In the seas,
marine crocodiles were still common and lived as far
north as Maryland. Many species of marine mammals also inhabited the
seas, including baleen and toothed whales, seals,
sea lions, walruses, and sea
cows. Both toothed and baleen whales were particularly diverse
during this period. One odd marine mammal that had appeared in the
Oligocene and was common in the North Pacific Ocean during the Miocene
was called Desmostylus. It had four legs
that were well adapted both for living on land and for swimming, and
its teeth were shaped like bundles of cylinders. Distant relatives
of elephants and sea cows, Desmostylus and its kin were entirely
extinct by the end of the Miocene.
Some of these marine mammals were undoubtedly preyed upon by the extinct
giant-toothed white shark, Carcharodon megalodon.
This giant shark is well-known from fossils along the east coast of
North America. In fact, by this time most types of living sharks
were present in all the world's seas. Many of the marine invertebrates
of the Miocene were similar to those present today-echinoderms, snails,
bivalves, crustaceans, and cephalopods were common. Marine animals
of the ancient Paleozoic Fauna, such as brachiopods
and crinoids, were rare. The first kelp forests are
known from this time.