ventral process
fibula
fibulare
intermedium
tibiale
femur
tibia
fibula
fibulare
intermedium
tibiale
Fig. 67. Hindfins of A) Ichthyosaurus communis (BMNH R33721) and B) Ophthalmosaurus icenicus (from Andrews [1910: fig. 41]). A) is
depicted as a left fin and B) as a right fin; both are shown in ventral views. Scales each equal 50 mm.
hindfins smaller, but they also tend to have fewer digits as
well as fewer elements in each digit (Fig. 67). In the large
and long-bodied Eurhinosaurus and Temnodontosaurus, the
disparity between fore and hindfins is much less pronounced.
Anatomical Diversification
Ichthyopterygian anatomy varied more extensively among
Triassic forms than in post-Triassic ones. This is not surprising
considering that the post-Triassic forms, all belonging
to the clade Parvipelvia, shared a single basic body
plan, which is only one of many that evolved during the
Triassic. The diversification of body plans, and of associated
anatomical details, was completed within the first 35
million years of the ichthyopterygian evolution, which
spanned 155 million years in total. Four basic body plans
may be recognized for convenience, and these are tentatively
referred to as basal, stem, mixosaurian, and •parvvpelvi-
an. There are, of course, intermediates between these
plans, and the first two represent a paraphyletic grouping
along the stem leading to the Parvipelvia (see the chapter
on Phylogeny). However, these names are useful for reviewing
character transformations, as long as the aforementioned
restrictions are acknowledged.
Ichthyosaurs appeared in the Early Triassic (Oleneki-
Overview of Four Body
Basal plan
(Fig. 68A-B)
Nearly complete skeletons representing the basal plan are
known for two Early Triassic ichthyosaurs, namely Chao-
husaurus and Utatsusaurus. Thèse ichthyosaurs were not
very much longer than 2.m in the largest adults, and
sometimes less than 1 m. They had a relatively small skull
compared to the body, which was slender. The limbs were
already fin-shaped, but retained elongated zeugopodials
and round carpals. The trunk vertebral centra were cylindrical,
with a height/length ratio of about 0.9 to 1.2 near
the pelvic girdle, and there were about 40 presacral vertebrae.
The caudal fin had a small dorsal lobe supported by
the anticlination of the neural spines (Motani et al., 1996;
Nicholls & Manabe, 1999), located near the peak of the
curved vertebral column (caudal peak). The pelvic girdle is
small relative to the body, but large compared to that in the
other body plans.
an) with the basal body plan, as exemplified by Utatsusaurus,
which quickly disappeared before the Middle Triassic.
The stem plan evolved by the end of the Olenekian and
coexisted with the basal plan at least for a short duration,
as seen in the Lower Saurian Level of Spitsbergen (i.e.,
coexistence of Isfjordosaurus and Besanosaurus-like forms).
This plan disappeared in the Norian (Late Triassic): an
undescribed Shonisaurus-kke form (ROM 44661) from the
middle Norian of Williston Lake is probably its last representative.
The mixosaurian plan is only known for the
clade Mixosauria, and thus restricted to the Middle Triassic.
The parvipelvian plan appeared in the late Carnian in
some precursors (e.g., Californosaurus), and became established
by the middle Norian (e.g., Macgozvania). This plan
survived for about 135 million years until the early Late
Cretaceous, and may be called the typical body plan for
ichthyosaurs. For this reason, this body plan was described
in detail in the previous chapter.
Plans and Intermediates
Stem plan
(Fig. 68C)
The stem plan is the least well known of the four. Because
of the paraphyletic nature of the ichthyopterygian stem-
group, a range of body designs is included under this
category. Ichthyosaurs with this body plan are usually
large, sometimes exceeding 10 m in length, but there were
smaller species that probably did not exceed 3 m. The skull
was relatively small as in the basal plan, and the body was
slender, partly because of the high presacral count of about
60 (+5), or more. The vertebral centra were mostly discoi-
dal, with height/length ratios of about 2.0 near the pelvic
girdle (3.0 in Shonisaurus). The limbs were fin-shaped, with
short zeugopodials lacking a complete shaft. The carpals
are indistinguishable in shape from metacarpals, which
lack shafts. The caudal fin is poorly known, but Besanosau-
rus seems to have lacked a true tailbend: the sudden narrowing
of the caudal centra, a typical indicator of a tail-
bend, appears to be absent (Dal Sasso & Pinna, 1996:
fig. 8).Cymbospondylus shared many general features of the
stem body plan, possibly because of a similarity in locomotor
style, but its anatomical details differed extensively
from later forms, such as Shastasaurus. For example, the
metacarpals seem to have retained shafts in Cymbospondy-
lus (Sander, 1989).