Period· The columns of time

Paleogene

66 Ma → 23 Ma

Forty-three million years in which the world turns from a greenhouse all but free of ice into one with a polar cap, and mammals grow into the space the asteroid left.

What the evidence shows

Its three epochs each carry a stage of the same movement: an emptied world in which the survivors grow large bodies first and large brains only later, then the hottest interval of the era, then a world that acquires a polar cap. The turn between the last two is sharp and dated: at 34 million years the air loses enough carbon dioxide for ice to survive an Antarctic summer, and a continental ice sheet spreads in about five hundred thousand years. The modern climate begins at that threshold, and the ice has been there, in one volume or another, ever since.

Bounds and conventions

Its base is the Cretaceous boundary, one of the sharpest lines on the chart; its top, at 23 million years, is an ordinary stratigraphic boundary and nothing like as vivid. Between them the period rests on three kinds of evidence that do not always agree to the same decimal: ash beds, isotope curves and fossil first appearances. Each epoch carries its own disputes.

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. Bertrand et al. (2022), Science · Brawn before brains in placental mammals after the end-Cretaceous extinction
3. Gutjahr et al. (2017), Nature · Very large release of mostly volcanic carbon during the Palaeocene-Eocene Thermal Maximum
4. Hutchinson et al. (2021), Climate of the Past · The Eocene-Oligocene transition: a review of proxy data, models and comparisons
5. Thewissen et al. (2007), Nature · Whales originated from aquatic artiodactyls in the Eocene epoch of India
6. Ni et al. (2013), Nature · The oldest known primate skeleton and early haplorhine evolution