Cochise College             Student Papers in Geology     
Geology Home Page                     physical geology  historical geology  planetary  gems
Roger Weller, geology instructor
wellerr@cochise.edu       


Pleistocene Extinctions
Simara Hubble

Physical Geology
Spring 2006

The Pleistocene Extinction

 

Visions of mastodons, mammoths and saber-tooth tigers, large mammals grouped under the title of mega fauna have inspired the human imagination since their exciting discovery. These creatures have long been gone from our Earth. They vanished quickly, in terms of geological time, at the end of the Pleistocene epoch. This epoch began approximately 1.6 million years ago and ended around 10,000 years later, preceding the Holocene Crecent epoch which is considered to be the Age of Man. By that time, more than two thirds of mega fauna in North America had become extinct (11). Mega fauna are defined loosely as animals over 500 kg, 100 pounds or anything larger than the current largest domesticated animal we see today.

 

The claw of a giant sloth. This ground-dwelling creature was, at its maximum, as big as the modern day elephant.

Photo courtesy Roger Weller/Cochise College  

            Specific species of mega fauna have captured the attention of the majority, the saber-toothed tiger and the wooly mammoth for example. However there was actually an incredible variety of giant creatures, incomprehensible by our perception of animals in the current world. Those that were not immense in size had distinctive features not shared by the smaller mammals of the Holocene epoch, such as the large serrated teeth of the saber toothed tiger. This Pleistocene extinction is not considered to be one of the Big 5 extinction events, because the number of extinct species is not as high. In fact, the Pleistocene extinction is lumped with the Holocene Crecent as part of the Holocene extinction event, which is seeing the continuing demise of many creatures (19). The last ice age extinction seems to have been confined, for the most part, to mammals. Below is a list of some of the immense creatures lost to the present day world.


















Image courtesy of The Illinois State Museum     www.museum.state.il.us

 

Herbivores

Megatherium

(Giant ground sloth)

20 feet long (6 m.)

Megalocerous giganteus

(Giant Irish elk)

7 feet high (2.1 m.)

Mammuthus primigenius

(Wooly mammoth)

11.5 feet long (3.5 m.)

Mammut americanum

(American mastodon)

8-10 feet high (2.5 - 3 m.)

Glyptodon

(Ancient armadillo)

10 feet long (3 m.)

Ursus spelaeus

(Cave bear) Omnivore

20 feet long

DoediFcurus

(Large armadillo)

13 feet long (4 m.)

Coelodonta

(Wooly rhinoceros)

11 feet long (3.5 m.)

Castoroides ohioensis

(Giant beaver)

3.3ft high (1 m.)

Camelops hesternus

(American camel)

12 feet high (3.6 m.)

 

Carnivores

Arctodus simus

(Short faced bear)

5.5 feet high (1.7 m.)

Smilodon fatalis

(Saber-toothed cat)

4-5 feet long (1.2-1.5 m.)

Canis dirus

(Dire wolf)

5 feet high (1.5 m.)

Ursus spelaeus

(Cave bear) Omnivore

20 feet long

Panthera leo spelaea

(Cave lion)

11.5 feet long (3.5 m.)

 

 

            According to scientific theory based on geologic evidence, the Earth has been slowly coming out of an Ice Age for the past 18,000 years. Geologist believe, based on glacial deposits and age dating of extracted ice cores, that the Earth has gone through many cycles of alternating glacial and interglacial periods over millions of years. When the polar ice caps expand, portions of the earth become glaciated, covered in large continental ice sheets. During the Pleistocene epoch, which began approximately 1.6 million years ago, the earth saw several such periods, and the expansion of large ice sheets, some which still exist today in places such as Greenland. These periods of interchanging temperature are partly attributed to Milankovitch cycles. The most recent period of glaciation, which peaked at about 18,000 years ago, preceded the demise of several legendary creatures.

 

Scotese,  C. R., 2001.   Atlas of Earth History, Volume 1, Paleogeography,

PALEOMAP Project, Arlington, Texas, 52 pp.

www.scotese.com

 

Scotese,  C. R., 2001.   Atlas of Earth History, Volume 1, Paleogeography,

PALEOMAP Project, Arlington, Texas, 52 pp.

www.scotese.com

 

            The widespread extinction of several species of mega fauna at the end of the Pleistocene epoch, around 12,000 years ago, has been attributed to several causes.  

            The first hypothesis is that the pressure of climactic changes forced some species into extinction. Through causes unknown, the Earth began to warm again after the last period of polar cap expansion. As the continental glaciers melted, the larger animals were seemingly unable to adapt to the changing environment fast enough. However, the flaw in this concept lies within the geologic evidence of both the previous landscape and these creatures. Over the thousands, sometimes million years of these species’ reign, it appears that they were well able to adapt to previous climactic changes. The Pleistocene epoch itself was, as stated earlier, marked by periods of expanding and contracting ice sheets. As one source observes “… the problem still remains as to why such an inability to respond (to selective pressures) should suddenly arise in hundreds of thousands of animals of different categories and all in the same geological time frame” (8).   

















