Eocene

American  

[ee-uh-seen] / ˈi əˌsin /

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.

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noun

1. the Eocene Epoch or Series.

Eocene British  

/ 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

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noun

1. the Eocene epoch or rock series

Eocene Scientific  

/ ē′ə-sēn′ /

1. 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.

2. See Chart at geologic time

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Other Word Forms

• post-Eocene
  adjective

Etymology

Origin of Eocene

First recorded in 1825–35; eo- + -cene

Example Sentences

These remains date back to the Middle Eocene period, roughly 47 million years ago.

From Science Daily • Apr. 29, 2026

Dr. Marc Jones, a curator of fossil reptiles and amphibians who co authored the research, notes that the Eocene was marked by major climate shifts worldwide.

From Science Daily • Dec. 31, 2025

Hyaenodon, extinct carnivorous mammals, late Eocene to middle Miocene.

From Salon • Feb. 17, 2025

This was a small crocodile, a testament to the warm and wet early Eocene climate, like central Africa today.

From Seattle Times • Sep. 15, 2023

Fossils of these ancients have been found dating from the Cretaceous era while in the Eocene and Miocene they were spread over England and Europe and America.

From "Travels with Charley in Search of America" by John Steinbeck