first major worldwide division of rocks and time of the Paleogene Period, spanning the interval between 65.5 million and 55.8 million years ago. The Paleocene Epoch was preceded by the Cretaceous Period and was followed by the Eocene Epoch. The Paleocene is subdivided into three ages and their corresponding rock stages: the Danian (Danian Stage), Selandian (Selandian Stage), and Thanetian (Thanetian Stage).

Marine rocks of Paleocene age are relatively limited in occurrence, and as a consequence much of the information about this epoch comes from terrestrial deposits. The most complete picture of Paleocene terrestrial life and environments is afforded by the rock record of North America; elsewhere, Paleocene animals, especially mammals (mammal), are lacking or rare or are only of late Paleocene age. Prominent faunal remains of the late Paleocene Epoch are known from the regions of Cernay, France; Gashato, Mongolia; and the Chico River of Patagonian Argentina.

The climate of North America during the Paleocene Epoch was characterized by a general warming trend with little or no frost. Seasonal variations probably can best be described as alternations of dry and wet seasons.

One of the most striking features of vertebrate life in the Paleocene Epoch was the complete absence of dinosaurs (dinosaur) and other reptilian (reptile) groups that were dominant during the preceding Cretaceous Period. Another striking feature was the rapid proliferation and evolution of mammals (mammal). Paleocene mammals included representatives of many groups or orders that still exist today, though the Paleocene forms were mostly archaic (that is, descended from yet earlier forms) or highly specialized. Paleocene mammals included Cretaceous species such as opossum-like marsupials (marsupial) and, especially, the archaic and unusual multituberculates (multituberculate)—herbivorous animals that had teeth very similar in some respects to those of the later, more advanced rodents. The condylarths (Condylarthra)—hoofed animals that were very important members of the Paleocene animal kingdom—included forms that were evolving toward herbivorousness while still retaining insectivorous-carnivorous traits of their Cretaceous ancestors. Primates (primate) became more abundant in the middle Paleocene; they displayed characteristics intermediate between the insectivores and the lemurs (lemur), especially in their dental anatomy.

Late in the Paleocene, mammalian evolution showed a trend toward larger forms and more varied assemblages. Primitive mammalian carnivores—notably the creodonts (Creodonta) (a group of catlike and doglike animals)—appeared, as did large herbivores, ancestral rodents, and the first known supposed primates. The Gashato fauna from Mongolia contains the remains of the earliest known hare (Eurymylus), and among Paleocene mammal remains from South America are many early representatives of animals that became dominant in subsequent epochs of the Paleogene Period.

Life in the early Paleocene oceans took hundreds of thousands to millions of years to recover from the mass extinction event at the end of the Cretaceous Period, but by Late Paleocene times many groups of marine invertebrate animals had diversified considerably, including mollusks and plankton. Highly fossiliferous marine sediments from the Upper Paleocene are well known along the Gulf and Atlantic coastal plains of North America.