The Moeritherium was a marsh-animal, and may very well have been
cousin to the branch of the family which pushed on to the seas,
and developed its fore limbs into paddles.
The Rodents are represented in primitive form early in the Eocene
period. The teeth are just beginning to show the characteristic
modification for gnawing. A large branch of the family, the
Tillodonts, attained some importance a little later. They are
described as combining the head and claws of a bear with the
teeth of a rodent and the general characters of an ungulate. In
the Oligocene we find primitive squirrels, beavers, rabbits, and
mice. The Insectivores also developed some of the present types
at an early date, and have since proved so unprogressive that
some regard them as the stock from which all the placental
mammals have arisen.
The Cetacea (whales, porpoises, etc.) are already represented in
the Eocene by a primitive whale-like animal (Zeuglodon) of
unknown origin. Some specimens of it are seventy feet in length.
It has large teeth, sometimes six inches long, and is clearly a
terrestrial mammal that has returned to the waters. Some forms
even of the modern whale develop rudimentary teeth, and in all
forms the bony structure of the fore limbs and degenerate relic
of a pelvis and back limbs plainly tell of the terrestrial
origin.
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