Epoch· The columns of time

Eocene

56 Ma → 33.9 Ma

The warmest stretch of the Cenozoic, in which bats fly, whales enter the sea and primates begin, and which closes with the freezing of Antarctica.

What the evidence shows

Hot at the start, it cools for twenty million years. A bat from Wyoming at 52 million years has a membrane between elongated fingers and a claw on every one of them, but an inner ear too small to have heard its own echoes: the wing came before the sonar. Whales walk into the sea between 50 and 38 million years. Primates begin at about thirty grams, with gripping hands, forward-facing eyes and nails where their ancestors had claws. Around 49 to 48 million years the Arctic Ocean lies under a skin of fresh water carrying a floating fern that doubles its mass in days, and the carbon that fern buries is one of the drawdowns invoked for the cooling. By 33.9 million years, carbon dioxide has fallen below the level at which ice survives an Antarctic summer.

Bounds and conventions

Both ends are well defined, the base on the carbon spike and the top near the first great Antarctic glaciation. How much the Azolla bloom contributed to the cooling is not settled, and the estimates vary by nearly an order of magnitude; nor is the share to be given to carbon dioxide against the opening of the Southern Ocean gateways, one of the longest-running arguments in Cenozoic climate.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Simmons et al. (2008), Nature · Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation
3. Thewissen et al. (2007), Nature · Whales originated from aquatic artiodactyls in the Eocene epoch of India
4. Ni et al. (2013), Nature · The oldest known primate skeleton and early haplorhine evolution
5. Speelman et al. (2009), Geobiology · The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown
6. Hutchinson et al. (2021), Climate of the Past · The Eocene-Oligocene transition: a review of proxy data, models and comparisons