Fig. 30. A) complete scleral ring, lateral view. B) a single scleral
plate, dorsal view. Based upon BMNH R8177 (Ichthyosaurus sp.).
Scale equals 20 mm.
margin of the postorbital is divided into two flanges of
bone that embrace the leading edge of the quadratojugal.
The quadratojugal is a fan-shaped element that lies immediately
anterior to the quadrate. After narrowing ventrally,
it expands, forming an elliptical hollow that locates upon
a swelling for its reception on the quadrate, immediately
above the articular surface for the mandible.
Although the shape of the orbit is subject to alteration
by post-mortem compression, it appears to be oval rather
than round, the major axis lying parallel with the longitudinal
axis of the skull. It is probably also somewhat narrower
anteriorly. Ichthyosaurs have a well-ossified scleral
ring, composed of a variable number of individual plates
or ossicles, usually numbering between 12 and 16 (Fig. 30),
though the number may have been as high as 19 in Mixo-
saurus. The ring has a three-dimensional configuration,
resembling a frisbee, with a large central aperture. Each
thin plate comprises a tapering lateral surface, the edge of
which contributes to the aperture. This lateral surface is set
off at something less than a right angle from a peripheral
surface that contributes to the lateral margin of the entire
ring. The internal surface of the scleral plate, which faced
towards the eye, is relatively flat and featureless, but the
exterior surfaces appear to be covered by a thin bony layer,
sculptured with fine striations. Adjacent plates meet in a
partly overlapping, partly abutting union.
süpratemporal
opisthotic
stapes
basioccipital
basisphenoid
Fig. 31. Posterior view of the skull, based largely upon BMNH R6697 (Ichthyosaurus sp.).
pterygoid
Scale equals 50 mm.
exoccipital
parietal postfrontal
quadrate
pterygoid
Basicranium and Otic Capsule anterior semicircular canal
The basicranium is formed by the basioccipital and basisphenoid,
the paired exoccipitals, opisthotics, and proot-
ics, and the unpaired supraoccipital. The stapes, a robust
element, is firmly braced between the basioccipital medially
and the quadrate laterally (Fig. 31). As there are discrete
articular facets between the basioccipital and basisphenoid,
as well as between the basioccipital, exoccipitals,
and supraoccipital, the relationships between these
elements are well established. There is also a fairly good fit
between the opisthotic and the basioccipital and supra-
temporal. However, there is no bony contact between the
opisthotic and prootic, the otic capsule being largely cartilaginous.
McGowan (1973B) attempted to resolve the
problem by relating the impressions of the membranous
Fig. 32. Hypothetical model of a membranous labyrinth of Ichthyosaurus
sp. (BMNH R1168), based upon latex casts taken from
the prootic and opisthotic impressions of this structure. Left lateral
view. Scale equals 20 mm.
supraoccipital
hypothetical
membranous
labyrinth
prootic
basisphenoid
exoccipital cartilaginous
opisthotic
basioccipital
hypothetical
otic capsule
Fig. 3 J ; Lateral view of occiput of lchthymtfrp sp. (BMNH 116697). A) stowing hypothetical membranous labyrinth, and B) showing
a reconstruction of the cartilaginous otic capsule. Scale equals 40 mm.
labyrinth, which are well preserved in each element, to a
generalized model of the same. This was facilitated by
making latex casts of the impressions and melding these
into a wax model of a generalized membranous labyrinth
(Figs. 3 2,33A). From this model it was possible to obtain a
relationship for the prootic, and an approximation of the
extent of the cartilaginous otic capsule (Fig. 33B).
The basioccipital, being an especially robust element, is
commonly found in the fossil record, and is often the only
evidence of ichthyosaurs at a given locality. It is therefore
fortuitous that it possesses some taxonomically diagnostic
features. As the basioccipital is known for very few taxa,
this has limited applicability at present, but it has been
useful, as will be seen later. The essential features of the