Posts Tagged ‘Bering Landbridge’

Did Elk (Cervus elephus) Live in North America Prior to 15,200 BP?

October 27, 2014

A recent thorough study, combining radio-carbon dating and genetic evidence, suggests elk did not cross the Bering Landbridge and colonize North America until about 15,200 years ago.  According to this study, “there is no unequivocal fossil evidence that elk lived in North America prior to this date.”  However, not every elk fossil in North America has been radio-carbon dated, and I’ll explain later how it’s possible elk could have colonized North America before this date without contradicting the data in this study.  But first, I’ll summarize the pertinent findings of this paper which was published in The Proceedings of the Royal Society of Biological Sciences last year.

The authors of this study had difficulty even finding North American elk fossils that were thought to be older than 15,000 years.  They did find 5 but determined from tests of bone collagen that 3 had been misidentified and were either bison or horse.  The radiocarbon date on the 1 that was correctly identified as coming from a member of the deer family was 13,100 BP…within the proposed timespan.

Radio-carbon dating shows that elk have lived in northeast Asia for over 50,000 years.  (Radio-carbon dating can’t date fossils older than this.)  Scientists examined the DNA of almost 100 Pleistocene-aged and modern elk specimens from northeast Asia and North America.   They found that all living North American elk are descended from elk originating in east Asia.  North American elk apparently split from their Siberian ancestors about 15,000 years ago.

The environment of the Bering Landbridge experienced many changes over time.  It was underwater from 135,000 BP- 70,000 BP.  It rose above sea level between 70,000 BP-60,000 BP.  Between 60,000 BP-30,000 BP, sea levels fluctuated so that some times the region was inundated and at other times it re-emerged.  From 30,000 BP-11,000 BP  the landbridge re-emerged, but since the latter date, it has been submerged once again.  However, until about 16,000 BP the environment of the landbridge had little favorable elk habitat.  A relatively warmer and drier climate occurred here between 16,000 BP- 11,000 BP, and elk were able to inhabit the landbridge and colonize the rest of North America from this starting point.  Curiously, elk no longer live in Alaska because a cooler wetter climate pattern has caused their favored habitats to deteriorate here.

Archaeologists believe humans also crossed the Bering landbridge during the same time period as the elk migration, but archaeological evidence is scarce.  The presence of elk fossils in Alaska during this time period and the timing of this migration as a proxy for human migration supports their belief.  Because the climate was warmer and drier, these paleo-indians may not have been as culturally adapted to living in arctic conditions as the more recent native American inhabitants are here.

Elk in Tennessee.  This species may be a relatively recent member of North America’s fauna.  There is no unequivocal evidence of their presence on this continent until 15,200 BP.

Range map of elk.  Elk no longer occur in Alaska because habitat is not favorable there for their survival.  However, between ~16,000 BP- ~11,000 BP, climate there was relatively warmer than it is today, and elk were able to live in the region.  This is when scientists believe they crossed the Bering Landbridge and began colonizing North America. Humans without an arctic-adapted culture also crossed the Bering Landbridge during this time period.

I don’t dispute the data and conclusions of this fine study.  However, I’m not entirely convinced that elk did not occur in some regions of North America prior to 15,200 BP.  The unique environmental conditions on the Bering Landbridge between 16,000 BP-11,000 BP that allowed elk to migrate to North America probably occurred and re-occurred periodically throughout the Pleistocene.  It’s possible and even likely that elk would have crossed the landbridge during an earlier incarnation of this climate phase.  Scientists have been wrong before about the exact dating of animal migrations into North America.  Formerly, scientists assumed grizzly bears (Ursus arctos) did not colonize North America south of the Ice Sheets until about 13,000 BP when the glaciers receded.  But a few years ago, fossil evidence of a grizzly bear, dating to 21,000 BP, was found in Edmonton, Alberta.  All it would take to prove that elk lived in North America prior to 15,200 BP would be 1 specimen dating to a greater age than that.  I’m aware of 1 candidate.  There is an elk vertebrae in the Charleston Museum of Natural History that was recovered from sediments thought to be of early Pleistocene age.  It was found associated with Eremotherium, a species of ground sloth thought to have become extirpated from the region 30,000 years ago.  As far as I know, this specimen has never been radio-carbon dated.  I think it merits dating.  Would the date support the findings of the new study or would it suggest a more complicated history for Pleistocene elk in North America?

