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.

---

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

---

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

---

Other Word Forms

• post-Eocene
  adjective

Etymology

Origin of Eocene

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

Example Sentences

See Examples For:

Eocene

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

In addition to these two impacts, existing evidence suggests three smaller asteroids also hit Earth during this time -- the late Eocene epoch -- pointing to a disturbance in our solar system's asteroid belt.

From Science Daily ● Dec. 4, 2024

Indeed, from the larger perspective of Earth’s planetary history, Antarctica was green for a large chunk of time, specifically during the period prior to the Eocene Epoch 56 million years ago.

From Salon ● Oct. 19, 2024

Further information about the Eocene ecosystem comes from the abundant leaf fossils.

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