The Paleocene is a Geologic Epoch that extends from about 58 to 65 million Years before the present. As with other older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are somewhat uncertain. Note the change from Period used in the Mesozoic and Paleozoic Eras to the term Epoch for the shorter subdivisions of the Tertiary Era. The name Paleocene was refers to the "old(er)-new faunas that arose after the demise of the dinosaurs and prior to the emergence of modern mammalian orders in the Eocene. The Paleocene follows the Cretaceous Period and is followed by the Eocene Epoch. The Paleocene is the first Epoch of the Tertiary Era.
The end of the Cretaceous/start of the Paleocene is marked by a major, and extensively studied, extinction event. It is marked over much of the Earth by a discontinuity with high Iridium levels, and an abrupt change in flora and fauna. There is some evidence that a substantial, but very short lived, climactic change may have occurred in the very early decades of the Paleocene. Majority opinion is that the changes are related to the impact of a large extraterrestrial object in the vicinity of Yucatan. The end of the Paleocene is marked by the emergence of modern orders of mammals.
The Paleocene is usually broken into Lower and Upper subdivisions. The Faunal stages from youngest to oldest are:
Danian/Montian/(lower Midway) Thanetian/Selandian/Landenian/Heersian/(Upper Midway)
During the Paleocene, the continents continued to drift toward their present positions. The mid continent sea receded from North America. Climates remained warm.
On land, plants became quite modern. Paleocene and later plant fossils generally are attributed to modern genera or to closely related taxa. Marine faunas also came to resemble modern faunas with only the marine mammals and the Charcharinid sharks missing. The major area of fast faunal evolution was among the land mammals which -- largely freed from reptile competition for niches -- exploded in size and diversity during the Paleocene.