More of North America was dry land during the Oligocene than in the preceding Eocene Epoch. The Gulf Coast was flooded, but the Atlantic Coast north of South Carolina became emergent. The principal formation of the Gulf Coast region is the Vicksburg Limestone which can be seen at Vicksburg National Military Park. The Pacific Coast, like the more northern Atlantic Coast, was largely elevated and its erosion led to the deposition of sediments. The great erosion of the Rocky Mountains was responsible for the deposition of the fossil-rich White River Group clay and sand over large areas of western Nebraska and northeastern Colorado and parts of Wyoming and the Dakotas, including Badlands National Park in South Dakota. Late in the Oligocene, the John Day deposits, notable for their fossils, formed in Oregon. During the Oligocene, volcanoes erupted in the San Juan Mountains of Colorado and the Absaroka Mountains of Wyoming where unrelated volcanism still persists in Yellowstone National Park.
The climate of the Oligocene was mild and temperate in North America. The virtual disappearance of the archaic mammals of the Paleocene marks Oligocene evolution in Europe and North America. Carnivorous mammals—ancestral dogs and cats—made their appearance, along with beavers, mice, rabbits, and squirrels. Camels, entelodonts (extinct ancestors of modern pigs and other hoofed animals), and a more highly developed type of horse appeared. Both terrestrial and aquatic rhinoceroses evolved, particularly notable in the National Park System are the fossil remains of the rhinoceros (Subhyrocodon occidentalis) from Badlands National Park in South Dakota.
Significant Paleogene events
The extinction event at the end of the Cretaceous Period opened numerous ecological niches. These were filled mostly by mammals, which underwent a dramatic evolutionary radiation. By the end of the Cenozoic Era, North America was home to mastodons, ground sloths, armadillos, camels, horses, saber-toothed cats, giant wolves, giant beavers, and giant bears. Birds also began to diversify and occupy new niches during this time. In addition, many familiar plants such as pines, cacti, and palms appeared. Also, grasses developed into expansive grasslands, which spread across the continent.
Learn more about events in the Paleogene Period
“Age of Mammals”
For more than 100 million years, mammals coexisted with dinosaurs, yet the fossil record indicates that during this entire time mammals were neither diverse nor abundant. Even during the Late Cretaceous, very near the end of the “Age of Reptiles,” only a few families of mammals existed. This situation changed drastically after the mass extinction event at the end of the Cretaceous Period, which eliminated the dinosaurs and many of their relatives and created numerous adaptive opportunities for mammals.
Studies of the fossil record show that within about 10 million years of the Cretaceous extinction, some 130 genera of mammals existed, as many as there have been at any time since. Bats, primates, rodents, whales—these and other forerunners of today’s animals were already present.
The Cenozoic evolutionary history of mammals is better known than the history of any other class of vertebrate. Two factors account for this. First, Cenozoic terrestrial deposits are more common than Mesozoic and Paleozoic deposits. Second, mammal fossils are easier to identify. For instance, in mammals the teeth are fully differentiated into distinctive types, and the chewing teeth, called premolars and molars, differ in each of the mammal orders. As such, a single mammal chewing tooth is commonly enough to identify the genus from which it came (Wicander and Monroe 2000).
Bird Diversification
The first members of many living orders and families of birds such as owls, hawks, ducks, penguins, and vultures evolved early in the Paleogene Period. A marked increase in the variety of songbirds took place during the next geologic period (Neogene), and by 10 to 5 million years ago many present-day genera of birds were present (Wicander and Monroe 2000).
Today birds vary considerably in size and adaptations, but their basic skeletal structure has not changed significantly since the beginning of the Cenozoic. This uniformity is not surprising because most birds are fliers, and adaptations for flying impose limitations on variations in structure. Birds adapted to numerous habitats and increased in diversity through the Paleogene and into the Neogene. More recently, their diversity has decreased slightly (Wicander and Monroe 2000).
One of the remarkable early adaptations of birds was the development of large, flightless predators. For example, the now-extinct Diatryma stood nearly 7 feet (2 m) tall, had a huge head and beak, short massive legs, toes with large claws, and small vestigial wings. Flightless birds notwithstanding, the true success story among birds belongs to the fliers. An amazing array of adaptive types arose during the Cenozoic Era, making them as successful as mammals.
Development of Grasses
Until recently, scientists thought that grasses first appeared during the Paleogene Period because the earliest (and only) grass fossils found were about 55 million years old. Because scientific progress is never final, however, and what is accepted as a fact today may be modified or even discarded tomorrow, this long-held “truth” was recently refuted. In November 2005, scientists published results that dinosaurs were digesting different varieties of grasses between 71 and 65 million years ago. These researchers analyzed fossilized dung, called coprolites, and found microscopic particles of silica, called phytoliths, which showed that at least five taxa from extant grasses were present on the Indian subcontinent during the latest Cretaceous (Prasad et al. 2005).
Nevertheless, as part of the continuing evolution of flowering plants, grasses continued to develop during the Paleogene Period. With new growth originating near the root, rather than at the tip of the plant, grasses are protected from otherwise catastrophic damage caused by grazing and fire. Therefore, unlike trees, they quickly regenerate and create renewable food sources for plant eaters.
Evidence of grasslands appeared during the Neogene Period (Miocene Epoch) about 23 million years ago. Grasslands developed in areas frequently ravaged by fire (e.g., from lightning strikes), and animals rapidly evolved to utilize this new habitat. During this time, mammals that graze on grasses, including both the even-toed hoofed mammals (e.g., antelope, deer, cattle, and sheep) and odd-toed mammals (e.g., rhinoceroses and horses), diversified greatly.
