CRANIAL ANATOMY OF RHOMALEOSAURUS THORNTONI ANDREWS 
fig. 2 Rhomaleosaurus thorntoni Andrews; ventral view of the skull; 
| scale bar = 100 mm. 
povered in a radiating set of shallow grooves, suggesting that they 
ielped anchor the buccal lining. 
These features of the internal narial region are different from 
those of Pliosaurus brachyspondylus and R. megacephalus, where 
the structure is more fully known (Cruickshank et al 1991; Taylor & 
Sruickshank 1993b). In P. brachyspondylus, the internal nares lie at 
the end of grooves in the roof of the mouth, with two prominent 
“oramina lying on the medial faces of the depression in front of each 
ternal naris, all equally spaced. This is markedly different from R. 
negacephalus, where the internal nares lie at the ends of grooves, 
medial to, and parallel with, supplementary grooves which end in 
oramina. In neither of these species is there a fan-shaped area 
nedial to the diastema, nor the extra foramina lying within the limits 
f the internal narial excavation, as illustrated here. As in all 
plesiosaurs which I have examined, the internal nares lie anterior to 
external nares, inviting the explanation that the narial system 
cted as a hydrodynamically driven olfactory system, and was not 
sed for respiration (Cruickshank ef al 1991). The internal nares in 
. zetlandicus and R. cramptoni are not visible, being obscured by 
he rami of the lower jaw (Taylor 1992b), or matrix. 
| The badly disrupted posterior palatal elements show that there 
as a prominent pterygoid boss in exactly the same position as in R. 
= (Taylor 1992a; b), and an ectopterygoid lying between 
e jugal and pterygoid. 
Mandible (Figs 3, 4). Parts of the lower jaw preserved include an 
Imost complete right ramus as far back as the end of the dentary, the 
ymphysis and the left ramus to just behind the symphysis, plus a 
=m 
111 
Fig. 3 Rhomaleosaurus thorntoni Andrews; lower jaw; 3a, dorsal view; 
3b, section through symphysis on line a—b; scale bar = 100 mm. 
portion of the middle of the left ramus and the left articular region 
(not illustrated). These portions of the lower jaw correspond almost 
exactly to the remains of the skull and no doubt represent what was 
saved during collection, from what must have been an almost 
complete cranium. 
The symphysis occupies five tooth positions and a further 26 tooth 
positions can be counted in the right dentary. The general obscurity 
of sutures makes it difficult to identify individual bones, but as far as 
can be seen, the structure of this lower jaw is the same as that of R. 
zetlandicus (Taylor 1992b). There are both mature and replacement 
teeth present in the lower jaw, with their associated primary and 
secondary alveoli. 
On the portion of the left jaw ramus containing the glenoid fossa, 
there is a large dorso-median trough on the prearticular and articular 
(Taylor 1992b: fig 7; Cruickshank 1994a: fig 7), which may be one 
of the determining characters of the genus Rhomaleosaurus (Taylor 
1992a; Cruickshank 1994a). 
