the extinctions of american megafauna at the pleistocene/holocene boundary and its causes

7/25/2019 The Extinctions of American Megafauna at the Pleistocene/Holocene Boundary and Its Causes

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The Extinctions of American Megafauna at thePleistocene/Holocene Boundary and Its Causes

Friday, March 20, 2015ERTH2312

Jake MacLean

100884185


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Abstract

The world has seen lare chanes in e!ol"#ion #hro"ho"# #i$e, %ro$ #he &rs#$icro'es #o c"rren# day( #here has 'een !as# chane and e!er !aryinoranis$s) *o$e#i$es li%e was really s$all, so$e#i$es i# was !ery lare, and

so$e#i$es i# was +"shed al$os# in#o none-is#ence 'y $ass e-#inc#ions).i/eren# e!en#s #hro"ho"# Ear#hs his#ory ha!e 'een #ho"h# o% as #heca"ses o% #hese e-#inc#ions) The di!ersi#y and hardiness o% li%e is wha# $akesEar#h so s+ecial and "ni"e) Towards #he end o% #he leis#ocene e+och,2)88$ya1100 years ao, we saw larer $a$$als in #he $ericas #han wehad e!er seen 'e%ore and $ay e!er see aain) They were #he 'ies#crea#"res, and so$e o% &erces# +reda#ors #o walk #he ear#h since #he #i$e o%#he dinosa"rs) They were s+ecialied and well ada+#ed #o #heir en!iron$en#,ye# so$e %orce o% na#"re was s#ill a'le #o dri!e #he !as# $a6ori#y o% #he$e-#inc#) The $any #heories as #o how #his co"ld ha!e ha++ened are widely!aryin) *o$e 'elie!e i# was ca"sed solely 'y #he $ira#ion o% h"$ans in#o

7or#h $erica as #he erin land 'ride 'eca$e ice%ree) 9i#h o"r #ools andad!anced in#ellience we were #oo danero"s a +reda#or %or #he $ea%a"na#o li!e alon side) :#hers 'elie!e #ha# %ac#ors s"ch as cli$a#e chane dro!e#hese $assi!e crea#"res #o e-#inc#ion) ;n reali#y, i# see$s #o 'e a co$'ina#iono% cli$a#e chane and #he in


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Throughout geologic time we have seen a great diversity of both fauna and flora that have seen

heights of dominance and many extinctions, both large and small. The Pleistocene is the secondmost recent epoch in geologic history, as a part of the larger quaternary era. The quaternary runs

from 2.588 million years ago to present, and is divided into the Pleistocene and olocene. The

Pleistocene came to and end roughly !!,"## years ago $%ollins& and with it we saw the extinctionof a large population of megafauna that roamed 'orth (merica and most of the world for varying

spans of the epoch. The megafauna were widespread and well adapted, but !)### years ago the

fossil record shows the beginnings of a mass extinction. $*eisman& These were animals whoroamed the world unscathed for upwards of a million years, and they were being +illed off faster

than at any previous time. These megafauna were similar to many of the animals we see

throughout the world today, often related in many cases, with the only maor difference being thatthey were much larger. There were bears and wolves almost twice the si-e of those we see today,

sloths as large as elephants, and lions roaming much of 'orth (merica $*eisman&. *hen we findthese fossils it ma+es us question our reality, our ancestors who discovered them, only a mere 2##years ago, were dumbfounded and confused with the sheer si-e of these animals, as they had

never seen anything close to this si-e. There have been many studies done to confirm the origins,

lifestyle, and climate in which these animals thrived for upwards of a million years $*eisman&.

owever, one of the most pu--ling question is how could these megabeasts be wiped out, whator who could have possibly caused such a mass extinction in creatures as large and widespread as

the Pleistocene megafauna/ There are multiple theories as to what caused the extinction of the

megafauna, and the answer may not be as simple as ust one of them, but more than li+ely it issome combination thereof $*eisman&. *e will loo+ at whether human hunting, climate change,

the array of large predators we +now to have existed could have caused the extinction.

Before the Humans

To begin, we need to understand the sheer si-e of the animals in comparison to those we see

today. ( polar bear is one of the largest carnivores to roam the 'orth (merican wilderness, malesweighing in around an average of 8## pounds, with some of the biggest +nown male individuals

reaching si-es of roughly !"## pounds, and females being slightly smaller. These polar bears are

roughly 80 feet from nose to tail. $1efenders !& ( wolf is a dangerous predator, so much so thatwe have almost wiped them out completely over the last !5# years. They used to be common

throughout much of 'orth (merica, before we drove them to the brin+ of extinction. ( large male

wolf may come in around !!# pounds. $1efenders 2& f we start to compare our carnivorous'orth (merican mammals of recent times to those of the Pleistocene, we will get a picture of

how large the predators really were, and the dangers they posed to the earliest settlers of 'orth

(merica. 3or instance, we can loo+ no further than the short faced bear.

