The Paleogene period, from 65.5 to 23 million years ago, marked the beginning of the Age of Mammals. Earth’s continents moved to their current configuration, forests expanded, and sea levels dropped. Mammals thrived and evolved, filling the ecological niche left by dinosaurs. Large flightless birds and aquatic mammals also evolved.
The Paleogene period is the first of two periods is the Cenozoic, which extends from the extinction of dinosaurs (except birds) 65.5 million years ago to the present day. The Paleogene itself spans from 65.5 million years ago to 23 million years ago. The Paleogene period is considered to be the beginning of the Age of Mammals, in contrast to the Age of Reptiles which preceded it. It is made up of three parts, each about 13 million years long: the Paleocene, Eocene and Oligocene.
During the Paleogene period, the Earth’s continents gradually moved from their old configuration, consolidated close together, to their current, more widely spaced configuration. 50 million years ago, Earth was getting close to what we would consider normal, even though India was an island and much of Central Asia was inundated. Earth’s average temperature started out as typical, then rapidly began to decline, a trend that continued until relatively recently.
In the Paleogene period, dense forests expanded to more extreme latitudes than during the Jurassic. Sea levels have dropped, exposing significant amounts of land that had previously been submerged. Africa and South America were both island continents during this period, developing their own unique flora and fauna.
Birds, ancestors of surviving dinosaurs, evolved into something similar to their present form during the Paleogene. Without dinosaurs to rule the earth, mammals thrived, some growing to great proportions. The ancestors of rhinos, horses, cats, primates and dogs all evolved during the Paleogene period. The most important mammals during the early Paleogene period come from groups that are now extinct, but by its end, many of the direct evolutionary ancestors of modern forms had evolved. Many of the small mammals resembled possums or raccoons in appearance. With the ecological niche formerly occupied by dinosaurs now vacant, mammals experimented with adaptive radiation in a variety of new forms.
Although mammals were not yet very large, there were large flightless birds that dominated the land in some areas. Some of these were up to 2 meters tall and were fierce predators. In the sea, aquatic mammals such as whales evolved, some of which reached 25 m in length. Few fossils of life in Australia or Antarctica have been discovered from this period.
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