The first fish appeared about 460 million years ago. They had no jaws. They fed on tiny animals that lay hidden in the mud with mouths that were simple openings.
Gradually, fish developed jaws from the bony supports around their gills, called gill arches. Jaws allowed the fish to eat more efficiently, and to feed on more type of food. Such fish are called Placoderms. Present day fish, including sharks, have evolved from the placoderms.
Ancient marine reptiles came in many shapes and sizes. The Plesiosaurs had stocky, barrel shaped bodies, short tails, and paddle-like limbs. Some had long, slender necks, while other had short, squat one.
The Pliosaurs had shorter necks, massive skulls, and enormous teeth. The Umoonasurus
stood out from other Plesiosaurs because of a serious of high, thin crests on its head and numerous fused vertebrae at the end of its tail.
