Period· The columns of time

Paleogene

66 Ma → 23 Ma

In the Paleogene, from 66 to 23 million years ago, the planet passed from a greenhouse almost without ice to a world with a polar ice sheet, while mammals took the niches the impact had emptied.

Period of forty-three million years in which the planet passes from a greenhouse regime almost without ice to one with a polar ice sheet, while mammals occupy the niches vacated by the impact.

What the evidence shows

Its three epochs each correspond to a stage of the same movement: an emptied world in which survivors first increase in size and only later increase their relative brain volume; then the warmest interval of the era; then a world carrying an ice sheet. The passage between the last two is sharp and dated: at 34 million years, carbon dioxide falls below the threshold that allows ice to survive an Antarctic summer, and a continental ice sheet spreads within some five hundred thousand years. The present climatic regime begins at that threshold, and ice has persisted since, in varying volume.

Bounds and conventions

Its base is the Cretaceous boundary, among the sharpest features on the chart; its top, at 23 million years, is an ordinary stratigraphic limit. Between them the period rests on three classes of evidence that do not always agree exactly: dated ash beds, isotopic curves and first fossil appearances. Each epoch carries discussions of its own.