The Paleogene Period is a geologic period that stretches from 65.5 million years ago, when the dinosaurs were blasted into extinction by an asteroid, to 23 million years ago, when the period was concluded with an extended episode of global cooling. Compared to the present day, the Palogene was a warm time, and as such, is sometimes called "a continuation of the Mesozoic, but with mammals."
Just as during the Mesozoic, during the Paleogene, the world was warm, with no polar ice caps, and dense forests extended further north and south, to places like Wyoming. The Paleogene had high sea levels, but not as high as during the Cretaceous before it, and except for the flooding of large portions of central Eurasia, the configuration of the continents was pretty much the same as it is today.
The Paleogene was characterized by the rapid diversification of mammals and the filling of numerous niches left open by the extinction of all non-avian dinosaurs, pterosaurs (flying reptiles), and large marine reptiles (plesiosaurs, mosasaurs, etc.) However, unlike the gigantic dinosaurs that came before them, the land mammals of the early Paleogene were relatively tiny, with none larger than a bear, and most much smaller, about the size of cats. Larger mammals, like the ancestors of rhinoceroses, evolved in the middle of the period.
Many of the groups that dominated the Paleogene are extinct today. The cougar-like animal Mesonychia is thought to have been a leading scavenger or predator of the early Palogene. In the early part of the Paleogene, cats, pigs, and whales evolved. In the middle of the Paleogene, bats, the ancestors of elephants, and Eohippus, the first horse, evolved. Towards the end of the period, rodents and the first primates evolved, along with condylarths, which are considered the ancestors of modern hoofed herbivores. Various early spinoffs of animals like the horse, like Propalaeotherium, whose evolutionary lines would die out without leaving any descendants, evolved during this period.
In the skies, birds were busy at work taking over the niche left empty by the death of the pterosaurs. Birds evolved into a variety of fantastic colors, shapes, and sizes, by the end of the period achieving a level of diversity similar to today's. In the oceans, with the death of the ammonites, the predominant mollusk became squid, while sharks grew in diversity and numbers to take up the niches left empty by the demise of large marine reptiles. These were soon joined by whales, which took tens of millions of years to achieve their present size.