Pleistocene megafauna
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| Quaternary Period |
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| ↑ Neogene |
Pliocene
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Pleistocene megafauna is the set of species of large animals — mammals, birds and reptiles — that lived on Earth during the Pleistocene epoch and are now extinct. These species appear to have died off as humans expanded out of Africa and Eurasia, the only continents that still retain a diversity of megafauna comparable to what was lost. Three theories have been given for these extinctions: hunting by the spreading humans, climatic change, and spreading disease. A combination of those explanations could be correct.
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[edit] Hunting
This theory holds Pleistocene humans responsible for the megafaunal extinction. One variant, often referred to as overkill, portrays humans as hunting the megafauna to extinction within a relatively short period of time. Some of the direct evidence for this includes: fossils of megafauna found in conjunction with human remains, embedded arrows and tool cut marks found in megafaunal bones, and cave paintings that depict such hunting. Biogeographical evidence is also suggestive; the areas of the world where humans evolved currently have more of their Pleistocene megafaunal diversity (the elephants and rhinos of Asia and Africa) compared to other areas such as Australia, the Americas, Madagascar and New Zealand, areas where early humans were non-existent. Based on this evidence, a picture arises of the megafauna of Asia and Africa evolving with humans, learning to be wary of them, and in other parts of the world the wildlife appearing ecologically naive and easier to hunt. This is particularly true of island fauna, which display a dangerous lack of fear of humans.
Circumstantially, the close correlation in time between the appearance of humans in an area and extinction there provides weight to this theory. This is perhaps the strongest evidence, as it is almost impossible that it could be coincidental when science has so many data points. For example, the woolly mammoth survived on islands despite worsening climatic conditions for thousands of years after the end of the last glaciation, but they died out when humans arrived around 1700 BC. The megafaunal extinctions covered a vast period of time and highly variable climatic situations. The earliest extinctions in Australia were complete approximately 30,000 BP, well before the last glacial maximum and before rises in temperature. The most recent extinction in New Zealand was complete no earlier than 500 BP and during a period of cooling. In between these extremes megafaunal extinctions have occurred progressively in such places as North America, South America and Madagascar with no climatic commonality. The only common factor that can be ascertained is the arrival of humans. [Martin, Paul S., Twilight of the Mammoths - Ice Age Extinctions and the Rewilding of America, 2005.]
[edit] Climate change
An alternative explanation offered by many scientists is that the extinctions were caused by climatic change following the last Ice Age. Critics object that since there were multiple Ice Ages in the evolutionary history of many of the megafauna, it is rather implausible that only after the last glacial would there be such extinctions.
Some evidence weighs against this theory as applied to Australia. It has been shown that the prevailing climate at the time of extinction (40,000–50,000 BP) was similar to that of today, and that the extinct animals were strongly adapted to an arid climate. The evidence indicates that all of the extinctions took place in the same short time period, which was the time when humans entered the landscape. The main mechanism for extinction was likely fire (started by humans) in a then much less fire-adapted landscape. Isotopic evidence shows sudden changes in the diet of surviving species, which could correspond to the stress they experienced before extinction [1] [2] [3].
[edit] Hyperdisease
Some research has looked at the possibility a pandemic or hyperdisease could have caused the megafaunal extinction.[1] According to this theory, large mammals were particularly vulnerable because they were fewer in number than smaller species. Body size and population size tend to be inversely related (because larger species require more food per individual to survive).
Diseases imported by people have been responsible for extinctions in the recent past; for example, bringing avian malaria to Hawaii has had a major impact on the isolated birds of the island. MacPhee is searching DNA in mammoth remains from Wrangel Island in Siberia; he hopes to find evidence of infection.
One suggestion is that pathogens were transmitted by the expanding humans via the domesticated dogs they brought with them.[citation needed] Unfortunately for such a theory it can not account for several major extinction events, notably Australia and North America. Dogs did not arrive in Australia until approximately 35,000 years after the first humans arrived and approximately 30,000 years after the megafaunal extinction was complete and as such can not be implicated. In contrast numerous species including wolves, mammoths, camelids and horses had emigrated continually between Asia and North America over the past 100,000 years. For the disease hypothesis to be applicable in the case of the Americas it would require that the population remain immunologically naive despite this constant transmission of genetic and pathogenic material.
[edit] Combined factors
Some experts claim that various combinations of these factors are responsible for the extinctions. For example, while it is certain in the case of the moa and some other species that hunting was primarily responsible, other species may have been 'pushed over the edge' by hunting when they were already struggling due to imported diseases or climate change.
[edit] Megafauna and legend
Throughout history, humans wondered about the large fossilized teeth and bones they discovered from time to time. During the classical age, the bones of giants were reputed to have been found. In medieval times, fossils were attributed to fantastic beasts such as dragons and unicorns. The Sioux have legends of Thunder Beasts, and the Aztec believed in giants.
With modern tools, paleontologists have reconstructed a world of beasts as amazing as the dragons and unicorns of folklore, including many larger versions of contemporary mammal, reptile and bird families. Scientists and historians, who long dismissed legends of monstrous creatures as merely myths, are beginning to look at these stories in a new light. The sites where the Greeks claimed to find the bones of giants today produce the bones of mammoths. Similar finds have been made in Central America. Since humans coexisted with these animals, some researchers wonder if the beasts might also be remembered in legend. For example the Australian Dreamtime legends refer to the megafaunal species. The mythical Rainbow serpent, source of creation, water and destruction, may be based on the extinct snake Wonambi naracoortensis. Fossils of this species are deposited with opal, producing a reflected colour spectrum. The Bunyip may be the Diprotodon, filtered through legend. Clearly it is difficult to translate the poetic vocabulary of myth into the technical language of paleontology.
[edit] By region
[edit] North America
Pleistocene fauna in the Americas included giant sloths, short faced bears, California tapirs, two species of now extinct llamas, peccaries, the American Lion, giant condors, Miracinonyx (American "cheetahs", not true cheetahs), saber-toothed cats (like the scimitar cat), dire wolves, saiga, camelops, at least two species of bison, stag-moose, horses, mammoths and mastodons, and giant beavers. In contrast today the largest North American land mammal is the American Bison.
[edit] South America
South American wildlife in the Pliestocene varied greatly, from the birds such as Phorusrhacos, to the giant ground sloth, Megatherium. The continent also had quite a few grazers such as Doedicurus, Macrauchenia, 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.
[edit] Australia
Australia was characterized by marsupials, monotremes, crocodilians, testudines and monitors and numerous large flightless birds. Pleistocene Australia supported large carnivorous 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.
[edit] Eurasia
As with South America, elements of North American fauna could be found in Eurasia. Among the most recognizable Eurasian species are the Woolly Mammoth, Aurochs, Cave Lion, Cave Bears, Irish Elk and woolly rhinoceros.
[edit] Insular
Many islands had unique megafauna that went extinct on the arrival of humans. These included giant bird forms in New Zealand such as the moas and the Harpagornis (giant eagle); Archaeoindris, a gorilla-sized lemur, two species of hippopotamus and the gigantous Aepyornis in Madagascar; dwarf Stegodon from Island of Flores; land turtles and crocodiles in New Caledonia; and giant geese and moa-nalo (giant ducks) in Hawaii.
[edit] References
- ^ MacPhee, Ross D.E.; Preston Marx (1997). "The 40,000-year Plague: Humans, Hyperdisease, and First-Contact Extinctions". Natural Change and Human Impact in Madagascar: 169-217, Washington, D.C.: Smithsonian Institution Press.
