Global Warming and the Extinction of the Ice Age Mammals

Russell Graham (Denver Museum of Nature and Science, Denver, Colorado, USA)

[now at the Earth and Mineral Sciences Museum at Penn State University]

Ohio Historical Society, Columbus, Ohio, USA

6 March 2001


~32 genera of mammals went extinct in North America at the end of the Pleistocene.  In fact, an end-Pleistocene extinction is seen in large land mammals on every continent except Africa.  Africa is the only continent where we don’t see a major mass extinction of mammals at the end of the Pleistocene.

Included in the Pleistocene large mammal fauna was the mastodon, weighing 4-5 tons, standing 8-10’ at the shoulder.  Also had the Columbia Mammoth, Jefferson’s Mammoth, and the Woolly Mammoth.

The mammoth and mastodon were 2 proboscideans that engaged in niche partitioning.  The mammoth was a grass eater, and lived in open environments - it had teeth designed for grinding grass over large surface areas.  The mastodon was a browser, eating leaves & twigs & aquatic vegetation, and was more of a forest dweller.


Other members of the extinct Pleistocene mammal fauna included the 1-ton ground sloth, of which there are 4 types in the North American Pleistocene.  This group was more diverse in the Central and South American Pleistocene.  Also the extinct muskox, the woodland muskox - a Pleistocene form different from the modern muskox, which is an open environment animal, and needs a cold environment to survive.  Antelopes and peccaries and four types of horses were also in the North American Pleistocene.  Horses originally evolved in North America, then migrated to the Old World, and went extinct in North America.  The horse was re-introduced in NA in historical times by the Spanish.  Camels, llamas, and tapirs are also in the NA Pleistocene.


There were several NA Pleistocene carnivores - the lion (probably the most widely distributed predator of all time; lion tracks are known in a cave from Missouri), two types of saber-tooth cat (the normal La Brea type saber-tooth, as well as the scimitar tooth, with smaller sabers); bears, including the short-faced bear, which was larger than the modern grizzly bear; and the dire wolf, which was larger than the modern gray wolf.


Bison - an example of a NA Pleistocene herbivore that did survive, but a 7’ spread of horns is known in Pleistocene forms.

Also had the giant beaver (which got as big as a modern black bear).

Rodents and shrews didn’t go extinct, but they changed their geographic distributions.

The end of the Pleistocene was a significant mammal event.  Several causes have been proposed for it:

1) human overkill model

2) climate change model

3) disease from Old World humans/dogs

4) keystone species model


The human overkill idea has considerable evidence behind it - Clovis culture artifacts are often associated with mammoths and mastodons, etc.  But, did human hunting actually result in extinction?  Some say yes.  Graham prefers the climate change idea, suggesting that the global warming that occurred at the end of the Pleistocene is the primary mechanism behind the extinction.

Giant sequoias were widespread in the Tertiary, but they are now limited to California, occurring only in a few protected (i.e., national parks) and non-protected areas in the Sierra Nevada foothills.

There are 2 models for how communities respond to climate change: 1) community unit model; 2) individual model.

Which response reflects what happened at the end of the Pleistocene in North America?


Did GIS and data compilation from the literature from paleontological and archaeological records for the last 40,000 years.  Seeing support for the individual model.  Saw some non-analog associations.  Why non-analog associations occur in the fossil record may be due to biological reasons (a true association that doesn’t have a modern analog) or due to non-biological reasons (mixing of fossils in deposit, rapid environmental change with accompanying rapidly changing fauna, etc.).


Modern dating methods are highly precise.  Can now date individual teeth (low quantity of material needed).  Also, modern dating methods result in very small error bars (Ex: 15,000 ± 60 years).  Dating of non-analog associations at Pleistocene fossil sites show that many of these are true biological associations (creatures living together in the same environmental conditions that do not live together now).  Conclusion: there were environments present in the North American Pleistocene that do not exist anymore.

We are seeing some real mammal non-analog associations.  Vegetation studies show non-analog floras also.


The Pleistocene boreal forest environment - dense coniferous forests, with little understory vegetation.  Few animals can survive on conifers (think about the tastiness of turpentine!).  The boreal forest today has a low diversity and a low biomass.  Pleistocene boreal forests had a higher diversity, because they were more open, with more low-growing vegetation for larger mammals to eat.  Pleistocene “open” boreal forests do exist today, but they are rare and occur in scattered areas only of parts of Newfoundland and Nova Scotia.


There were more grasslands/open forests in eastern North America during the Pleistocene.  “Prairie Peninsula” - the bulge in modern (late Holocene) prairie percentage maps near Chicago.  This Prairie Peninsula extended into Ohio during the early Holocene.


Wrangell Island Mammoths (off Siberia) - dated (confirmed & reconfirmed) to 4000 years (!!), the time of the pharaohs!

Mammoths/mastodons/short-faced bears were the few large Pleistocene mammals living past 11 k.y., living past the time when most Pleistocene mammals went extinct.  Most Clovis associations are with mammoth and mastodon remains, but not very much with other animals.  Clovis people may have pushed the mammoth and mastodon over the edge of extinction, but most Pleistocene mammals were gone by the time of the Clovis peoples (~10,800 to 10,950 yrs).


Of the other two proposals for Pleistocene extinctions, the disease model (from humans and dogs migrating from the Old World, introducing diseases to New World animals) is not testable.  The keystone species model requires the Pleistocene mammals to go extinct after the keystone species (= mammoths/mastodons).  The data show the opposite pattern.  So, this idea has been falsified.



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