Recapitulation of Cenozoic Events

Paleocene: Radiation of Mammals, birds, and insects. Topical conditions

Eocene: Radiation of flowering plants, most modern mammalian orders are represented.

Oligocene: Monkeys appeared, mild climates

Miocene: Apes arise, climate cooler, plains and grasslands

Pliocene: Hominids appear, large carnivores.

Pleistocene: Ice ages, modern humans (Genus Homo) appear

Holocene (Recent): Human civilization

>The marine invertebrate groups that survived the Mesozoic extinctions diversified throughout the Cenozoic. Bivalves, gastropods, corals, and several kinds of phytoplankton such as foraminifera proliferated.

>During much of the Early Cenozoic, North America was covered by subtropical and tropical forests, but the climate became drier by Oligocene and Miocene time, especially in the mid-continent region.

>Birds belonging to the living orders and families evolved during the Paleogene System. Large, flightless predatory birds of the Paleogene were eventually replaced by mammalian predators.

>Evolutionary history is better known for mammals than for other classes of vertebrates, because mammals have a good fossil record and their teeth are so distinctive.

>Egg-laying mammals (monotremes) and marsupials exist mostly in the Australian region. The placental mammals¬ by far the most common mammals owe their success to their method of reproduction.

>All placental and marsupial mammals descended from shrew like ancestors that existed from Upper Cretaceous to Paleogene time.

>Small mammals such as insectivores, rodents, and rabbits occupy the microhabitats unavailable to larger mammals. Bats, the only flying mammals, have forelimbs modified into wings but otherwise differ little from their ancestors.

>Most carnivorous mammals have well-developed canines and specialized shearing teeth, although some aquatic carnivores such as seals have peg-like teeth.

>The most common ungulates are the even-toed hoofed mammals (artiodactyls) and odd-toed hoofed mammals (perissodactyls), both of which evolved during the Eocene. Many ungulates show evolutionary trends such as molarization of the premolars as well as lengthening of the legs for speed.

>During the Paleogene, perissodactyls were more common than artiodactyls but now their 16 living species constitute only about 10% of the world's hoofed mammal fauna.

>Although present-day Equus differs considerably from the oldest known member of the horse family, Hyracotherium, an excellent fossil record shows a continuous series of animals linking the two.

>Even though horses, rhinoceroses, and tapirs as well as the extinct titanotheres and chalicotheres do not closely re¬semble one another, fossils show they diverged from a common ancestor during the Eocene.

>The fossil record for whales is now complete enough to verify that they evolved from land-dwelling ancestors.

>Elephants evolved from rather small ancestors, became quite diverse and abundant, especially on the Northern Hemisphere continents, and then dwindled to only two living species.

>Horses, camels, elephants, and other mammals spread across the northern continents during the Cenozoic because land connections existed between those land¬masses at various times.

>During most of the Cenozoic, South America was isolated, and its mammal fauna was unique. A land connection was established between the Americas during the Late Cenozoic, and migrations in both directions took place.

>One important evolutionary trend in Pleistocene mammals and some birds was toward gigantism. Many of these large species died out, beginning about 40,000 years ago.

>Changes in habitat and prehistoric overkill are the two hypotheses explaining Pleistocene extinctions.
 

THE INTERCONTINENTAL MIGRATIONS
The mammalian faunas of North America, Europe, and northern Asia exhibited many similarities throughout the Cenozoic. Even today, Asia and North America are only narrowly separated at the Bering Strait, which at several times during the Cenozoic formed a land corridor across which mammals migrated. During the early Cenozoic, a land connection between Europe and North America allowed mammals to roam across all the northern continents. Many did; camels and horses are only two examples. However, the southern continents were largely separate island continents during much of the Cenozoic. Africa remained fairly close to Eurasia, and at times faunal interchange between those two continents was possible. For example, elephants first evolved in Africa, but they migrated to all the northern continents.

South America was isolated from all other landmasses from Late Cretaceous until a land connection with North America formed about 5 million years ago. Before the connection was established, the South American fauna was made up of marsupials and several orders of placental mammals that lived nowhere else. These animals thrived in isolation and showed remarkable convergence with North American placental mammals. When the Isthmus of Panama formed, migrants from North America soon replaced many of the indigenous South American mammals, whereas fewer migrants from the south were successful in North America. As a result of this great American interchange, today about 50% of South America's mammalian fauna came from the north, but in North America only 20% of its mammals came from the south. Even today, the coyote (Canis latrans) is extending its range from the north through Central America.

Most living species of marsupials are restricted to the Australian region.  Marsupials occupied Australia before its separation from Gondwana, but apparently placentals, other than bats and a few rodents, never got there until they were introduced by humans. So, unlike South America, which now has a connection with another continent, Australia has remained isolated, and its fauna is unique.