Onto the Land and Back to the Sea: The Amniotes 251Evolution of the Great Sea Dragons
There were no real sea serpents in the Mesozoic Era, but the plesiosaurs were the next
thing to it. The plesiosaurs were reptiles who had gone back to the water because
it seemed like a good idea at the time. As they knew little or nothing about swim-
ming, they rowed themselves around in the water with their four paddles, instead of
using their tails for propulsion like the brighter marine animals. (Such as the ichthyo-
saurs, who used their paddles for balancing and steering. The plesiosaurs did every-
thing wrong). This made them too slow to catch fish, so they kept adding vertebrae to
their necks until their necks were longer than all the rest of their body. . . . There was
nobody to scare except fish, and that was hardly worthwhile. Their heart was not in
their work. As they were made so poorly, plesiosaurs had little fun. They had to go
ashore to lay their eggs and that sort of thing. (The ichthyosaurs stayed right in the
water and gave birth to living young. It can be done if you know how.)
—Will Cuppy, How to Become ExtinctDuring the “age of the dinosaurs” or the Mesozoic Era, the seas teemed with many different
types of life: enormous numbers of plankton, gigantic clams on the seafloor, as well as weird
oysters like Gryphaea (fig. 8.10), many types of sea urchins and heart urchins (fig. 8.9), squid-
like belemnites and ammonites, a great diversity of “holostean” and eventually teleost fish
(see fig. 9.12), but the dominant predators of all these creatures were marine reptiles. These
include not only huge sea turtles up to 7 meters (22 feet) long and crocodiles known as geo-
saurs, which had webbed feet and a tail fin, but also three major groups that were unique to
the Mesozoic seas: the dolphin-like ichthyosaurs, the long-necked paddling plesiosaurs, and
the seagoing Komodo dragons known as mosasaurs.
In marine beds over many parts of the world, we have excellent, often complete skel-
etons of these creatures. In the western Great Plains (especially South Dakota, Colorado, and
Kansas), there are extensive exposures of marine sediments that were deposited in the Creta-
ceous when a great inland sea covered the Plains region from the Gulf of Mexico to Hudson’s
Bay. Each of these creatures is so distinctive that it would clearly be a “created kind” to a
creationist. And yet we have excellent evidence of the origins and relationships of all three
groups both from transitional fossils and from the cladistic analysis of their relationships.
If they were not extinct, we might check their molecular phylogeny as well.
The most amazing thing about all three groups is that they were clearly reptiles, so they
were descended from terrestrial creatures that developed a land egg, yet these three all inde-
pendently returned to the oceans (as did crocodilians, sea turtles, sea snakes, seals and sea
lions, and whales, of course). Apparently, the food resources were so great in the oceans that
land-dwelling reptiles found their way to reap this bountiful harvest in at least five different
groups. This seems amazing in itself, yet the fact that it has happened many times shows
how powerful the selection forces for this lifestyle must be. Such a radical change in ecology
usually caused much convergence in body form as well, so we can see how ichthyosaurs and
whales have independently evolved the streamlined torpedo-like shape that is also found in
fish. Returning to the ocean makes certain reproductive and physiological demands, in addi-
tion to streamlining the body for swimming and modifying the hands and feet into flippers.
For example, marine reptiles must still reproduce somehow. We know that sea turtles and
saltwater crocodilians crawl out on land and lay eggs in a nest, and presumably mosasaurs