: 
_CRANIAL ANATOMY OF RHOMALEOSAURUS THORNTONI ANDREWS 
a b 
= 100 mm. 
The differences in orientation of the zygapophyses in the cervical 
vertebrae of Plesiosauria depend on their relative position in the 
neck. In general the zygapophyses of the anterior cervical vertebrae 
are horizontally oriented, becoming inclined after the first ten or so. 
For instance in MANCH LL8004, a specimen of Macroplata 
longirostris (Blake) (Broadhurst & Duffy 1971), there are about 32 
cervical vertebrae, of which the first ten have horizontal zygapophy- 
ses, while the remainder have zygapophyses angled at about 45° to 
the horizontal. Liassic plesiosaurs in general seem to have between 
28 and 32 cervical vertebrae. Even in the posteriormost cervicals, 
the rib articulations are placed close to the lower rim of the centra 
(Taylor & Cruickshank 1993a), and therefore could still appear to be 
from a more anterior position. Therefore, it is not always obvious 
from which part of the neck any single vertebra might come, and 
hence to draw conclusions about zygapophyseal orientation is pre- 
mature. 
The question of the characters of the humeri may well depend on 
the state of preservation of each. The skull and skeleton of R. 
cramptoni are very much less damaged than those of R. thorntont, 
and it seems unwise to make strict taxonomic statements on this 
character without knowing more about individual variation within 
the genus Rhomaleosaurus. 
L Therefore, the principal points of difference between the two 
species can be interpreted as being due to either their relative state of 
preservation, their size, or to an unreliable character, as in the case of 
‘he neck vertebrae. On the basis of the foregoing discussion, both R. 
_eramptoni and R. thorntoni are seen to belong to the same species. In 
ddition they come from approximately the same horizon, in the 
i= stage of the Liassic (Lower Jurassic) of England. 
One other similar pliosauroid is known from the Yorkshire (Eng- 
and) Toarcian, R. zetlandicus (Phillips, in Anon, 1854) (Taylor 
19920) Reconstructions of part of the skulls of R. thorntoni, R. 
-elandicus and R. cramptoni are shown for comparison (Figs 6a-c). 
The relevant differences lie in the overall size of each and in the 
\pparent width of the postorbital bar; in R. thorntoni it is relatively 
ider than in R. zetlandicus and R. cramptoni, but as all specimens 
re variously damaged in that area, no firm conclusions can be 
eached on this character. All specimens have the same short, broad 
nout, which contrasts with the more slender, relatively longer snout 
113 
Fig.6 Outline reconstructions of the anterior portion of skulls; 6a, Rhomaleosaurus cramptoni (Carte & Baily, 1863), from a photograph of the type 
NMING F8785; 6b, Rhomaleosaurus zetlandicus (Phillips, in Anon, 1854), after Taylor 1992b; 6c, Rhomaleosaurus thorntoni Andrews, 1922; scale bars 
of the Hettangian R. megacephalus (LEICS G221.1851) 
(Cruickshank 1994a). The Toarcian specimens have similar denti- 
tion, possessing sparsely ridged teeth, which also contrasts with 
those of R. megacephalus. 
Taking all three Toarcian species together (Fig. 6), it is probable 
that they represent only size variants of the same species. They are 
conspecific and should be referred to the single species 
Rhomaleosaurus zetlandicus (Phillips, in Anon, 1854), which has 
date priority. 
In Fig. 6, which compares that part of the skull preserved in 
R4853 with the other two types, it will be noted that the premaxillaries 
of R4853 are apparently narrower than those of the other two 
specimens. The reconstruction was effected using the most con- 
servative measurements, and perhaps this is reflected in a false 
narrowing of the premaxillary facial processes. It is not likely that, 
for instance, any conclusions can be drawn from such a reconstruc- 
tion concerning growth rates, or sexual dimorphism. 
SUMMARY AND CONCLUSIONS 
1 The skull of the type specimen of Rhomaleosaurus thorntoni 
Andrews, 1922, from the Toarcian of Northamptonshire, is illus- 
trated for the first time. Additional information concerning details 
of its external nares, and reassessment of other characters dis- 
cussed in the original description, make it difficult to sustain its 
supposed differences from R. cramptoni (Carte & Baily, 1863) 
from the Toarcian of Yorkshire. 
Comparisons with the type of R. zetlandicus (Phillips, in Anon, 
1854), also from the Toarcian of Yorkshire, indicate that R. 
thorntoni is merely a larger specimen of R. zetlandicus. 
3 Since all three specimens are shown here to belong to the same 
species, the correct name for it is Rhomaleosaurus zetlandicus 
(Phillips, in Anon, 1854). 
4 Rhomaleosaurus zetlandicus was the top predator in the Upper 
Lias of England. R. megacephalus from the Rhaetian or Hettangian 
(Lower Lias) has a longer, more slender snout, and different 
dentition. 
tO 
