NEOGENE PERIOD: 23 million years ago to recent
Miocene Epoch: 23 to 5 million years ago
Pliocene Epoch: 5 to 2 million years ago
Pleistocene Epoch: 2 million years ago to 12,000 years ago
Recent/Holocene: since 12,000 years ago
As the Cenozoic progressed, the continents approached their modern positions. India collided with Asia, creating the Himalayas and North and South America collided. The fusion of the Middle East and India with Asia closed the great Tethys Seaway. Antarctica occupied the South Pole. At the end of the Miocene, evidence indicates the Mediterranean evaporated after its western border was sealed, creating a basin 3 km deep and resulting in the deposition of 1 million cubic km of salt. When the Straits of Gibraltar opened, a waterfall 1,000 times the volume of Niagara would have refilled the Mediterranean basin. In the middle of the Tertiary, Arabia began to separate from Africa, Baja California from Mexico, and Sardenia and Corsica from Europe. In the past 5 million years, the San Andreas fault in California has experienced about 240 km of movement. Tectonic movements and uplift resulted in the modern height of the Andes in South America. The Hawaiian Islands were formed by volcanic activity (Seyfert, 1979).
Throughout the Cenozoic, modern groups emerged in addition to a number of extinct forms. Relatives of condors achieved wingspans of 8 meters and beavers reached lengths of more than 3 meters.
The diversity of hoofed mammals included short-necked giraffes, deer with enormous antlers, and long-necked camels.
A diversity of elephants evolved ranging from mammoths to deinotheres.
Predators included giant cave bears and saber tooth cats.
South America hosted a unique fauna which included native ungulates, giant armadillos, and giant ground sloths until North American immigrants caused their extinctions.
Odd-toed herbivores included 6 meter tall rhinos and a diversity of three toed horses.
A large number of apes existed in the mid- to late-Cenozoic. Beginning about 5-7 million years ago, a diversity of upright hominids evolved whose only surviving descendant are modern humans.
THE GREAT AMERICAN INTERCHANGE
South America was isolated from North America from the Late Cretaceous to the Late Pliocene, about 3 million years ago. The South American fauna was unique and many of the primitive groups of mammals survived there without pressures from advanced placental predators. When the isthmus of Panama formed to fuse the two American continents 3 million years ago, at least 16 genera of mammals migrated north and at least 23 genera migrated south.
The effect on North America was minimal. Glyptodonts and giant sloths did colonize North America (as did at least one kind of terror bird, Titanis) but they eventually became extinct, perhaps because of being hunted by early humans. Today, the only North American survivors of this interchange are the opossum, a porcupine, an armadillo, and 2 species of rodents.
The effect on the South American fauna was enormous. The North American immigrants were very successful although not all of the migrating species survived to modern times (such as a few three-toed horses, bears, and elephants). The South American ungulates and terror birds became extinct as did many of the edentates and marsupials, in part because of the advanced placental predators. More than half of the modern South American mammalian genera are descended from the North American species of this interchange.
At least some of this unique South American fauna survived until early
human groups arrived, such as the giant sloths. The mythological figures
of native Paraguayans include a fierce creature known as the Ao-Ao. Although
this seems a bit odd given that Ao-Ao is the guarani word for sloth and
modern sloths are anything but fierce, it is quite possible that this
legend has its origin in the hunting experiences of the early native South
Americans which encountered the much larger relatives of the modern sloth.
Below is a Paraguayan representation of the mythological Ao-Ao.
In some areas of the world, oil from previously marine sediments is exposed on land. As lighter elements evaporate, the remaining material forms a sticky tar. In some places, such as Peru and the famous La Brea tar pits of California, this tar became a natural animal trap. Animals which became trapped (perhaps because they thought the tar would provide a drink) attracted insects, predators and scavengers (including saber tooth cats, dire wolves, golden eagles, and giant teratorn birds). The bones of carnivores far outnumber other groups of animals. Dating techniques indicated that different tar pits were active during different time periods and some could be active during intervals over a course of thousands of years (Kurten, 1988).
