Paleocene Epoch

Paleocene Epoch, first of the divisions of the Paleogene Period of the Cenozoic Era on the geologic time scale, spanning an interval from about 65 million to 56 million years ago, and originally defined, as were the epochs that followed it, on the basis of the percentage of modern species of shellfish found in the fossil record. The Paleocene is immediately preceded by the Upper Epoch of the Cretaceous Period and is followed by the Eocene Epoch.

The Paleocene marks the final stage in dismemberment of the ancestral supercontinent Pangaea, which had begun tearing apart early in the Mesozoic Era (see Geology: The Geologic Time Scale). Tectonic plate motions finally separated Antarctica from Australia; in the northern hemisphere, seafloor spreading of the widening North Atlantic wedged North America and Greenland apart. The dinosaurs having disappeared at the end of the preceding Cretaceous Period, mammalian life now began to dominate the planet. Chief among the early mammals were marsupials, insectivores, lemuroids, creodonts (a carnivorous group separate from modern carnivores such as cats and dogs), and primitive hoofed animals from which such diverse groups as the horses, rhinoceroses, pigs, and camels were to evolve in later years. See Paleontology.