Fig.4 Rhomaleosaurus thorntoni Andrews; symphysis of lower jaw; 4a, 
ventral view; 4b, section through symphysis on line cd; scale bar = 100 
mm. 
Fig.5 Rhomaleosaurus thorntoni Andrews; teeth; 5a, replacement tooth 
crown, position 9, right maxilla; 5b, replacement tooth crown, position 
24, right dentary; 5e, mature tooth, position 12, right dentary; 5d, 
mature tooth, position 8, right dentary; teeth are oriented with crowns 
towards top; drawn with an Abbé drawing apparatus on a Wild M3 
stereomicroscope; scale bar = 5 mm. 
A.R.I. CRUICKSHANK 
Dentition (Fig. 5). The dentition is that of a powerful predator, 
with a rosette of interlocking, procumbent teeth in the premaxillae 
and lower jaw symphysis, followed by tooth-rows which, after two 
small median teeth, have large caniniforms in the upper jaw overlap- | 
ping somewhat smaller teeth in the lower jaw. The tooth adjacent to 
the midline in both the upper and lower dentitions is much smaller | 
than the more mesial teeth. In the lower jaw there is a marked } 
reduction in size of teeth immediately behind the fifth position, | 
which continues in a regular manner to the end of the tooth row on | 
the dentary. In the upper jaw the fifth tooth position is very small, 
and is followed behind the diastema by another small tooth. Tooth | 
positions seven, eight, nine and ten are very much larger caniniforms. | 
Thereafter there is an even more marked reduction in tooth size, | 
when compared with the lower dentition, until the sockets become 
difficult to distinguish. This arangement is very similar to that of R. | 
zetlandicus (Taylor 19925), allowing for the incompleteness of that |) 
specimen. 
It is possible to amplify the description of the individual teeth 
offered by Taylor (1992b), for R. zetlandicus. Those illustrated come | 
from the 9th position on the right maxilla, showing the buccal | 
surface (Fig. 5a); lying across the root of the 23rd tooth on the right |) 
ramus of the lower jaw (Fig. 5b); the 12th position of the right ramus 
of the lower jaw (Fig. 5c); and the 8th position of the right ramus of 
the lower jaw (Fig. 5d). Figs 5a and 5b are replacement teeth, 
whereas Figs 5c and 5d are erupted, mature teeth. 
The crowns are covered in a coarse ornament, which reduces in | 
number of ridges towards the tooth-tip, but which all seem to have 
carinae on mesial and distal surfaces. The ornament on these teeth is 
identical with those illustrated by Taylor (1992b: fig. 9), but quite 
different from the tooth illustrated by Cruickshank (1994a: fig. 10) 
for R. megacephalus, where the ornament is much finer and more - 
closely spaced. The ridges are triangular in section, and some start 
slightly below the crown-root boundary. 
DISCUSSION 
Andrews (1922: 413) compared R. thorntoni with R. cramptont, 
regretting that the shoulder girdle of the latter was not visible and |; 
that he could not therefore use it for comparative taxonomic pur‘ 
poses. The skull and vertebral column of each species seemed to be 
much the same, but he drew attention to the following differences 
between them. Firstly, he thought that the external nasal openings 
were much further in front of the eyes in R. thorntoni than in R. 
cramptoni. Secondly, he recognized differences in the platforms of 
their cervical neural arches: in R. thorntoni these are nearly horizon- | 
tal, but in R. cramptoni they are strongly inclined. Thirdly, he}’. 
pointed out that the humerus in R. thorntoni was relatively larger. 
with a more expanded distal end. 
Neither of the external nasal openings are very obvious in R. 
thorntoni: that on the right side is obscured by the displaced facial 
processes of the premaxillae, and that on the left is only partly)}) 
preserved and probably invisible before the skull was recently 
cleaned properly. However, there is the depression some distance in 
front of the right orbit which could have been mistaken for af 
external naris prior to full cleaning of the specimen, and this woul 
agree with Andrews’ identification of an unusually anteriorly placeq 
external nasal opening. This depression is floored with crushec 
bone, and does not penetrate onto the underside of the dermal bone: 
of the snout. Restoration of the snout region (Fig. 6c) using informa 
tion now available, shows the external nares to be situated in i 
normal position relative to the orbits. 
- ge wF 
