nasal
w
\ dentary
spenial
maxilla premaxilla
Fig. 24. i. breviceps (based on BMNH 39263), a small, short-snouted species. The detail depicts a transverse section through the snout,
taken from Sollas (1916: fig. 2). Scale equals 100 mm.
Skull
With a few notable exceptions, which will be discussed
later, the skulls of post-Triassic ichthyosaurs are all very
similar, one species differing from another largely in cranial
proportions. However, many early workers, including
Conybeare (1822), attempted to differentiate between species
using discrete characters, primarily of the teeth. But, as
discussed below, dental features are unreliable. It is the
scarcity of discrete, meristic characters that has frustrated
attempts to give a phylogeny for the diverse assemblage of
Jurassic species.
Detailed accounts of the cranial anatomy of Jurassic
ichthyosaurs are available in the literature, and the present
treatment is intended only as an overview. For more detailed
information the reader is directed to two sources:
Sollas (1916) and McGowan (1973B). Sollas’s singular
study was made by serially sectioning a three-dimension-
ally preserved skull of Ichthyosaurus communis, and reconstructing
its structure from wax slices, each slice being
constructed by reference to the serial images. McGowan’s
study was largely based upon some exquisitely preserved
and uncrushed partial skulls, which were extracted from
limestone nodules with acetic acid.
Although some cranial sutures, like those of the maxilla,
are plain to see, others are difficult or impossible to
discern. Consequently, many of the sutures in the figures
that follow have been drawn by reference to the two studies
mentioned above.
Skull Roof
The skull, even of short-snouted species like Ichthyosaurus
breviceps (Fig. 24), has an extensive rostrum, formed largely
by the premaxilla. This rostral segment, referred to in descriptive
treatments as the snout, supports a single row of
marginal teeth, which lie in a dental groove. The dentigerous
margin is formed by the premaxilla and maxilla, their
respective contributions varying from taxon to taxon. In
Ichthyosaurus, for example, the maxilla is a fairly small
splint of bone that has only a minor contribution (Fig. 24).
The maxilla is more prominent in Temnodontosaurus, forming
about half of the dental margin (Fig. 25). The external
naris, usually approximating to an ellipsoid in shape, lies
well back, just in front of the orbit. It is bounded dorsally
and ventrally by the premaxilla anteriorly, and by the nasal
and lacrimal posteriorly (Fig. 26A). In contrast to certain
other aquatic vertebrates, like cetaceans and crocodylians,
there was no tendency for the nares to migrate towards the
top of the skull. Indeed, the nares were placed more dorsally
in basal ichthyosaurs than in later ones. Most of the
dorsal aspect of the snout is formed by the union of the two
premaxillae, but they diverge posteriorly, the triangular
gap being filled by the paired nasals (Fig. 26A). The nasals
extend well toward the tip of the snout, concealed beneath
the overlapping premaxillae (Fig. 26B). Such extensive
overlapping and underlapping of the dermal elements of
the skull and mandible is characteristic of ichthyosaurs,
and is clearly illustrated by Sollas’s (1916) serial sections.
The paired nasals come together in midline in a butt joint.
maxilla
Fig. 25. Temnodontosaurus platyodon (figured in McGowan [1994A]) a large, fairly long-snouted species. Scale equals 100 mm.
Fig. 26. Skull roof of Ichthyosaurus, redrawn from Sollas (1916: figs. 5-6). A ) dorsal view. B) ventral view, to show the extensive
overlapping and underlapping. Note how the nasals overlap the frontals, being extensively overlapped themselves by the premaxillae.
Scale equals 100 mm.
Their contact edges often lie ventral to their lateral edges,
so a shallow depression is formed between them. There is
also often a loss of contact, or even a slight emargination,
along the medial edges of the nasals over a short distance,
thereby forming a narrow intemasal foramen. However,
the presence of this foramen is difficult to establish because
this part of the skull is usually not exposed. Thus it cannot
be seen in any of the laterally compressed individuals,