Pleistocene Extinctions - The Pleistocene or Ice Age Extinction Event

The Pleistocene or Ice Age Extinction Event

The Late Pleistocene extinction event saw the extinction of many mammals weighing more than 40 kg.

• In North America around 33 of 45 genera of large mammals became extinct.
• In South America 46 of 58
• In Australia 15 of 16
• In Europe 7 of 23
• In Subsaharan Africa only 2 of 44

The extinctions in the Americas entailed the elimination of all the larger (over 100 kg) mammalian species of South American origin, including those that had migrated north in the Great American Interchange. Only in North America, South America and Australia did the extinction occur at family taxonomic levels or higher.

There are two main hypotheses concerning the Pleistocene extinction:

• The animals died off due to climate change associated with the advance and retreat of major ice caps or ice sheets.
• The animals were exterminated by humans: the "prehistoric overkill hypothesis" (Martin, 1967).

There are some inconsistencies between the current available data and the prehistoric overkill hypothesis. For instance, there are ambiguities around the timing of sudden extinctions of Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa and South or Southeast Asia, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval.

Evidence supporting the prehistoric overkill hypothesis includes the persistence of certain island megafauna for several millennia past the disappearance of their continental cousins. Ground sloths survived on the Antilles long after North and South American ground sloths were extinct. The later disappearance of the island species correlates with the later colonization of these islands by humans. Similarly, woolly mammoths died out on remote Wrangel Island 7000 years after their mainland extinction. Steller's sea cows also persisted off the isolated and uninhabited Commander Islands for thousands of years after they vanished from continental shores of the north Pacific.

Alternative hypotheses to the theory of human responsibility include climate change associated with the last glacial period and the Younger Dryas event, as well as Tollmann's hypothetical bolide, which claim that the extinctions resulted from bolide impact(s). Simplified versions of such hypotheses would predict an instantaneous, regional extinction(s), and thus cannot account for the planet-wide species losses that occurred over an interval of thousands of years. However in the case of such impact(s) on the North American ice sheet, for example, there is growing evidence to suggest that the initial post-impact cooling from blocked sunlight may have been compounded by adverse effects on the Thermohaline circulation by a substantial impulse of fresh water to the ocean(s) through a massive combination of runoff and precipitation from that impact.

Such a scenario has been proposed as a contributing cause of the 1,300 year cold period known as the Younger Dryas stadial. This impact extinction hypothesis is still in debate due to the exacting field techniques required to extract minuscule particles of extra terrestrial impact markers such as Iridium at a high resolution from very thin strata in a repeatable fashion, as is necessary to conclusively distinguish the event peak from the local background level of the marker. The debate seems to be exacerbated by infighting between the Uniformitarianism camp and the Catastrophism camp.