Terrestrial Life During the Paleogene Period
Mammals. Mammals didn't suddenly appear on the scene at the start of the Paleogene period; primitive mammals 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 epochs, 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 (hoofed mammals). 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 that they had evolved from recently extinct dinosaurs). One early evolutionary trend was toward large, flightless, predatory birds like Gastornis, which superficially resembled meat-eating dinosaurs, but subsequent eons saw the appearance of more diverse flying species, which were similar in many respects to modern birds.
Reptiles. Although dinosaurs, pterosaurs and marine reptiles had gone 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 underfoot.
Mammals. Mammals didn't suddenly appear on the scene at the start of the Paleogene period; primitive mammals 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 epochs, 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 (hoofed mammals). 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 that they had evolved from recently extinct dinosaurs). One early evolutionary trend was toward large, flightless, predatory birds like Gastornis, which superficially resembled meat-eating dinosaurs, but subsequent eons saw the appearance of more diverse flying species, which were similar in many respects to modern birds.
Reptiles. Although dinosaurs, pterosaurs and marine reptiles had gone 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 underfoot.
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