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EnvironmentNorth America

The Bighorn Basin warms

56.0 million yrs ago

Animals that shrank when the world got too hot

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In depth

The Palaeocene–Eocene Thermal Maximum is an abrupt global warming 56 million years ago, triggered by a rapid release of carbon into the atmosphere. The Bighorn Basin provides the most continuous land-based record of the Palaeocene–Eocene Thermal Maximum, and the locality is therefore widely used as an analogue in climate research: a rapid carbon release, five to eight degrees of warming, and a recovery that took some two hundred thousand years. It is also here that the first true primates and the first hoofed mammals appear in North America, almost simultaneously, as part of the same faunal turnover.

For children

Fifty-six million years ago the Earth suddenly became much warmer. In Wyoming you can follow it through the beds: the ancestors of horses and other mammals become noticeably smaller through the warming, and grow again afterwards. Smaller bodies shed heat more easily.

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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org · Last changed 2026-08-16