Eocene

[ee-uh-seen] /ˈi əˌsin/ Geology

adjective
1.
noting or pertaining to an epoch of the Tertiary Period, occurring from 55 to 40 million years ago and characterized by the advent of the modern mammalian orders.
noun
2.
the Eocene Epoch or Series.
/ˈiːəʊˌsiːn/
adjective
1.
of, denoting, or formed in the second epoch of the Tertiary period, which lasted for 20 000 000 years, during which hooved mammals appeared
noun
2.
the Eocene, the Eocene epoch or rock series
adj.

in reference to the second epoch of the Tertiary Period, coined in English 1831, from eo- + Greek kainos “new” (see recent); along with Miocene and Pliocene, by the Rev. William Whewell (1794-1866), English polymath.
Eocene
(ē’ə-sēn’)
The second epoch of the Tertiary Period, from about 58 to 37 million years ago. During the earliest part of this epoch, land connections existed between Antarctica and Australia, between Europe and North America, and between North America and Asia, and the climate was warm. The land connection between Antarctica and Australia disappeared in the mid-Eocene and early Oligocene, resulting in a change in the predominant oceanic currents and a cooler climate. With this change, the average size of mammals changed from less than 10 kg (22 lbs) to more than 10 kg. The Himalayas also formed during the Eocene, and most modern orders of mammals appeared. See Chart at geologic time.