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Pleistocene megafauna is the set of species of large animals that lived on Earth during the Pleistocene epoch and became extinct in a Quaternary extinction event. These species appear to have died off as humans expanded out of Africa and southern Asia, the only continents that still retain a diversity of megafauna comparable to what was lost. The Americas, northern Eurasia, Australia and many larger islands lost the vast majority of their larger and all of their largest mammals. Four theories have been advanced as likely causes of these extinctions: hunting by the spreading humans, climatic change, spreading disease, and an impact from an asteroid or comet. These factors are not necessarily exclusive: two or more may have combined to cause the extinctions.
Pleistocene fauna in North America included giant sloths; short-faced bears; several species of tapirs; peccaries (Including the long nosed and flat-headed peccaries); the American lion; giant condors; Miracinonyx ("American cheetahs", not true cheetahs); saber-toothed cats like Smilodon and the scimitar cat, Homotherium; dire wolves; saiga; camelids such as two species of now extinct llamas and Camelops; at least two species of bison; stag-moose; the shrub-ox and Harlan's muskox; 14 species of pronghorn (of which 13 are now extinct); horses; mammoths and mastodons; the beautiful armadillo and the giant armadillo-like Glyptotherium  and giant beavers as well as birds like teratorns and giant tortoises. The nine-foot sabertooth salmon lived at the time as well. In contrast, today the largest North American land animal is the American bison.
South American wildlife in the Pleistocene varied greatly, an example is the giant ground sloth, Megatherium. The continent also had quite a few grazers and mixed feeders such as the camel-like litoptern Macrauchenia, Cuvieronius, Stegomastodon, Doedicurus, Glyptodon, Hippidion and Toxodon. The main predator of the region was Smilodon, which crossed the land bridge between North and South America one million years ago.
Australia was characterized by marsupials, monotremes, crocodilians, testudines, monitors and numerous large flightless birds. Pleistocene Australia also supported the large Short-faced kangaroo (Procoptodon goliah), Diprotodon (a giant wombat), the Marsupial Lion (Thylacoleo carnifex), the flightless birds Genyornis and Dromornis, the 5-meter snake Wonambi and the giant lizard, the megalania.
As with South America, some elements of Eurasian megafauna were similar to those of North America. Among the most recognizable Eurasian species are the woolly mammoth, steppe mammoth, straight-tusked elephant, aurochs, steppe bison, cave lion, cave bear, Cave hyena, Homotherium, Irish elk, giant polar bears, woolly rhinoceros, Merck's rhinoceros, narrow-nosed rhinoceros, and Elasmotherium. In contrast today the largest European land mammal is the European bison or Wisent.
Many islands had unique megafauna that went extinct upon the arrival of humans more recently (over the last few millennia and continuing into recent centuries). These included giant bird forms in New Zealand such as the moas and Harpagornis (a giant eagle); giant lemurs, including Megaladapis and Palaeopropithecus species and Archaeoindris, a gorilla-sized lemur, two species of hippopotamus, a Giant tortoise, the Voay-crocodile and the gigantic Aepyornis in Madagascar; various Giant tortoise species from the Mascarenes, a dwarf Stegodon on Flores and a number of other islands; dwarf woolly mammoths on Wrangel Island and St. Paul Island; land turtles and crocodiles in New Caledonia; giant owls and dwarf ground sloths in the Caribbean; giant geese and moa-nalo (giant ducks) in Hawaii; and dwarf elephants and dwarf hippos from the Mediterranean islands.