Chapter 101: Age of mammoth |
After this follows the Neogene Period. The Neogene encompasses two epochs, beginning with the Miocene (23.03-5.33 Mya) and followed by the Pliocene (5.33-1.806 Mya).
The Pleistocene (the "Ice age", 1.806-0.0115 Mya) and the current epoch, the Holocene, beginning eleven thousand five hundred years ago are now (2009) included in the Quaternary Period.
Though traditionally the Holocene is treated separately, it may in fact just be the latest interglacial of the Pleistocene. This display includes the Miocene through the Pleistocene Epochs of the Neogene and Quaternary Periods.
The Neogene Period started with the replacement of vast areas of forest by grasslands and savannahs.
New food sources and niches on the grasslands and savannahs fostered further evolution of mammals and birds. Whales diversified in the seas, and sharks reached their largest size during the Miocene.
Complex patterns of mammalian evolution resulted from changing climates and continental separations. More modern mammals evolved as grasslands became widespread and the climate cooled and dried.
Additional information about the mammals of these epochs can be found in our Prehistoric Mammals of the Cenozoic exhibits.
The Neogene saw a gradual closing of the Tethys Sea as the continents moved into their modern positions. The dramatic cooling phases of the Neogene lead to more distinctive latitudinal biotic zones.
The Miocene comprised most of the Neogene Period making it the second longest Epoch of the Cenozoic Era. Wide expanses of grasslands formed across the Northern Hemisphere and supported a variety of new types of mammals. Horses moved from browsing in forests and meadows to grazing (eating grass) in grasslands. In the oceans the first known kelp forests appeared.
The new circulation patterns in turn lead to the evolution and spread of diverse marine mammals including a variety of toothed and baleen whales, sea lions, seals, walruses and sea cows. Non-mammalian predators included marine crocodiles and the largest known shark, Carcharodon megalodon.
Marine invertebrates were similar to today, in fact half of the species are unchanged.The Miocene began with a short warming, followed by a return to the general Cenozoic cooling trend. The once great Tethys ocean was reduced to the Mediterranean Sea and closed at both ends, bringing the circumglobal circulation of warm waters to an end.
The mural above emphasizes the variety of mammals that evolved to occupy the grasslands and savannahs of North America in the Miocene epoch. On the far left are the giant (8' at the shoulder) perrisodactyl Moropus, a clawed herbivore related to horses, confronting Daphoenodon dogs. In front of them are a group of ruminant oreodonts (Merychyus).
Next to them in the foreground is a herd of extinct horses, Parahippus, that was evolving from a browsing habit to a grazing (eating grass) one.
A group of semiaquatic Promerycochoerus are in the background and behind them a Daeodon, a large (12 ft long) pig-like scavenger or predator. Finally, on the right of this panel is a group of the small camelid Stenomylus.
A dramatic event at the beginning of the Pliocene was the catastrophic filling of the Mediterranian sea. The collisions of the African and Eurasian continents during the Miocene had closed the Mediterannian basin both in the east and at the Straits of Gibraltar resulting in the basin drying up and converting to grasslands.
When the barrier on the western end was breached it refilled catastrophically from the Atlantic ocean. Also during the Pliocene the Panamanian bridge was formed between North and South America allowing for the migration of animals both north and south in what is known as the "Great American Faunal Interchange."
Giant ground sloths, armadillos and marsupials among others came north, while cats, dogs, bears, camels and others went south. Many of the South American species were replaced by northern species and eventually went extinct. Bringing the two continents together also stopped the exchange between the Caribbean and the Pacific allowing these faunal provinces to evolve apart.
In Africa early hominids appear for the first time in the fossil record. Famous hominid fossils such as "Lucy" a female Australopithecus aferensis and footprints from a pair of hominids alive 3Ma have been found in Pliocene deposits in Africa. Marine and freshwater aquatic Pliocene fossils are now numerous around the globe.
