Chapter 100: Eocene Period

In the middle Eocene, grasslands had not yet spread North America was dominated by forest and forest mammals as seen above. The odd-toed perissodactyls, such as the Palaeosyops in the rear center had already appeared. In the trees next to them are the primitive rodent Ischyrotomys and the primate Smilodectes.

A group of primitive tapirs (Helatetes) are in the center of the frame. Just in front of them, looking at a group of Orohippus (a perrisodactyl), is a predatory Patreofelis. Just below a saber-toothed Machaeroides defends its kill (a lizard, Echmatemys) from the fox-size carnivore, Sinopa.

A couple of artiodactyls, the tiny Homacodon and Helohyus, are seen just above an edentate Metacheiromys leaving the frame above the saber-tooth and its kill.

The global cooling that eventually leads to later ice ages begins during this epoch. Forests begin to shrink and grasslands expand at the expense of forests.

Anthropoid apes make their first appearance. The newly opened landscapes favor the evolution of fast running prey and predators. Oreodonts were very common in North America along with three toed horses and a variety of rhinoceros species. 

The giant Indricothere, a type of rhinoceros the size of a medium sauropod and the largest land mammal ever, lived in central Asia. The anthropoid apes appeared during this epoch.

South America, isolated for a few million years, evolves a distinct fauna including giant carnivorous birds and predatory marsupials. Both cats and dogs were represented among carnivores, including saber tooth cats. In the ocean both seals and sea lions appear late in this epoch. 

The two suborders of whales alive today, the toothed and baleen whales evolved. Invertebrates continued to look more modern, while coral reefs spread in the southern hemisphere as far as New Zealand. South America breaks away from Antarctic, allowing an isolating circum-antarctic current and a permanent ice cap to form, lowering world temperatures.

Dominating this scene in the distance are giant Brontotherium, the largest land animals to ever live in North America. Just in front of them is one the the first true rhinoceros, Subhyracodon, and in front of it is a group of Merycoidodon herbivorous oreodonts. Next to them, in the center of the frame, is a Protapirus, the first true tapir, that in turn is above a group of Poëbrotherium, small ancestral camels. 

Two Hyaenodon fight over a Glyptosaurus lizard, while a saber-tooth cat, Hoplophoneus lies in the bushes in the center foreground. Moving across the front from left to right a series of small animals are seen: the primitive rabbit Paleolagus, the insectivore Ictops, three kinds of ruminants, Hypisodus, a couple of Leptomeryx, and on the far right lower corner, a group of Hypertragulus.

Lasting nearly 22 million years, the Eocene is the longest epoch of the Cenozoic. During this time, the first ancestors and close relatives of modern mammal species appeared. Many of these modern mammal groups appear at the beginning of the Eocene, a period of dramatic global warming called the Paleocene-Eocene thermal maximum.

Among the mammal groups that first appear in the fossil record during this period are the perissodactyls, artiodactyls, proboscideans, rodents, and primates. It is thought that the intense global warming allowed warm-adapted mammals to migrate between continents via land connections at very high latitudes. Improved global dispersal allowed mammals to spread into areas in which fossils were being preserved. One such place was what is now the Rocky Mountain basins of North America. Uplift of the Rockies, which began in the Mesozoic, continued in the Eocene. It was these conditions that led to abundant preservation of fossils in this area, including the earliest perissodactyls.

Early perissodactyls, such as the horse relative Hyracotherium, appear right at the very beginning of the Eocene. Hyracotherium and other perissodactyls probably originated in Europe or Asia and migrated from Asia to North America via a high latitude land bridge. 

The Eocene is the heyday of the perissodactyls, with more kinds of this ungulate group present than in any other epoch. As the Eocene progressed, the Earth's temperature gradually cooled. The rainforest-like habitats that covered much of the continents gave way to more open woodland.

By the end of the epoch, the planet was much cooler, seasons were more extreme, and Antarctic ice caps had begun to form. Many of the more primitive mammalian groups that dominated the Paleocene went extinct and were replaced by more modern mammal groups of mammal such as the perissodactyls.

