Life in the Eocene
One of the most remarkable aspects
of the Eocene marine fossil record is the presence of the first whales.
These earliest fossil whales, from rocks in Pakistan and India, suggest
that whales are closely related to the even-toed ungulates (artiodactyls).
Both artiodactyls and early whales have an unusual feature in that
one of the ankle bones (the astragalus) is shaped like a double pulley.
Archaic whales such as Ambulocetus and Pakicetus
demonstrate the transition from a terrestrial to an aquatic way of
life. Later in the Eocene, whales and sea cows (sirenians) adapted
more completely to ocean life and spread worldwide. One type of archaic
whale (Basilosaurus) achieved lengths of 60 feet.
Among the cartilaginous fishes, modern forms such as requiem sharks
increased, while sand tiger sharks decreased. By the Middle Eocene,
the giant sand tiger shark Otodus obliquus was extinct. Mako
and giant-toothed white sharks first appeared at this time as well.
However, bony fishes continued to dominate the seas,
as they do today. Marine invertebrates were also more modern. Paleozoic
Fauna animals such as brachiopods were uncommon, while
cephalopods, echinoderms, snails, and bivalves thrived.