252 Evolution? The Fossils Say YES!
and plesiosaurs could have done so too. But ichthyosaurs are so dolphin-like in body form
that they could not have wriggled onto a beach and dug a nest with their flippers. We know
that whales and dolphins give live birth, expelling the young from the womb and raising it
up to take its first breath, after which it can swim on its own. Apparently, ichthyosaurs could
also, since there are several remarkable specimens from the Jurassic Holzmaden shales in
Germany that appear to have been in the process of giving birth to live young when they
died and were fossilized (fig. 11.6).
Let us focus on the ichthyosaurs first. They represent the greatest challenge because
they are the most highly modified and specialized for marine life. They have highly fishlike
streamlined bodies with long toothy snouts for catching fish and squid, huge eyes for seeing
in dark murky waters, a well-developed dorsal fin, and both the hands and feet are fully
modified into flippers. Finally, their tails also have a vertically oriented tail fin. In contrast to
the tailfin of most fish, the supporting spinal column of ichthyosaurs flexes downward into
the lower lobe of the fin, not upward as in sharks and other primitive fish. A detailed analysis
of the skeletal characters of the group shows they belong to the Euryapsida, along with the
plesiosaurs, which we’ll discuss next (fig. 11.1). The Euryapsida, in turn, are the sister group
of the rest of the reptiles.
A number of striking intermediate forms are known from the early Mesozoic (fig. 11.7).
First, there is Nanchangosaurus from the Triassic of China. Although it has a slightly stream-
lined body and a long (but toothless) snout like an ichthyosaur, all of the rest of the features
of the skeleton are primitive, including the vertebrae; the limbs, which are not modified into
flippers but have normal proportions with all the regular wrist and ankle and toe bones; and
a long straight tail with no sign of a tail fin. The original authors were not sure where to put
this fossil because it is so primitive, but based on the skull, it seems to be an aquatic lizard on
the way to becoming an ichthyosaur.
The oldest known fossil that can definitely be called an ichthyosaur is Utatsusaurus from
the Early Triassic of Japan (fig. 11.7B). Although it has the general body form of an ichthyo-
saur, it has a mosaic of primitive features found in its reptilian ancestors. These include a
skull with only a short snout and unspecialized teeth, very primitive vertebrae (especially
in the neck), hands and feet that are not yet highly modified into flippers but still have dis-
crete fingers and toes, and a long straight tail with no evidence of the downward flexion to
FIGURE 11.6. The famous specimen of a female ichthyosaur giving birth to a live baby, getting caught in the act,
and then being fossilized in the Jurassic Holzmaden Shale of Germany. (Image courtesy of the State Museum
of Natural History, Stuttgart)
Baby