Paleocene epoch [key], first epoch of the Tertiary period in the Cenozoic era of geologic time (see geologic timescale) between 60 to 66 million years ago. In W North America, the uplift of the Rocky Mts. that marked the end of the Mesozoic era continued throughout the Paleocene, and the Cretaceous inland seas gradually withdrew from the Great Plains area and central and SW California. In Montana and Wyoming the Fort Union shales and sandstones, laid down during this epoch, are noteworthy because they overlie undeformed upper Cretaceous sediments, thus recording the demise of the dinosaurs and the rise of mammals. The Paleocene mammals were mostly small herbivores similar to their Mesozoic ancestors. By mid-Paleocene, the ungulates, or hoofed mammals of mostly five-toed forms, became abundant. Prosimian primates (tree shrews and tarsiers) also increased in number. Except for part of N France, Europe was largely emergent (i.e., above water). During this epoch, the opening of the Norwegian Greenland Sea eventually resulted in a much more significant mixing of waters, creating the cold North Atlantic Deep waters. Greenland began separating from Europe as the northern mid-Atlantic Ridge formed. On the other side of the world, Antarctica and Australia had separated; India had completed its separation with Africa, resulting in an outpouring of basalts; and India, Africa, and Australia were about to collide with Eurasia. By the end of Paleocene time, N America's last large sea retreated to the Gulf of Mexico. Some of the fossil evidence from Paleocene sediments is difficult to explain; Alaska, for example, clearly had broad-leafed evergreen floras that typically grow in tropical forests. As the land has not changed significantly in latitude since the Paleocene, the evidence of these floras is a puzzle.
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