Grasslands and savannahs continued to be commonplace as shown in the mural above emphasizing common Pliocene American mammals. In the background a herd of gomphotheres, Amebelodon (a primitive proboscidian with flattened shovel-like lower tusks) amble across the plain next to extinct grazing rhinoceros, teloceres. In the foreground three extinct deer-like artiodactyls with multiple horns, (Cranioceras) are seen next to a pair of Synthetoceras, artiodactyls with large forked horns on the snout. On the ground in front of them are a group of horned gophers (Ceratogaulus) the smallest known horned mammal and the only known horned rodent. A prehistoric horse can also be seen at the upper right.
Modern humans including Homo erectus, H. neanderthalensis and H. sapiens evolved during the Pleistocene. The first recorded migrations of humans out of Africa occurred during this epoch.
At least 20 cycles of glaciations occurred during the Pleistocene, with some glaciations covering up to 30% of Earth's surface. North American ice sheets in the Wisconsin glaciation of 18,000 year ago were over 3900 meters (2.4 miles) thick.
These and other massive ice sheets tied up so much water that sea levels dropped 140 meters. Many large mammals went extinct near the end of the Pleistocene (~11,000 years ago), leaving our modern flora and fauna.
Though controversial, a widely held theory explains these extinctions as a result of human hunters in both Europe and the Americas.
Now it was time for the beginning of the Pilocence Epoch or the age of Mamoths.
The Pliocene is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch.
As with other older geologic periods, the geological strata that define the start and end are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The boundaries defining the Pliocene are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Miocene and the relatively cooler Pleistocene. The upper boundary was set at the start of the Pleistocene glaciations.
In the official timescale of the ICS, the Pliocene is subdivided into two stages. From youngest to oldest they are:
Piacenzian (3.600–2.588 Ma)
Zanclean (5.332–3.600 Ma)
The Piacenzian is sometimes referred to as the Late Pliocene, whereas the Zanclean is referred to as the Early Pliocene.In the system of
North American Land Mammal Ages (NALMA) include Hemphillian (9–4.75 Ma), and Blancan (4.75–1.806 Ma). The Blancan extends forward into the Pleistocene.
South American Land Mammal Ages (SALMA) include Montehermosan (6.8–4.0 Ma), Chapadmalalan (4.0–3.0 Ma) and Uquian (3.0–1.2 Ma).
In the Paratethys area (central Europe and parts of western Asia) the Pliocene contains the Dacian (roughly equal to the Zanclean) and Romanian (roughly equal to the Piacenzian and Gelasian together) stages. As usual in stratigraphy, there are many other regional and local subdivisions in use.In Britain the Pliocene is divided into the following stages (old to young): Gedgravian, Waltonian, Pre-Ludhamian, Ludhamian, Thurnian, Bramertonian or Antian, Pre-Pastonian or Baventian, Pastonian and Beestonian. In the Netherlands the Pliocene is divided into these stages (old to young): Brunssumian C, Reuverian A, Reuverian B, Reuverian C, Praetiglian, Tiglian A, Tiglian B, Tiglian C1-4b, Tiglian C4c, Tiglian C5, Tiglian C6 and Eburonian. The exact correlations between these local stages and the ICS stages is still a matter of detail.
The global average temperature in the mid-Pliocene (3.3–3 mya) was 2–3 °C higher than today,global sea level 25 m higher and the Northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma. The formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios and ice-rafted cobbles in the North Atlantic and North Pacific ocean beds. Mid-latitude glaciation was probably underway before the end of the epoch. The global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas.
Continents continued to drift, moving from positions possibly as far as 250 km from their present locations to positions only 70 km from their current locations. South America became linked to North America through the Isthmus of Panama during the Pliocene, making possible the Great American Interchange and bringing a nearly complete end to South America's distinctive large marsupial predator and native ungulate faunas. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters dropping temperatures in the now-isolated Atlantic Ocean.
Africa's collision with Europe formed the Mediterranean Sea, cutting off the remnants of the Tethys Ocean. The border between the Miocene and the Pliocene is also the time of the Messinian salinity crisis.
Sea level changes exposed the land-bridge between Alaska and Asia.
Pliocene marine rocks are well exposed in the Mediterranean, India, and China. Elsewhere, they are exposed largely near shores.