The shortfaced bear was the largest bear to have ever existed and also the fastest. 4stimations

put the weights of the average bear anywhere between !##!0## pounds, both male and female.

These bears were also much taller than any bear we currently +now, with average heights being!!!2 feet, with some of the largest growing up to as much as !6!5 feet. $4xtinct& 3igure !

shows a s+eleton of a shortfaced bear in comparison to the modern gri--ly bear and brown bear.


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$'elson and arrington& *hile +eeping this si-e in mind, researchers also estimate that these

bears could cover as much as )#6# miles an hour, due to forward pointing toes, different fromthe pigeontoe characteristic we see on modern bears. $4xtinct& 7ome further research of the bone

structure and composition indicates high nitrogen levels, which in turn describe a diet that was

mostly if not entirely carnivorous. This carnivorous diet was shared with most of the large

predators of the time, with the smaller animals such as a blac+ bear opting for an omnivorousdiet. $Prehistoric& owever, the structure and thic+ness of the bones indicate that this animal was

li+ely not the predator, but a voracious scavenger, as the front leg bones would li+ely have

shattered rather easily if the bear attempted an attac+. $Prehistoric& eep in mind that a bear ofthis si-e moving this quic+ly and widely would have had to consume upwards of )5 pounds of

meat per day, ust to survive. $Prehistoric&

(nother example of a large Pleistocene predator would be the dire wolf. The dire wolf is the

largest member of the canis genus to have ever lived. t was much larger than the grey wolves we

see today, weighing anywhere from !25!"5 pounds. $*orld& The dire wolf 9canis dirus: is

considered to be one of the most populous predators of the Pleistocene, being discovered at asmany as !)0 sites across 'orth (merica alone. $1undas& *e start to see a pattern when we loo+

at the areas of mass extinction for the dire wolves, as the carbon dating puts most of the fossils at

an age of anywhere from !),### to !#,### years ago. 7ome of these sites have been called intoquestion due to +nown errors with the methods of dating, however, there are no fossils found

once we move from the Pleistocene into the olocene. $1undas&

The carnivores of the Pleistocene are not the only species that were much larger than those of

today. The omnivores and herbivores were much larger than current ones as well. n 'orth

(merica today, the largest mammal is the wood bison, weighing in at nearly 2### pounds.$oren& Their numbers have been nearly wiped due to the spread of agriculture through their

habitat, nearly forcing them to the brin+ of extinction. 4stimates say there are 66## in %anada

today, when you compare that to the estimated !8,### in the !8##;s it is easy to see the impact

and velocity with which humans can decimate a population. $oren&

The Pleistocene saw even larger omnivores and herbivores throughout 'orth (merica, such the

ground sloth, giant beaver and woolly mammoth. The ground sloth, +nown as megalonyxjeffersonii, was massive in comparison to any sloth today. This particular sloth was discovered in

part and named for Thomas


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point, and other similar weapons, were the first ones of their +ind in 'orth (merica. These new

predators were un+nown to the megafauna living here, and this unfamiliarity was the beginningof the end. $aynes&

The Human Impact

4vidence suggests that humans migrated into 'orth (merica, at the earliest, !,### years ago.

This is when glaciation in the northwest would have subsided enough to allow settlers to pass

through the >ering straight. 9(t the time it was a land bridge: $1ixon& Through radiocarbondating of human remains, we start to see some of the earliest evidence of settlements occurring

around !),### years ago. $1ixon& 3urther evidence suggests settlements along the northwest

coastal area as early as !5,)## years ago. $1ixon& 3igure 2 shows the retreat of the ice sheetscovering 'orth (merica, and how there would have been clear travel pathways for humans

roughly !### years ago. $itchen& umans have always been very resourceful and adaptable,

which has led to our advancement into a species that has the power to alter the planet in ways that

were never seen before we evolved to our current state. *e have left a mar+ on every corner ofthe globe, changing landscapes and ecosystems for our benefits, and not always regarding the

natural order of things. ?ur impacts are of great si-e, and sometimes they are on things of great

si-e. n many opinions, this was the case as we made that fateful ourney across the >ering landbridge onto the unexploited terrains of 'orth (merica. *e brought a new and un+nown threat to

the inhabitants, the clovis spear. Pictures of the different clovis spear points can be seen in figure