 

 

 

 

           

                                        Image courtesy of The Illinois State Museum     www.museum.state.il.us

           The idea that climactic changes were the primary cause is also weakened by the hypothesis of increased human hunting, or “overkill.” According to Amos Esty, as the Homo sapiens population grew in size, spreading throughout the continents, the accompanying range of the mega fauna shrunk accordingly (10). This is in opposition to the idea that climactic conditions drove the extinction, because in such a scenario, Homo sapiens and mega fauna would have been able to coexist in the same area. According to the study conducted under the overkill hypothesis, fossils were found along the boundary between the human population and the mega fauna (10).

 

            Basically this implies that the advancement of a Homo sapiens species and their ability to prey on these large mega fauna drove the creatures rapidly to extinction. The initial basis for this theory is the evidence that at about the same time the larger mammals were disappearing, Homo sapiens were spreading across the globe from Africa (10). In North America, where the ice sheet reached down to present day Ohio, animals and humans alike were able to cross over from Asia because the ice also formed a bridge with which to cross the Bering Strait. However, there is not conclusive evidence for this hypothesis in every area in which a species of mega fauna became extinct. While evidence is strong that it may have been a factor for those mega fauna indigenous to North America, there is very little evidence for such an occurrence in places like Australia. There is also conflicting evidence showing that humans and mega-fauna may have coexisted for several thousands of years. Additionally, according to another study in Siberia, it has been shown that the fossils mastodons found there do not show any indication of “butchery or dismemberment” (11). 

            The offspring of the overkill hypothesis is the second order overkill hypothesis. The proposal offered here is that as the New World was populated by mankind, the large carnivores were the primary prey. This in turn led to a dramatic increase of herbivorous mega fauna, which in turn depleted the food supply for these creatures. Since the largest herbivorous mega-fauna are presumed to have been browsers, such as the giant sloth which would browse the tops of trees, they were the first to be affected. Eventually grazing creatures were affected. This overall decrease in available prey, both herbivores and carnivores, in turn affected the human population (1). The strengths offered by this theory is that it explains the disappearance of animals that were not hunted by homo-sapiens and it also explains the changes in the human population in North America. However, once again this hypothesis has not yet been successfully applied to all areas which witnessed mass extinction.












 

 

 


                              Image courtesy of The Illinois State Museum     www.museum.state.il.us  
 

            Both of the overkill hypotheses have been tested with computer-simulated models. The results of these models have lent evidence for each explanation, without conclusively proving either.

 

            A recent, not yet tested explanation that is being explored in Siberia is the hyper disease hypothesis. This hypothesis too, depends on the rapid expansion of mankind throughout the Americas and Eurasia. However, in this scenario, instead of “overkill” causing extinction, a lethal “super” disease spread by the humans did the job instead. According to Ross MacPhee, Chairman of the Department of Mammalogy of the American Museum of Natural History, in order for such a disease to be responsible for the extinctions, it must infect all animals within a species, regardless of age, it must “kill rapidly,” it must be carried by either a human or something human-related and it must simultaneously not kill off the human population (11).

 

            The one all-encompassing conclusive theory for these mysterious extinctions continues to elude the scientific community, lending the subject to spirited debate and speculation. Many observations already recorded can be applied validly to specific land areas. It appears that the current assessment held is that some combination of all the aforementioned factors drove these impressive animals to their eventual demise.

 

Glypyo#1: Well, why don't they call it The Big Chill? Or The Nippy Era? I'm just sayin', how do we know it's an Ice Age?

Glypyo#2: Because... of all... the ice!

Glypyo#1: Well, things just got a little chillier.

- Ice Age (2002)

 

1.       http://www.well.com/user/elin/extinct.htm

2.       http://www.ucmp.berkeley.edu/quaternary/ple.html

3.       http://www.scotese.com

4.       http://www.museum.state.il.us/exhibits/larson/ice_age_animals.html

5.       http://www.kokogiak.com/megafauna/inames.asp

6.       http://www.kokogiak.com/megafauna/default.asp

7.       http://www.enchantedlearning.com/subjects/mammals/iceage/Iceagemammalsprintouts.shtml

8.       http://www.custance.org/Library/Volume4/Part_I/chapter10.html

9.       http://www.bagheera.com/inthewild/ext_woollym.htm

10.   http://www.americanscientist.org/template/AssetDetail/assetid/45941;_pM2rgmYMmN

11.   http://sciencebulletins.amnh.org/biobulletin/biobulletin/story981.html

12.   http://gallery.in-tch.com/~earthhistory/pleistocene%20page.html

13.   http://en.wikipedia.org/wiki/Pleistocene

14.   http://en.wikipedia.org/wiki/Megafauna

15.   http://en.wikipedia.org/wiki/Megafauna

16.   http://en.wikipedia.org/wiki/Ice_age

17.   http://en.wikipedia.org/wiki/Holocene_extinction_event

18.   http://www.weathernotebook.org/transcripts/1999/10/20.html

19.   http://en.wikipedia.org/wiki/Mass_extinctions