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Pliocene

The '''Pliocene''' epoch (geology)|epoch (spelled '''Pleiocene''' in some older texts) is the period in the geologic timescale that extends from 5.332 million to 1.806 million years before present. The Pliocene is the second epoch of the Neogene period in the Cenozoic era. The Pliocene follows the Miocene epoch and is followed by the Pleistocene epoch. The Pliocene was named by Sir Charles Lyell. The name comes from the Greek words (''pleion'', "more") and (''kainos'', "new") and means roughly "continuation of the recent", referring to the essentially modern marine mollusc faunas. As with other older geologic periods, the Stratum|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 onset of the Pliocene are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Miocene and the relatively cooler Pliocene. The upper boundary was set at the start of the Pleistocene glaciations. A recent proposal for a revision in the geologic timescale has the Pleistocene beginning at 1.8 million years ago,Gradstein ''et al.'' (2004) the proposal is however heavily disputed. Astronomer Narciso Benítez of Johns Hopkins University and his team suggest that a supernova is a plausible but unproven candidate for the marine extinctions that characterize the Pliocene-Pleistocene boundary, by causing a significant breakdown of the ozone layer.

Subdivisions

The Pliocene faunal stages from youngest to oldest according to International Commission on Stratigraphy|ICS classification are: Africa was dominated by hoofed animals, and primates continued their evolution, with australopithecines (some of the first Hominidae|hominids) appearing in the late Pliocene. Rodents were successful, and elephant populations increased. Cows and antelopes continued diversification and overtaking pigs in numbers of species. Early giraffes appeared, and camels migrated via Asia from North America. Horses and modern rhinos came onto the scene. Bears, dogs and weasels (originally from North America) joined cats, hyaenas and civets as the African predators, forcing hyaenas to adapt as specialized scavengers. South America was invaded by North American species for the first time since the Cretaceous, with North American rodents and primates mixing with southern forms. Litopterns and the Notoungulata|notoungulates, South American natives, did well. Small weasel-like carnivorous mustelids and coatis migrated from the north. Grazing glyptodonts, browsing giant ground sloths and smaller armadillos did well. The marsupials remained the dominant Australian mammals, with herbivore forms including wombats and kangaroos, and the huge diprotodonts. Carnivorous marsupials continued hunting in the Pliocene, including dasyurids, the dog-like thylacine and cat-like ''Thylacoleo''. The first rodents arrived, while bats did well, as did ocean-going whales. The modern platypus, a monotreme, appeared.

Birds

The predatory South American phorusrhacids were rare in this time; among the last was ''Titanis'', a large phorusrhacid that migrated to North America and rivaled mammals as top predator. Its distinct feature was its claws, which had re-evolved for grasping prey, such as ''Hipparion''. Other birds probably evolved at this time, some modern, some now extinct.

Reptiles

Alligators and crocodiles died out in Europe as the climate cooled. Venomous snake genera continued to increase as more rodents and birds evolved.

Oceans

Oceans continued to be relatively warm during the Pliocene, though they continued cooling. The Sea ice|Arctic ice cap formed, drying the climate and increasing cool shallow currents in the North Atlantic. Deep cold currents flowed from the Antarctic. The formation of the Isthmus of Panama about 3.5 million years ago cut off the final remnant of what was once essentially a circum-equatorial current that had existed since the Cretaceous and the early Cenozoic. This may have contributed to further cooling of the oceans worldwide. The Pliocene seas were alive with sea cows, Pinniped|seals and sea lions.

Supernovae

In 2002, astronomers discovered that roughly 2 million years ago, around the end of the Pliocene epoch, a group of bright Stellar classification|O and B stars called the Scorpius-Centaurus Association|Scorpius-Centaurus OB association passed within 150 light-years of Earth and that one or more supernovae may have occurred in this group at that time. Such a close explosion could have damaged the Earth's ozone layer and caused the extinction of some ocean life (consider that at its peak, a supernova of this size could have the same absolute magnitude as an entire galaxy of 200 billion stars).Comins & Kaufmann (2005), p. 359.