The Early Holocene Survival of Late Pleistocene Megafauna in the Americas

Most Pleistocene species of North American megafauna stopped occurring in the fossil record about 12,000 calender years ago.  It is remarkable how consistently the latest terminal radiocarbon dates of Pleistocene megafauna cluster around this time boundary.  Specimens that date to younger than this boundary are always questioned and resubmitted for another round of radiocarbon dating till a more believable result is attained, or they are dismissed as “contaminated” samples.  One notable exception to this rule was the discovery that mammoths survived on the Pribiloff Islands, located between Siberia and Alaska, until about 4000 calender years ago.  (See:https://markgelbart.wordpress.com/2013/09/15/the-late-extinction-of-the-pribiloff-island-mammoths/).  Another accepted exception are some of the fossils of dwarf ground sloths found on Carribean islands.  They also date to just a few thousand years ago.  However, these late survival sites are considered special cases explained by the isolation of islands.  The consistent terminal radiocarbon dates on the mainland suggested to scientists a fairly rapid extinction event.  Explanations include sudden climate change, a “blitzkrieg” human hunting overkill scenario, or a comet impact.  Any evidence that Pleistocene megafauna survived for thousands of years past the boundary of 12,000 BP  destroys all 3 of these explanations.

The fossil record is reliably incomplete because over 99% of animals that ever lived never became fossilized or preserved.  I hypothesize the final radiocarbon dates cluster around 12,000 BP because this is the last time these species were common enough in the environment to be preserved.  After this date, many of these species still existed but in isolated populations in areas with low human density and therefore were less likely to become preserved.  My hypothesis is supported by studies of sedaDNA in Alaska permafrost that show mammoths lived for 2200 years later than the last dated bone for this species, and horses were present here for 3700 years past the commonly accepted extinction date for this species in America. (See:https://markgelbart.wordpress.com/2011/02/24/csi-pleistocene-alaska/).

The Devil’s Den site in Florida also yielded early Holocene radiocarbon dates on megafauna bones. (See:https://markgelbart.wordpress.com/2012/07/08/the-devils-den-fossil-site-may-have-been-located-in-one-of-the-last-refuges-of-the-megafauna/).  These dates were obtained during the early 1970’s before improvements in radiocarbon dating were developed.  As far as I can determine, these specimens have not been redated.  If they haven’t, they should be.

Fossil remains of now extinct megafauna from 2 sites in South America also date to the early Holocene.  At the La Moderna site in Argentina, scientists excavated the remains of guanacos, paleollama, Amerihippus (an extinct species of horse), onohippus (a half-ass), deer, macrauchenia, toxodon, armadillo, glyptodont,  megatherium, and glossotherium.  Humans butchered many guanacos at this site as well as horse,  ground sloth, and glyptodont.  I think this is the only site where evidence of a human-butchered glyptodont has ever been found.  Only the bones of the most edible parts of the ground sloth were found here–evidence the beast was killed elsewhere, and the choice cuts carried to this site.  The calender year dates at this site average to between 7800 BP-8300 BP.

Evidence humans butchered a glyptodont was found at the La Moderna site in Argentina.

At the Camp Laborde site also in Argentina, scientists excavated rhea (an ostrich like bird), the fossil remains of 2 species of glyptodont, a megatherium associated with 2 quartz tools, peccary, llama, cavy, viscacha, and a canid closely related to the extinct Falkland’s wolf.  Humans evidentally butchered the megatherium ground sloth here too.  The average calender year dates found here range between 8500 BP-9000 BP. 

The Falkland Island’s wolf became extinct in 1876.  Fossils of a closely related species were found at the Camp Laborde site.

The cavy or Patagonian mara (Dolichotis patagonus).  Fossils of this species were also found at the Camp Laborde site.  Although it looks like a hare, it is actually in the rodent family.  An example of convergent evolution. 

The plains viscacha (Lagostomus maximus).  It is closely related to the chinchilla.  Fossils of this species were found at Camp Laborde.

The Falkland Island’s Wolf was hunted to extinction on the island by 1876.  A close relative lived on the mainland of South America until about 1600 years ago.  The species was nicknamed “the foolish dog” because it showed no fear of humans.  This canid was the only native terrestrial mammal on the Falkland Islands.  DNA evidence suggests it colonized the Falklands by traversing a narrow landbridge that must have existed 16,000 years ago.