It is possible to estimate global climatic conditions from the past 650,000 years from the ice and air trapped with ice sheets. The isotopes ratios of certain elements in rocks (such as oxygen) can allow estimates of more ancient time periods. Since ice sheets leave traces on underlying rock, it is possible to estimate the extent of past glaciations. Combined data indicate that the earth has been warmer and completely devoid of ice sheets for most of the past 500 million years (IPCC, Document I, 2007).
Louis Agassiz first proposed the possibility of Ice Age in 1837. A series of Ice Ages occurred over the past 1.5 million years. In North America, two ice sheets formed: one originating in the Northwest Rocky Mountains (the Cordilleran ice sheet) and another originating around the Hudson Bay (the Laurentide ice sheet). When the glaciations reached their greatest extent, these two ice sheets joined and formed an ice area of 6 million square milesan area greater than the ice of Antarctica and possibly representing the largest ice sheet to ever form in earths history. Ice sheets also covered parts of Europe, Asia, Greenland, Antarctica, and the southern regions of South America, Africa, and Australia. Because so much water was locked up in ice sheets, sea level dropped an estimated 300 feet. As a result, many regions which are currently covered by ocean were exposed such as an ice-free land bridge which connected Alaska and Siberia (the land bridge is referred to as Beringia) (Kurten, 1988).
The Late Tertiary was cooler than the earlier part had been and a variation of cooler and warmer periods occurred although the magnitude was less than the more recent glaciations and interglacial periods. In the past 1.5 million years, a series of glaciations were interrupted by interglacial periods of comparable warmth. The Nebraskan Glaciation began 1.5 million years ago and ended about 1.1 million years ago. After the Aftonian interglacial period, the Kansan Glaciation began about 900,000 years ago and ended about 600,000 years ago. Following the Yarmouthian Interglacial, the Illinoian Glaciation began about 600,000 years ago and ended 100,000 years ago. After the Sangamonian interglacial period, the Wisconsonian Glaciation began about 100,000 years ago and ended only recently (about 10,000 years ago). The last interglacial period lasted 11,000 years and the prior interglacial period lasted 16,000 years. Since it has been 10,000 years since the last glaciation ended, the current period might be another interglacial period which will end in another Ice Age. The alternation between glacial and interglacial periods is largely due to variations in the earths orbit (which changes regularly from elliptical to circular over a period of about 100,000 years), the tilt of the earths axis (which changes over a period of about 41,000 years), and the variation of the time period in which the earth is closest to the sun (the precession of the equinox) (Kurten, 1988).
During the last interglacial period (130 to 116 thousand years ago), there was less ice on earth than there is today. During the glacial and interglacial periods, more than 20 abrupt climatic changes have been recorded. For example, the Dansgaard-Oeschger events included an 8 degree Celsius temperature increase in Greenland over a few decades. The Heinreich events were rapid periods of warming in which ice sheet melting in the North Atlantic led to sea level rises of up to 15 meters and dramatic changes of the major currents. Sea level during the last glacial maximum was 140 feet below modern sea level. About 8,200 years ago, a sudden drop of temperature of 2 to 6 degrees Celsius apparently resulted from a reorganization of the Atlantics MOC following the massive release of 10 14 cubic meters of water from melting glaciers in the Hudson Bay area (the postglacial Lake Agassiz) (IPCC, Document I, 2007).
The earths climate of the period prior to 2.6 million years ago was typically warmer than it is today. Three million years ago, the temperature is estimated to have been 2 to 3 degrees warmer than modern times (IPCC, Document I, 2007).
More than fifty New World sites contain remains of humans which have been dated as older than 12,000 years and some have been dated at 32,000 to 40,000 years. From the period of 12,000 to 8,000 years ago, more than 25 species of large herbivorous mammals became extinct. Overhunting and the climate change associated with the end of an Ice Age were probably both contributing factors. A number of mammoth kill sites are known in North America. Several examples of mastodon hunting, in both North and South America, are also known (Kurten, 1988).
Because of modern human population growth and the resulting loss of habitat,
the modern mass extinction will soon be ranked as the second worst in
earth's history, greater than that which ended the dinosaurs although
not as severe as the end-Permian extinctions.