and
encased in concrete in Dublin ever since, the specimen
became the holotype of R. cramptoni (Carte and Bailey
1863). The total length of NMING F8785 was around 7 m
(Fig. 2); skull length along the dorsal midline is 88 cm. A
cladistic analysis was performed using PAUP (version 3.04;
Swofford 2000; see S1 for characters and S2 for the data
matrix) comprising 39 taxa, 93 characters and the saurop-
terygian Cymatosaurus (Rieppel 2000) as an out-group.
Systematic paleontology
–Plesiosauria de Blainville 1835
–Pliosauroidea Seeley 1874
–Rhomaleosauridae (Nopsca 1928)
–Rhomaleosaurus Seeley 1874
–R. cramptoni (Carte and Bailey 1863)
Revised diagnosis
Rhomaleosaurus is a large pliosaur characterised by the
following apomorphic combination of skull characters, as
identified by phylogenetic analysis: (1) rounded dorsome-
dian foramen situated between the nares, (2) foramina
present on the frontals, (3) premaxilla–maxilla sutures run
parallel to each other anterior to the nares, (4) palatine
excluded from internal naris (choanae), (5) length and width
of premaxillary rostrum subequal, (6) short mandibular
symphysis (length/width=0.60–0.89), (7) rounded bulb/
bump protruding from the medial margin of the retroarticular
process and (8) presence of a quadrate foramen.
Description and comparison
The skull of NMING F8785 is preserved three dimension-
ally (Fig. 1a–c). The premaxillae form a short rounded
spatulate rostrum with a total of five tooth positions each.
The occiput is deep and the jaws as preserved are fully
Fig. 1 Photos and drawings of the skull of NMING F8785, R.
cramptoni:adorsal view, bventral (palatal) view, and cposterior
view. Cross-hatching in aand bindicates restored areas, in cit
represents matrix; dotted lines indicate ridges (scale bar is 30 cm)
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occluded. Large oval nares are positioned close to (53 mm
away from) the orbits as in all plesiosaurs and there is a
lozenge-shaped dorsomedian foramen bordered by a raised
margin on the midline between the external nares (Fig. 1a);
this character is also seen in R. zetlandicus and R.
propinquus. Taylor (1992) noted the presence of this feature
in R. victor and a cleft was also figured for this taxon by
Fraas (1910; Taf. X).
Anterior to the external nares is a sharp median ridge;
this feature is also developed in all species of Rhomaleo-
saurus and forms a crest in Umoonasaurus (Kear et al.
2006). Unlike most plesiosaurs, the premaxillae do not
diverge immediately anterior to the nares and the premax-
illa–maxilla sutures run parallel, as is also the case in R.
zetlandicus,R. propinquus,R. thorntoni and Maresaurus
(Smith 2007). Each maxilla produces a triangular flange
that protrudes dorsally between the frontal and the
prefrontal. The frontals are elongate bones separated
medially by a long posterior premaxillary process; each
sends a broad anterior process to the external naris margin
and almost excludes the premaxilla from the margin of the
external naris (Fig. 1a). The frontals are also separated in all
other Toarcian pliosaurs, whereas they contact in the older
R. megacephalus (Cruickshank 1994).
The anteromedial border of the orbit is formed by the
prefrontal. There is a distinct postorbital ridge and the parietal
bears a square lateral process. A lozenge-shaped pineal
foramen is situated on the midline between the fused parietals
(Fig. 1a). The postfrontals are small triangular elements that
contribute to the posteromedial margins of the orbit. Each
postorbital ridge forms the majority of the postorbital bar
while the jugal is elongate and forms the posterolateral
margin of each orbit. The squamosals are large tri-radiate
elements forming the entire posterior margins of the
supratemporal fenestrae and expand into a squamosal–
parietal plate as described for R. zetalandicus (Taylor
1992) and also present in R. propinquus and R. mega-
cephalus (Smith 2007).
A convex posterior bulb is formed where the squamosals
meet on the midline; anteriorly, they contact the parietal
along a transverse inter-digitating suture forming an
expanded plate (Fig. 1a). There is a quadrate foramen
between the squamosal and quadrate at the dorsal tip of the
quadrate; this foramen is also present in R. zetlandicus
(Taylor 1992).
The palate of NMING F8785 is complete (Fig. 1b). The
fused vomers extend posteriorly between the choanae and
expand laterally to wrap around the posterior margin of
Fig. 2 Full body reconstruction
of Rhomaleosaurus in adorsal
and blateral view (scale bar is
1m)
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each to contact the maxilla. This is also the case in R.
zetlandicus,R. thorntoni and R. victor, whereas in R.
megacephalus, the palatines in these specimens do contact
the internal naris. A raised bump is formed at the anterior
part of the vomer–palatine suture (Fig. 1b), also present in
R. zetlandicus (Smith 2007). Posteriorly, the vomers
contact the pterygoids on the midline along a straight,
transversely orientated suture and the maxilla forms the
lateral margin of each internal naris. The pterygoids are
large plate-like elements; there is no open anterior
interpterygoid vacuity, although the pterygoids are sepa-
rated on the midline and filled by bone (parasphenoid?).
This contrasts with the narrow open anterior interpter-
ygoid vacuities in R. megacephalus (Cruickshank 1994),
as well as the broad anterior interpterygoid vacuity in R.
victor (Fraas 1910). The pterygoids form a square plate
caudal to the posterior interpterygoid vacuities and meet
medially (Fig. 1b). The cultriform process is exposed on the
palate; it is short with a ventrally facing concave surface.