The authors of the below referenced study use evidence from Camp Laborde  to reject the “blitzkrieg” overkill model of extinction.  (It’s kind of ironic that they use evidence of humans exploiting megafauna as evidence against overkill.)  They believe humans didn’t rapidly overkill megafauna because many of these species still existed in areas of South America with low human density until well into the Holocene.  However, they do believe a combination of climatic and anthropogenic change doomed South American megafauna.  I disagree with their reasoning and believe that if humans never colonized South America, most of these species would still be extant. Humans were ultimately responsible for megafaunal extinctions, but it took thousands of years rather than hundreds to completely wipe them out.  Pollen evidence from this study does not support climate change as a cause of extinction.  This site was a humid grassland during the early Holocene…just like the pampas of modern day Argentina.

Reference:

Politis, Gustavo; Pablo Messeneo

“The Camp Laborde Site: New Evidence for the Holocene Survival of Pleistocene Megafauna in the Argentine Pampas.”

Quaternary International 191 2008

Advertisement

The Devil’s Den Fossil Site may have been Located in One of the Last Refuges of the Megafauna

The Devil’s Den fossil site is located near Williston, Florida in Levy County.  It’s a sinkhole that served as a natural trap between 9,000 BP and 8,000 BP.  Apparently, the sinkhole closed until a few hundred years ago when it reopened and began trapping animals again.  The sinkhole ranges between 5-90 feet below the average water table and accordingly is filled with water.  It also serves as a bat roosting site, explaining the numerous chiropteran remains.  Rainwater dissolving limestone bedrock creates sinkholes such as this one.

Location of Levy County (red), site of the Devil’s Den sinkhole.

The area immediately adjacent to the sinkhole originally consisted of a dense mesic forest of hardwood, but beyond this moist environment, a grassland savannah with widely spaced live oaks predominated.  Scientists believe this was also the type of environment that occurred here 9,000 years ago at the time of the fossil depositions.  Indians later periodically cultivated some of the land.  Though watermelons didn’t originate in North America, Indians in Florida grew large quantities of them from seeds they obtained from Europeans.  The earliest recorded use of the land around Devil’s Den was as a watermelon field.  The next owner used the sinkhole as a garbage dump.  By the time scientists were granted permission to excavate fossils here in the late 1960’s, it was in the middle of a cow pasture.  Today, it is a private resort and a destination for scuba divers and campers.  There are 4 chambers in the sinkhole that scuba divers enjoy exploring.  The excavated fossils came from chamber #3, but scuba divers report seeing fossils all over the bottom still.  

On lists of megafauna terminal extinction dates, the data from the Devil’s Den fossil site is always excluded.  I never understood this, but I think I’ve finally been able to piece together enough information to assume an answer.  H.K. Brooks wrote a research paper detailing the radiocarbon dates and the geological information that made him conclude the fauna from this site was between 7,000-8,000 years old.  As far as I can determine, this paper never was published, though the information has been mentioned in several published works including the one referenced below.  Because this information was never published in a peer-reviewed publication, it’s  disregarded by scientists arguing over the cause of Pleistocene megafauna extinctions. But that doesn’t mean the data is wrong.  One of the reasons he may have had a hard time publishing his data was because it contradicted the preconceived notion of when megafauna became extinct.  It also contradicted both competing models of megafauna extinction.  In 1974 most vertebrate zoologists believed climate change caused the extinction of the megafauna.  But the climate and environment of 9,000 BP in north Florida were about the same as it is during the present time.  Yet, many notable species of megafauna were apparently still extant.  His data also conflicted with Paul Martin’s sudden overkill “blitzkrieg” model of extinction in which he proposed all the megafauna were wiped out in a few hundred years between 12,000 BP and 11,000 BP.  Without either school of thought in his corner he found it impossible to get his data published.  The later extinction dates do fit within a protracted overkill model of extinction that had not yet been proposed or considered.  Evidence at Devil’s Den suggests declining populations of some megafauna, while others are already extinct.  This perfectly supports a model of extinction that includes gradual and haphazard human overhunting. SedaDNA found in Alaska permafrost also suggests younger extinction dates for megafauna than are commonly accepted.  (See: https://markgelbart.wordpress.com/2011/02/)

H.K. Brooks’ study was in 1974 before radiocarbon dating was recalibrated.  This means the dates of the fossils at Devil’s Den probably are from 8,000 BP-9,000 BP rather than 8000 BP-7,000 BP, but this is still 3,000 years younger than the commonly accepted terminal extinction dates of most megafauna species.

The Devil’s Den sinkhole.  Imagine a deer being chased by a dire wolf and falling in this hole.