One of the best sites for late Eocene mammal fossils is an extensive area of arid, heavily-eroded terrain covering parts of Nebraska, South Dakota, and Wyoming, which is known as the White River Badlands.

After Eocene it was time for Oligocene Epoch. According to Xin the Oligocene Epoch, right smack in the middle of the Tertiary Period (and end of the Paleogene), lasted from about 33.9 to 23 million years ago.* Although it lasted a "short" 11 million years, a number of major changes occurred during this time. These changes include the appearance of the first elephants with trunks, early horses, and the appearance of many grasses — plants that would produce extensive grasslands in the following epoch, the Miocene.

As a result of the cooling trend prevalent throughout the Oligocene Epoch, the lives and habitats of many organisms were directly affected. In the oceans, marine biotic provinces became more fragmented as sea dwellers capable of withstanding cooler temperatures congregated to places further from the warmer equator, where other species could better survive. The cooling trend was also responsible for the reduced diversity in marine plankton, the foundation of the food chain.

On land, mammals such as horses, deer, camel, elephants, cats, dogs, and primates began to dominate, except in Australia. The continuation of land mammal faunal migration between Asia and North America was responsible for the dispersion of several lineages to new continents. Early forms of amphicyonids, canids, camels, tayassuids, protoceratids, and anthracotheres appeared, as did caprimulgiformes, birds that possess gaping mouths for catching insects. Diurnal raptors, such as falcons, eagles, and hawks, along with seven to ten families of rodents also first appeared during the Oligocene. The "bulk feeding" in the open grasslands and savannas that occurred in this period resulted in the increase of general herbivore size. As an example, ungulates continued to get larger throughout the Oligocene.

The early Oligocene was marked by a multitude of different events ranging from the appearance of new groups such as elephants to the decline in taxonomic diversity in middle- and high-latitude forests. "Micro-mammals" experienced a period of diversification, as did the marsupials in Australia. This period was also marked by a relative free change of animals among northern continents, as evidenced by the similarity in vertebrate faunas.

In North America, the cricetids (voles and hamsters) first appeared while the mesothermal dicotyledons (a group of flowering plants) went extinct. South America became dominated by forests, and the first primates appeared in Africa. Primates found in Southeast Asia during this period represent primitive members of the New World and Old World higher primates.

In western Europe, an extraordinary, sudden change in the fauna, known as the Grand Coupure, occurred. This event involved the immigration of many new taxa, artiodactyls and perissodactyls in particular (e.g., rhinocerotoids, chalicotheriids, anthracotheres, and tayassuids), from areas to the east and the extinction of many Eocene genera and species. At least 17 generic extinctions, 20 first appearances, and 25 unaffected genera of mammals are represented across the Eocene-Oligocene boundary in western Europe.

On a global scale, broad-leaved evergreen vegetation became restricted to 35° latitude around the equator, and megathermal, multistratal vegetation was confined to 15° latitude around the equator. Broad-leaved evergreen plants became increasingly confined to lower latitudes in Eurasia, and microthermal, broad-leaved forest became common over large regions of the Northern Hemisphere.

The mid-Oligocene was marked by a worldwide marine regression; this included a decline in the total number of marine species. On land, the first of the open grassland faunas appeared in Mongolia while in North America, microthermal broad-leaved deciduous forests extended further into southern regions typified before by evergreen species and for the first time in history covered vast regions of the Northern Hemisphere.

The late Oligocene was marked by the expansion of grasslands and prairies that were intimately linked to the expansion of grazing animals. Grasses and composites increased in abundance on the global scale, and humid forests became increasingly common in the southern parts of South America. Horses experienced a period of diversification; anatomical modifications in horses indicate an increase in cursoriality compared to more primitive ancestors. Primitive beavers appeared and the earliest of the New World monkeys inhabited in South America.

The late Oligocene Deseadan record includes two major groups that are thought to represent early waif dispersals from other continents. 

One of these, the caviomorph rodents (e.g., porcupines, capybaras, chinchillas, and a wide assortment of smaller forms), was the only group of rodents in South America until the Plio-Pleistocene. They diversified into 16 families, only two of which are now extinct. The second group of early immigrants was the primates.