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.