Businessmen built a a stairwell down to the sinkhole, providing easier access for scuba divers. http://www.devilsden.com/  

If a person could go back in time and camp at this site 9,000 years BP, they would find a wilderness rich in wildlife.  Several species of megafauna may have already become extinct while others were in decline, yet they still existed.  Bison, horses, and flat-headed peccaries roamed the savannahs and rested in the shade of centuries-old live oaks.  Great droves of white-tail deer outnumbered all the other ungulates because they were better able to withstand human hunting pressure.  White-tail deer fossils were by far the most common of the ungulates at this locality.  This is unusual compared to other Pleistocene fossil sites in Florida.  Dr. Webb suggests competing tapirs, llamas, and long-nosed peccaries had become extinct by this time.  All of these species favored forest or forest edge habitat.  Without them deer had this niche to themselves.  Mastodons and Jefferson’s ground sloths browsed the moist woods.  A camper going on a hike would need to carry a firearm.  Saber-tooths, jaguars, and dire wolves still stalked the range.  Two species of bears might prove troublesome too, but by this time they probably had learned to fear man.  Paleo-indians and their dogs occasionally passed through here as well.  But the most common carnivore was the striped skunk.  Scientists found 36 striped skunk skulls at Devil’s Den.  Either they were abundant, or they had a knack for falling into sinkholes.  The same might be true for cottontail rabbits which left more fossils at Devil’s Den than any other animal.  Below is a list of mammal species recovered from the Devil’s Den fossil site. * denotes extinct species.

opossum

least shrew

short-tailed shrew

eastern mole

southeastern myotis bat

gray myotis bat

Florida yellow bat

eastern pipistrelle

human

*Jefferson’s ground sloth. (Dr. Webb erroneously considered the sloth fossil found here as Wheatley’s ground sloth.  Bjorn Kurten corrected this misidentification.)

gray squirrel

fox squirrel

southern flying squirrel

southeastern pocket gopher

old field mouse

cotton mouse

gopher mouse

golden mouse

rice rat

woodrat

cotton rat

meadow vole

pine vole

muskrat–Archaeological records suggest muskrats lived in Florida as recently as 3100 years BP.)

Florida round-tailed muskrat

*southern bog lemming

cottontail rabbit

*dire wolf

red wolf

domesticated dog

gray fox

raccoon

*Florida spectacled bear

black bear

long-tailed weasel

striped skunk

spotted skunk

bobcat

jaguar

*saber-tooth cat

*mastodon

horse

*flat-headed peccary

white-tailed deer

bison

In addition cows and pigs from more recent times had fallen in the sinkhole and they left bones.

Although a study of bird fossils from Devil’s Den was conducted, the results were never published.

J. Alan Holman did publish his study of reptile and amphibian fossils from Devil’s Den in Herpetologica 34 (2) in 1978.  The abstract of that paper claims 1 salamander, 5 species of frogs, alligators, 1 species of lizard, and 15 species of snakes were recovered here.  Devil’s Den is also the  youngest known record of both species of extinct giant tortoise–Hespertestudo crassicutata and Hesperotestudo incisa.

Reference:

Webb, David

Pleistocene Mammals of Florida

University of Florida Press 1974

Interestingly, in addition to discussing the site and the fossils, Dr. Webb came up with a long convoluted statistical analysis that allegedly showed how the evidence from Devil’s Den supported climate change as a cause of megafauna extinction.  His analysis came to the conclusion that the mass wave of extinction at the end of the Wisconsinian Ice Age was not unusual–a ludicrous claim.  Dr. Webb is retired now, but in his later work he did come to accept that man played an important role in the extinction of at least some of the megafauna.

CSI: Pleistocene Alaska

“Science” programs on cable networks are often idiotic.  I suppose the television producers want to boost ratings and make their presentation dramatic for entertainment value.  But for those of us who know a good bit more about the subject than the average couch potato, these programs are laughable.  I think it was the History Channel that featured a special about the Pleistocene megafauna that concluded the beasts became extinct because of global warming.  The imbecilic producers actually showed computer images of the beasts dropping dead due to a heat wave.  They made no mention of overhunting by humans which is a much more plausible cause of extinction.  Instead, they wanted to jump on the bandwagon of global warming phobia, unware that all of these “Ice Age” species survived the Sangamonian Interglacial which was on average warmer than present day temperatures.