). $>ostrom& They are long pointed roc+s anywhere from !5@ that made for deadly weaponswhen attached to the spear. This allowed for humans to extend their reach and attac+ with a

velocity that otherwise would not have been possible. $%lovis& (s the new settlers evolved their

hunting styles, they were even fashioning a ma+e shift throwing +nife, further extending theirrange and ability to attac+. $%lovis& To the megafauna of 'orth (merica, the humans were an

un+nown, new predator, and therefore they lac+ed the survival instincts that would have

otherwise identified them as dangerous, and were not adaptable enough to quic+ly adust to this

new threat. $aynes& There are many sites where we see mass graves of mammoths andmastodons that are full of clovis points, indicating the efficiency this tool allowed for the humans

to have in their hunting tribes. $aynes, *eisman& *hile it is clear that we can see a relationship

between the arrival of humans and the decline of the megafauna in 'orth (merica, there are stillpeople who are of the opinion that humans alone could not have possibly wiped out the massive

populations that existed before their arrival. $aynes&

*hile the large herbivorous megafauna may have not recogni-ed the humans as predators due to

their new arrival, that still does not account for such a large extinction, including some very

ferocious predators that were more than agile enough to survive against humans even with their

clovis points. Aosimann and Aartin published findings in !0"5 that it would only ta+e roughly!2 years of human nomads living in an area to cause a local extinction. $


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severely disable their abilities to reproduce and add increasing competition for survival on

carnivores, omnivores, and herbivores left within the ecosystem after the nomads had moved on.$


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the most notable megafauna were already !### years extinct. $aynes& This means that while

climate change did play a factor in the extinctions of some megafauna, the rapid warming periodmar+ing the end of the Pleistocene cannot be seen as the full and total cause for the extinctions.

$aynes&

Other Causes of Mass Death

There are many other ha-ards connected to the extinction of many megafauna in 'orth (mericaas the Pleistocene came to an end. The Ca >rea tar pits in what is now southern %alifornia are no

exception. Tar pits are areas where oil deposits have come close to the surface and are lea+ing out

to the surface as asphalt li+e material. $arris and


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Conclusion

E-#inc#ions are no#hin new #o Ear#h, i# has e-+erienced $ass e-#inc#ion a%#er

$ass e-#inc#ion #hro"ho"# eoloic his#ory) 7a#"ral %orces ca"sed $any o%#hese e!en#s, whe#her #hose were as#eroid i$+ac#s, !olcanis$, cli$a#echane %or #he war$er or colder, or so$e co$'ina#ion o% ca"ses) Mos# o%#hese e!en#s occ"rred lon 'e%ore h"$ans had e!ol!ed) The $os# recen#$ass e-#inc#ion o% #he leis#ocene $erican $ea%a"na, howe!er, occ"rredin#ri"inly +arallel #o #he #i$e i# is 'elie!ed h"$ans s+read #hro"ho"# #he$ericas) Many +eo+le ha!e "es#ioned whe#her #his e-#inc#ion e!en# co"ld'e con#ri'"#ed direc#ly #o #he in


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Fi"re 1) The shor#%aced 'ear in co$+arison #o #he a!erae sie o% #he$odern rily and 'rown 'ears, res+ec#i!ely) =7elson and Harrin#on>


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Fi"re 2) The re#rea# o% #he ice shee#s %ro$ 43kya #o 1kya) # 1kya, #herewere clear +a#hways #hro"h which h"$ans co"ld ha!e en#ered 7or#h$erica) =?i#chen>

Fi"re 3) collec#ion o% clo!is s+ear +oin#s %o"nd in Eas#ern 7or#h $erica)Es#i$a#ed #o 'e o% an ae o% ro"hly 10,50012,000 years old) =os#ro$>


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Fi"re 4) The $a+ shows #he loca#ion o% #he La rea #ar +i#s in $odern day@ali%ornia, si#"a#ion rih# in #he $iddle o% Los neles) =Harris, John>


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Fi"re 5) Early e-ca!a#ions o% #he La rea #ar +i#s in #he early 1A00s) Thisshows #he concen#ra#ion o% 'ones #ha# ha!e 'een %o"nd here, ro"hly #o#ally1 $illion 'ones were %o"nd across all o% #he #ar +i#s si#es) =Harris, John>

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the extinctions of american megafauna at the pleistocene/holocene boundary and its causes

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