The 43 million years of the Paleogene period represent a crucial interval in the evolution of mammals, birds and reptiles, which were free to occupy new ecological niches after the demise of the dinosaurs in the K/T Extinction. The Paleogene was the first period of the Cenozoic Era (65 million years ago to the present), followed by the Neogene period (23-2.6 million years ago), and is itself divided into three epochs: the Paleocene (65-56 million years ago), the Eocene (56-34 million years ago) and the Oligocene (34-23 million years ago).
Climate and Geography. With some significant hiccups, the Paleogene period saw a steady cooling of the earth's climate from the hothouse conditions of the preceding Cretaceous period. Ice began to form at the North and South poles and seasonal changes were more pronounced in the northern and southern hemispheres, which had a significant impact on plant and animal life. The northern supercontinent of Laurasia gradually broke apart into North America and Eurasia, while its southern counterpart Gondwana continued to split off into South America, Africa, Australia and Antarctica, all of which began drifting to their present positions.
Terrestrial Life During the Paleogene Period
Mammals. Mammals didn't suddenly appear on the scene at the start of the Paleogene period; primitive forms dated back as far as the Triassic period, 230 million years ago. In the absence of dinosaurs, though, mammals were free to radiate into a variety of open ecological niches. During the Paleocene and Eocene periods, mammals still tended to be fairly small, but had already started evolving along definite lines: the Paleogene is when you can find the earliest ancestors of whales, elephants, and odd- and even-toed ungulates. By the Oligocene epoch, at least some mammals had begun to grow to respectable sizes, though they weren't nearly as impressive as their descendants of the ensuing Neogene period.
Birds. During the early part of the Paleogene period, birds, and not mammals, were the dominant land animals on earth (which shouldn't be all that surprising, given their kinship with recently extinct dinosaurs). One early evolutionary trend was toward large, flightless, predatory birds like Gastornis, but subsequent eons saw the appearance of more diverse flying species, which were similar in many respects to modern birds.
Reptiles. Although dinosaurs were completely extinct by the start of the Paleogene period, the same wasn't true for their close cousins, the crocodiles, which not only managed to survive the K/T Extinction but actually flourished in its aftermath. The deepest roots of snake and turtle evolution can be located in the later Paleogene, and small, inoffensive lizards continued to scurry all over the place.
Marine Life During the Paleogene Period
Not only the dinosaurs went extinct 65 million years ago; so did their vicious marine cousins, the mosasaurs, along with the last remaining plesiosaurs and pliosaurs. This sudden vacuum at the top of the aquatic food chain naturally spurred the evolution of sharks (which had already been around for hundreds of millions of years). Mammals had yet to fully venture into the water, but the earliest, land-dwelling ancestors of whales prowled the Paleogene landscape, most notably in central Asia.
Plant Life During the Paleogene Period
Flowering plants, which had already made an appearance toward the end of the Cretaceous period, continued to flourish during the Paleogene. The gradual cooling of the earth's climate paved the way for deciduous forests, mostly on the northern continents, with jungles and rain forests increasingly restricted to equatorial regions. Toward the end of the Paleogene period, the first grasses appeared, which would have a significant impact on animal life during the ensuing Neogene period.