Another program about megafauna extinction (either on Discovery or PBS) did give equal time to overkill, climate change, hyperdisease, and comet impact.  The latter two, in my opinion, are crackpot theories.  While it is clear that an extraterrestrial impact probably led to the extinction of the dinosaurs, the evidence that a comet caused the recent megafauna extinction event is not at all convincing.  Supposedly, the comet hit a glacier–explaining why there’s no impact crater.  The theory is on shaky legs to start, if it’s proponents have to make an excuse for the lack of a smoking gun.  Other independent scientists have been unable to replicate their findings, perhaps relegating the comet impact theory to the trash can of illogical ideas.  When interviewed for the above mentioned program, one of the comet impact proponents made an astoundingly illogical comment.  He stated that the overkill theory didn’t make sense to him because he couldn’t believe humans could kill every last individual of a species.

His statement demonstrates a remarkable ignorance of basic ecology.  Humans would not have to kill every last individual of a species to render it extinct.  All wild species of animals already have high rates of natural mortality.  Let’s take the mastodon for example.  Mastodons had a long lifespan and low birthrate.  Saber-tooths and scimitar-toothed cats caused a certain amount of mortality.  Diseases, such as tuberculosis, took a percentage more.  Accidents (falling into quagmires, males killing each other in battles for mates, etc.) took a toll as well.  But this was mortality the mastodon birthrate could keep up with, and they maintained their existence for millions of years.  However, the addition of human hunting was a variable that increased a mortality rate that exceeded their birthrate.   So man + natural mortality = extinction.  Remove man from the equation and it would be natural mortality = a still extant species.

In my opinion overhunting by man is the only cause of the demise of the Pleistocene megafauna.  There is no other logical solution to this mystery. But I don’t agree with Paul Martin’s blitzkrieg model of extinction.  I believe the process took thousands of years rather than hundreds.  A recent study supports my belief in a protracted overkill scenario.

Researchers taking cores of permafrost in Alaska’s north slope.  There is forensic evidence of extinct Pleistocene megafauna in some of these cores.  This forensic evidence dates thousands of years later than the most recent dated fossil evidence of these species.  That means these animals lived more recently than previously thought.  Photo from google images.

A group of scientists led by James Halle examined cores of permafrost found in Alaska.  They were looking for megafauna DNA.    The cores were dated using carbon dating and optically stimulated luminescence (see  http://crustal.usgs.gov/laboratories/luminescence_dating/technique.htmlxplanation for an explanation of OSL dating).  The dates of the cores with the megafauna DNA ranged from 11,000 to 8,000 years old.  This sediment is more recent than the youngest known dated megafauna fossils from the interior of Alaska.  (A population of dwarf mammoths is known to have survived on the Pribiloff Islands until ~4000 BP.)  These cores harbored DNA from mammoth, horse, bison, moose, and snowshoe hare.  This DNA is referred to as sedaDNA and comes from hair, feces, and urine.  It proves the existence of these species in the environment, even though they left no fossil evidence from this time period.  The mammoth DNA is 2600 years younger than the youngest known mammoth fossil from the interior of Alaska; the horse DNA is 3700 years younger than the youngest known horse fossil from this region.  Using sedaDNA is uniquely possible in Alaska, thanks to permafrost conditions, and it’s a technique probably not possible anywhere else. 

The scientists concluded that the continued existence of megafauna until ~7500 BP rules out climate change, the blitzkrieg model of overhunting, comet impact, and hyperdisease as the ultimate causes of megafauna extinction, though they do concede that anyone of these factors could’ve caused an initial population collapse.  Curiously, they made no mention of a protracted overkill model of extinction, which in my opinion this study strongly supports.

It occurs to me that previously disregarded recent dates of megafauna fossils my not be in error.  For example the Devil’s Den fossil site in Levy County, Florida yields remains of Jefferson’s ground sloth, dire wolf, Florida spectacled bear, southeastern lemming, saber-tooth cat, mastodon, horse, and flat headed peccary.  The fossils from this site were radiocarbon dated to be 7,000-8,000 years old–several thousand years later than when scientists believe these species became extinct.  Therefore, scientists disregard these dates as inaccurate due to some kind of contamination or obsolete carbon dating techniques.  I suggest scientists redate these fossils, as well as those from other sites with unusually young dates, using the more updated and improved methods of carbon dating.  It seems likely the Pleistocene megafauna may have survived in southeastern North America for several thousand years longer than previously thought.

References

Halle, James et. al.

“Ancient DNA reveals late survival of mammoth and horse in interior Alaska”

http://www.pnas.org/content/early/2009/12/16/0912510106.full.pdf

www.devilsden.com/