achieved in cetaceans by an insulatory lay er o f soft tissue
between the tw o otic capsules. Directional hearing ability
is thus negated, a t least for Ichthyosaurus, by the existence
o f direc t bone-to-bone contac t between the two otic capsules
(McGowan, 1973B).
The earliest known ichthyosaurs were among the smallest,
and Chaohusaurus chaoxianensis, from the Early Triassic,
was well under 1 m in length (Motani et al.( 1996). Mixo-
saurus comalianus, from the Middle Triassic, was of similar
size. Many others, including Ichthyosaurus and Stenoptery-
gius, were dolphin-sized. Shonisaurus, from the Late Triassic,
is the largest of all ichthyosaurs, reaching lengths of up
to 15 m (McGowan & Motani, 1999), and possibly more.
There is also a similar-sized but unnamed taxon from the
Early Jurassic (McGowan, 1996A). These giants were of the
same size order as extant baleen whales, but there is no
evidence that ichthyosaurs ever exploited the rich food
resources offered by plankton. Collin & Janis (1997) made
a persuasive argument that this is because of the constraint
of their soft anatomy that prevented them from forming a
water-tight seal at the back of the throat. This would have
made it impossible for them to force water through a
straining device without also forcing it down the throat.
Ichthyosaurs, especially post-Triassic forms, were highly
modified for life in the marine environment. As a consequence
their skeletal structure tends to be conservative.
Different taxa therefore look much alike, often making it
difficult to find diagnostic features for distinguishing between
them. Furthermore, although their skeletons are
often well preserved, some features are frequently obscured
because of the extensive compression that most
skeletons have undergone during fossilization. For example,
the ribs, which are well developed and numerous,
almost always overlie the vertebral column, making it
difficult, if not impossible, to see details of the centra. The
rib heads themselves are also usually obscured, either by
other ribs, or by the vertebral column. The condition of the
rib articulation - bicipital or unicipital - has been used in
the past as an important taxonomic feature, probably without
justification. However, because of these preservational
problems, the feature can seldom be assessed. Cranial sutures
are also often difficult to discern, hampering interpretations
of cranial structure. For all these reasons, the
anatomical distinctions between one taxon and another are
not always well defined. This is why relative proportions,
as expressed by ratios, are so frequently used for distinguishing
between species.
Given their long paleontological history, and the abundance
of material, it is not surprising that a large number
of species have been described. To date, 235 Species of
ichthyosaurs, or purported ichthyosaurs, have been formally
named in the literature. Of these, only 80 are here
considered valid, 15 of which are species inquirendae. The
remainder comprises synonyms, nomina dubia, nomina
nuda, and misidentifications. Furthermore, 76 genera have
been proposed, of which only 36 are considered valid here.
As three of these .are represented by. monotypic species
inquirendae, the count of valid genera will further decrease
if any are shown to be invalid.
Anatomy of a Typical Ichthyosaur
The following account of a “typical” ichthyosaur is based
upon the Early Jurassic genus Ichthyosaurus, except where
stated otherwise.
Vertebral Column
The vertebral column is often preserved in its entirety in
Early Jurassic ichthyosaurs, as exemplified by articulated
skeletons from Lyme Regis and from Holzmaden. However,
because of the way that most of these skeletons are
preserved, the anatomical details of their vertebrae are
often obscured. Thus ribs frequently overlay the presacral
vertebrae, obscuring all but the neural spines (Fig. 1A).
And even when unobstructed by ribs, the vertebrae are
usually preserved predominantly from the lateral aspect.
This is because their centra lie on edge, in continuity, like
a row of stacked coins. Sometimes, however, centra became
toppled during preservation, allowing them to be
seen in anterior or in posterior view, at least obliquely (Fig.
IB).
The neural arch rarely fuses with the centrum in ichthyosaurs.
As the neural arch and spine is far less robust
than the centrum, it is frequently poorly preserved, or lost,
in disarticulated material. The vertebral centrum, in contrast,
is one of the most robust parts of the skeleton. Consequently,
isolated centra are common elements of the
vertebrate assemblages of Early Jurassic marine deposits.
They are frequently well preserved, yielding additional
data on vertebral structure. However, since the taxonomic
identity of isolated centra is often in doubt, and since their
relative position in the vertebral column can only be estimated,
isolated finds are of limited value.
Ophthalmosaurus is typically represented by material
that has been completely freed from matrix, and several
skeletons are available with a complete or near complete
vertebral series. These specimens are a valuable source of
three-dimensional information. However, the relationships
between individual elements are generally wanting,
so it is usually not possible to determine the precise order
of the vertebrae throughout the series. Nevertheless, these
specimens have been illuminating.
A particularly valuable source of information is provided
by several partial skeletons from the Lower Lias of
Dorset, that were acid-prepared from undistorted calcareous
nodules. Although most of them are too incomplete to
be identified to species with any certainty, they are proba-
F*§- B A) Part of the pre-sacral vertebral column, showing how the ribs frequently overlay the centra, largely obscuring them - notice
that they are exposed from the lateral aspect (neural spines not shown). B) sometimes the vertebral centra are displaced, revealing more
than their lateral aspect.