Fig. 21. Dorsal rib, with a transverse section to show the shallow furrows on the anterior and posterior surfaces. Based on BMNH R8737
(Ophthalmosaurus). Scales equal 100 mm and 10 mm, respectively.
right rib
Fig. 22. A-C) anterior views of a pair of ribs, articulated with a vertebra, to show how the frontal shape is determined by the nature
of the articulation and the curvature of the rib. In A the left and right ribs have the same curvature but different capitular lengths. In
B the difference between left and right is due to differences in the lengths of the diapophysis and parapophysis. In C the difference
between the two sides is due to curvature. D) a pair of ribs of Ophthalmosaurus (BMNH 8737), articulated with a vertebra from the
corresponding level. Scale equals 100 mm.
the ribs trend from being bicipital anteriorly to unicipital
posteriorly. As in other vertebrates, the ribs of the dorsal
region are gently curved, contributing to the cross-sectional
shape of the body. The caudal ribs, in contrast, are
straight, and the anterior ones may have markedly expanded
distal ends. In some specimens, as in BMNH 2013,
a remarkably well-preserved, mature specimen of Ichthyosaurus
communis, the distal expansions are especially broad
(Fig. 20). These robust anterior caudal ribs, which are
about as long as the centra are high, probably provided an
extensive attachment area for the muscles that moved the
tail. The ribs diminish in size posteriorly, becoming quite
small well before the level of the tailbend.
Surprisingly, the anterior caudal ribs of Eurhinosaurus,
as exemplified by BMNH R5465, are diminutive compared
with the large size of the skeleton. They are less than half
the height of the corresponding centra, and barely as long
as the anteroposterior centrum length. These ribs also lack
a prominent distal expansion. Temnodontosaurus, as exemplified
by SMNS 15950, is somewhat similar, though the
ribs are not so diminished in size. Thus, although the
anterior caudal ribs are less than the height of the centra,
they are about one and a half times their width.
The proximal portion of each dorsal rib is anteroposte-
riorly compressed, with a median furrow on both the anterior
and posterior surfaces (Fig. 21). This gives the ribs an
hourglass-shaped cross-section. There is much variation in
the extent of this grooving and flattening. In Ophthalmosaurus,
for example, these features continue for essentially the
entire length of the rib, though somewhat less pronounced.
However, in Ichthyosaurus the features tend to be more
restricted to the proximal end of the rib, although there
appears to be a wide degree of individual variation which
does not seem to be correlated with maturity. For example,
grooving and flattening is restricted to the proximal portion
of the ribs in two fairly small specimens of Ichthyosaurus
communis (BMNH R10020 and R1162), and in two large,
obviously mature individuals of the same species (BMNH
R3372 and 2013). However, in another small individual of
I communis (BMNH R 2562), the ribs are prominently
grooved, and these appear to extend for most of the length
of the ribs. Similarly, in one specimen of Ichthyosaurus
breviceps (BMNH R216), the grooves appear to extend for
most of the rib length, whereas, in a second specimen of the
same species (BMNH R3367), the ribs, which are few and
incomplete, appear to have more restricted grooves.
It is difficult to get much of an idea of the frontal shape
of the ichthyosaurian body by looking at its ribs. The shape
of an animal’s body - whether it is barrel-chested like a
whale or narrow-chested like a herring - is determined
both by the curvature of the ribs in the vertical plane, and
in the nature of the rib articulation (Fig. 22A-C). Most
articulated ichthyosaur skeletons are laterally compressed.
Their ribs, which in life were curved in the transverse
Fig. 23. Anterior (frontal) view of the mounted skeleton of Ophthalmosaurus
(composite of BMNH R3702, R3893 and R4124).
plane, have flipped through 90 degrees so their curvature
lies in the sagittal plane. Thus preserved, the ribs give a
good idea of their curvature. However, the proximal ends
of the ribs are invariably obscured by other ribs, and by the
vertebrae overlying them, concealing the nature of the rib
articulation. Any estimate of the frontal shape of the body
in these specimens would thus be speculative. Only by
reconstructing three-dimensionally preserved material is
it possible to get a good idea of body shape, and some of
the specimens of Ophthalmosaurus housed in the Natural
History Museum, London, are suitable for this purpose.
They include BMNH 8737, which has many seemingly
undistorted ribs and vertebrae. Articulating one of the
more complete ribs with an appropriate vertebra gives a
good idea of the cross-sectional shape of the body. It is
remarkably barrel-chested (Fig. 22D), rather than laterally
compressed as might be expected for an animal so superficially
similar to a fish. But our perception of the laterally
compressed piscine body is biased by our familiarity with
common fishes like the salmon and mackerel. Other fishes,
like the swordfish (Xiphias), a member of the Scombroidei,
have remarkably broad bodies, with an almost square
frontal section.
There are very few three-dimensionally mounted skeletons
of ichthyosaurs in existence, and the exemplar of
Ophthalmosaurus on display in the Natural History Museum,
London is probably the most lifelike. The specimen -
a composite of three individuals (R3702, R3893, and R4124;
Andrews 1910) - appears undistorted, and the relationships
between the ribs and vertebrae look authentic. The
skeleton has a broad frontal profile (Fig. 23), though not
quite so markedly inflated as in the articulated ribs (Fig.
22D). Such a broad cross-section may have been typical of
post-Triassic taxa.