The cultriform process is short as in all other species of
Rhomaleosaurus, but it is completely absent in R. victor.
There are small (20 mm long) but distinct lateral palatinal
vacuities between the palatines and pterygoids, and the
suborbital fenestrae are large and elongate, widest anterior-
ly, slightly pinched towards the posterior and expanded
again slightly at the posterior border (Fig. 1b). The flat face
of the prominent ectopterygoid boss is directed ventrolat-
erally and the surface is distinctly ornamented with pits (as
in crocodilians) indicating the presence of a plate of
cartilage.
The basicranium of NMING F8785 is exposed in
ventral and posterior view (Fig. 1c). The parasphenoid
forms a sharp ventral keel and merges into the basi-
sphenoid posteriorly, while the occipital condyle of the
basioccipital extends beyond the posterior margin of
the pterygoid plates (Fig. 1c). There is no evidence for
the exposure of the basioccipital on the posterior-most
ventral surface of the palate between the pterygoids in R.
cramptoni, as inferred for R. zetlandicus (Taylor 1992). A
dorsoventrally orientated oval pit is present on the
posterior of the occipital condyle. The occipital condyle
is situated dorsal to the level of the pterygoid plates of the
quadrate–pterygoid flange. The posterior margin of the
foramen magnum slopes posteriorly so that the dorsal parts
of each exoccipital–opisthotic and all of the supraoccipital
are obscured by matrix. The paroccipital process is a splint
of bone angled posteriorly and slightly ventrally. Distally,
this process forms a spatula, which contacts the quadrate–
pterygoid flange. Proximally, the paroccipital process
and quadrate–pterygoid flange are separated but, distally,
they unite for half of their length before broadly contacting
the medial wall of the squamosal and quadrate flange
(Fig. 1c).
Discussion
Results of the cladistic analysis are presented in Fig. 3a. A
heuristic search strategy implemented in PAUP led to the
generation of 2,114 equally most parsimonious trees
(MPTs), each of 364 steps in length when a standard
character ordering sequence is used (re-running the matrix
by use of unordered characters results in no changes to
major clade topologies).
Our hypothesis suggests the presence of three clades of
pliosaurs—Pliosauridae, Rhomaleosauridae and Leptoclei-
doidea—in broad agreement with earlier studies (O’Keefe
2001; Druckenmiller 2006). The rhomaleosaurid lineage is
proposed to be the sister taxon of the Pliosauridae and
Leptocleidoidea, which comprise the traditionally defined
‘pliosauromorph’plesiosaurs (O’Keefe 2002; Fig. 3).
The strict consensus of the MPTs is shown in Fig. 3a—
R. cramptoni is nested within a monophyletic Rhomaleo-
sauridae, with the other named species in this genus and
alongside the taxa Archaeonectrus,Macroplata,Euyclei-
dus,Sthenarosaurus and Maresaurus (Fig. 3b). The genus
Rhomaleosaurus is, however, not monophyletic and should
therefore be restricted to encompass just R. cramptoni and
its Toarcian relatives (i.e. R. zetlandicus,R. thorntoni and
R. propinquus). R. megacephalus and R. victor do not
belong to this genus; both taxa are currently under revision
(Smith 2007). Although recovered in all MPTs, Rhomaleo-
sauridae is supported by just one unambiguous synapomor-
phy, the presence of accessory grooves on the palatal
surface (character 34; S1 characters). Additional characters
that also support this lineage in the cladistic analysis are
given in the caption to Fig. 3.
On the basis of our phylogenetic hypothesis (Fig. 3a),
Rhomaleosauridae is restricted in age to the Lower Jurassic
and early Middle Jurassic and comprises a short-lived early
radiation of pliosaurs. Maresaurus is the youngest known
member of this clade. Subsequent to their extinction, the
open-water top-predator niche occupied by these marine
reptiles was filled in the mid-late Jurassic by the shorter-
necked pliosaurids; as suggested by O’Keefe (2002), these
Fig. 3 Strict consensus tree for pliosaurs (a) and close up of the
Rhomaleosauridae (50% majority-rule consensus) (b)withskull
cartoons. This clade is supported by the following apomorphies:
diminutive contact of premaxilla with external naris (except Archae-
onectrus; confidence interval (CI)=0.667), accessory grooves on the
palatal surface (CI=1.0); bowed mandible (except Archaeonectrus,
shared with Plesiosaurus,Simolestes; CI=0.5); between 27 and 29
cervical vertebrae (except Macroplata; CI= 0.571); nutritive foramina
on cervical vertebrae sunk in deep depressions (shared with L. clemai,
Dolychorhynchops; CI=0.5); large nutritive foramina on cervical
vertebrae (except Macroplata, shared with Dolychorhynchops,L.
clemai; CI=0.286)
b
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marine reptiles diversified rapidly as marine predators,
experimenting with a range of morphologies.
Acknowledgements We thank N. Monaghan, M. Parkes and J. Sigwart
(NMI) for supporting this project, S. Moore-Fay and C. Collins for the
preparation, K. Grimes for photographs and R. O’Keefe, R. Schouten,
R. Forrest and M. Taylor for the assistance. G. Mayr, M. Caldwell and
three referees provided comments in review.
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