The Palaeogene (meaning “old generation” in Greek) is the first geological period of the Cainozoic and the tenth period in the Phanerozoic eon. It began 65 million years ago, and ended 23 million years ago. Before it was the Cretaceous. The Palaeogene was followed by the Neogene.
The climate was warmer than today. The world was largely forested, and the larger land animals were browsers, not grazers. Grasses existed, but they had not yet become the dominant ecology they became later. The change to a modern climate came in the Miocene period, which started off like the Paleogene, but changed towards a modern-type ecology with less rainfall and widespread grasslands.