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

The majority of known Podonominae fossils originate from the Northern Hemisphere, with only two prior exceptions documented from the Southern Hemisphere: an Eocene specimen from Australia and a Paleocene record from India.

From Science Daily • Oct. 15, 2025

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

From Salon • Feb. 17, 2025

Washington in the Eocene looked different from today, with a coastline well east of where Seattle sits today and a chain of volcanic islands dotting the horizon offshore.

From Science Daily • Feb. 6, 2024

These tracks might have been made by a creodont, a predatory mammal about the size of a house cat, which flourished in the Paleocene and Eocene in Europe, Africa and North America.

From Seattle Times • Sep. 15, 2023

Archie wanted to discuss the Eocene skull he was holding, but all the others could talk about was the game.

From "Stargirl" by Jerry Spinelli