Epoch· The columns of time

Eocene

56 Ma → 33.9 Ma

The Eocene, from 56 to 34 million years ago, was the warmest epoch of the Cenozoic, in which bats, whales and primates appeared, and it ended with the freezing of Antarctica.

Warmest epoch of the Cenozoic, in which bats, cetaceans and primates appear, and closed by the glaciation of Antarctica.

What the evidence shows

Warm at its beginning, the epoch cools for twenty million years. A bat from Wyoming, dated to 52 million years, has a membrane between elongated fingers and a claw on each of them, but a cochlea too small to have processed its own echoes: the wing precedes echolocation. Cetaceans enter the sea between 50 and 38 million years from hoofed land mammals. Primates appear at some thirty grams, with prehensile hands, forward-facing orbits and nails where their ancestors carried claws. Around 49 to 48 million years, the Arctic Ocean carries a layer of fresh water in which the fern Azolla proliferates, doubling its mass within days, and the carbon it buries is among the sinks invoked to explain the cooling. At 33.9 million years, carbon dioxide has fallen below the threshold allowing ice to survive an Antarctic summer.

Bounds and conventions

Both ends are well defined, the base on the carbon injection and the top near the first major Antarctic glaciation. The contribution of the Azolla bloom to the cooling is not established and estimates vary by nearly an order of magnitude; the respective weight of carbon dioxide and of the opening of the Southern Ocean gateways, Drake Passage and the Tasman gateway, which allow the circumpolar current to establish itself, remains one of the longest controversies in Cenozoic climate.