254 Evolution? The Fossils Say YES!
support a vertical tail fin. Another specimen, Grippia from the Early Triassic of Spitsbergen, is
known primarily from the skull, but it shows a relatively short snout, small eyes, and simple
knob-like teeth for crushing mollusks, not the spiky teeth of most fish-eating ichthyosaurs.
Yet another Early Triassic form from China, Chaohusaurus (fig. 11.7A and B) also has a short
snout, simple teeth, primitive vertebrae, and robust limbs that are beginning to form a pad-
dle but still have discrete rows of finger bones with the normal count (not the extra bones of
an advanced ichthyosaur paddle—see fig. 11.7A). Cymbospondylus from the Middle Triassic
of Nevada still retains the primitive hand and foot structure as well and has a relatively short
snout with small eyes, and the tail is beginning to show the downward bend that indicates
the presence of a small tail fin.
The best known and best preserved of the early ichthyosaurs is Mixosaurus from the
Middle Triassic of Germany (fig. 11.7B) and many other places. The body has the classic ich-
thyosaur shape, with the long snout, large eyes, and dorsal fin. The hands and feet are begin-
ning to form flippers, although they have still not multiplied the finger and toe bones as in
later ichthyosaurs. And the tail shows just a slight downward bend, with some specimens
preserving the body outline and showing that it had a small upper lobe on its tail. Thus it
is advanced in many features but still retains the primitive hands and feet and does not yet
have the fully bilobed ichthyosaurian tail.
If the ichthyosaurs seem very highly specialized, the plesiosaurs are specialized in a
different direction. All of the advanced forms had stout bodies with robust shoulder and
hip bones and four large well-developed paddles (fig. 11.8). Some plesiosaurs (especially
the elasmosaurs) had long serpentine necks and small heads, while another group (the plio-
saurs) had long heads and snouts and much shorter, more robust necks. Instead of employ-
ing speedy dolphin-like swimming like ichthyosaurs, plesiosaurs were apparently adapted
for slow steady swimming by rowing with their fins (like a sea turtle does) and used their
long heads and necks to snap at prey that came within reach.
How could such peculiar “kinds” evolve? We have an even better series of intermedi-
ates for plesiosaurs than we do for ichthyosaurs. They start with the Late Permian fossil
Claudiosaurus from Madagascar (fig. 11.8A). It is so primitive that it may actually be the
first known euryapsid, and a sister group to both the plesiosaurs and the ichthyosaurs.
However, it shows the condition from which these euryapsids evolved. In most features,
it looks just like many other primitive reptiles of the Permian, except that the skull has the
characteristic holes, features of the palate, and loss of the temporal bar that earmark it as
a euryapsid. In addition, it seems to show the beginning of aquatic adaptations with the
loss of the breastbone. This enables it to swim with both limbs moving at the same time
in a swimming stroke, not alternating like the gait of a lizard. It also has relatively long
limbs and especially long toes, indicative of webbed feet. In fact, its limb proportions closely
resemble those of modern aquatic lizards, such as the Galapagos marine iguana. Finally, a
large part of the skeleton was reduced to cartilage, another indication of an aquatic lifestyle,
since it reduces the weight of bone in a body supported by water and not needing such a
robust bony skeleton.
From Claudiosaurus we next see a group of Triassic marine reptiles known as nothosaurs
(fig. 11.8B). These animals had skulls and bodies that were not noticeably different from
primitive euryapsids like Claudiosaurus. The biggest difference occurs in the neck, which is
much longer and anticipates the long necks of many plesiosaurs. The limbs are not much
more specialized for aquatic locomotion than those of its primitive relatives, but they have