Daphne'sdorm

Photo of elk fossils found in South Carolina from the book Additions to the Pleistocene Mammal Faunas of South Carolina, North Carolina, and Georgia by Albert Sanders.  The vertebrae comes from sediments thought to pre-date 15,200 BP.

The DNA evidence from the study make it clear–all North American elk alive today descend from the elk migration across the Bering Landbridge that began about 15,000 years ago.  But this doesn’t rule out the possibility that a now extinct Pleistocene ecomorph of the elk occurred in North America prior to this date.  Both cougars and jaguars lived in North America during the Pleistocene.  DNA evidence proves the North American ecomorphs of these 2 big cats went extinct about 11,000 years ago.  (See: https://markgelbart.wordpress.com/2014/04/11/extinct-pleistocene-ecomorphs-of-the-cougar-puma-concolor-and-the-timber-wolf-canis-lupus/)  All modern individuals of these 2 species descend from populations that lived in eastern South America at the end of the Pleistocene.  It’s not unprecedented for populations of a species to become extirpated, then replaced by other populations of the same species.

Reference:

Meirav, Meiri; et. al.

“Faunal Record Identifies Bering Isthmus Conditions as Constraint to end-Pleistocene Migrations to the New World”

Proceedings of the Royal Society of Biological Sciences 281 (1776) December 2013

The Beringian Buckle Stopped Rhinos from Recolonizing North America During the Pleistocene

November 26, 2012

Many magnificent mammals roamed the Americas during the Pleistocene but one of the most spectacular was conspicuously absent.  The abscence of rhinos from Pleistocene America was for a long time an ecological mystery.  The Bering Landbridge has intermittently served as a gateway between Eurasian and American fauna.  Bison, mammoths, elk, saiga antelope, brown bears, and lions crossed from Siberia to Alaska while horses and camels crossed from Alaska to Siberia.  The Bering Landbridge emerges above sea level during Ice Ages transforming the Bering Straight from ocean to habitable land where this faunal interchange can take place.  

Full-size image (83 K)

Map of the Bering Landbridge.  Note how vast it was.  It comprised tens of thousands of square miles. The southern half was good quality wildlife habitat but some species of animals, such as rhinos, could not survive on it, explaining why a certain proportion of animal species were filtered out of the transcontinental faunal exchange.

Recently, some paleoecological studies of areas in Alaska and Siberia that are immediately adjacent to the Bering Straight yielded evidence explaining why some animals, such as the woolly rhino (Coleodonta antiquatatas), never crossed the Bering Landbridge.  The northern half of the landbridge was likely blocked by glaciers.  The southern half consisted of moist shrubby maritime habitat drastically differing from the vast grassy steppes that existed on both sides of the Landbridge.  R. Dale Guthrie calls this habitat a “buckle in the belt of mammoth steppe,” a biome that existed from Europe across Asia and continued again in most of Alaska with the exception of the coastal regions.  The Beringian Buckle provided a barrier for some mammals, stopping woolly rhinos from colonizing America but also preventing such American species as ground sloths, short-faced bears, American donkeys, late Pleistocene camels, bonnet-horned musk-oxen, and badgers from colonizing Eurasia.  The studies also found different species of steppe-grass adapted beetles on each side of the buckle.

A riparian willow habitat in the Rocky Mountains.  This might have been similar to the kind of habitat in Beringia that woolly rhinos and certain kinds of grass-dependent beetles couldn’t survive in long enough to traverse, but woolly mammoth, bison, horses, and elk could.  On the east and west sides of the Beringian Buckle were vast steppe grasslands suitable for woolly rhinos.  However, they never could get to the east side.

Artist’s rendition of the Woolly rhino.  Note the size of its horn.

Climatic conditions over the interior regions of the continents during the Ice Ages created clearer skies and drier conditions than occur presently in Siberia and Alaska.  Temperatures were even colder than they are today, but there was less precipitation and cloud cover, creating an environment of grass interspersed with sand dunes.  The greater amount of sunlight thawed the permafrost.  Unlike today’s Alaska and Siberia, there were no spruce forests or any trees at all.  But the Beringian Buckle experienced more cloud cover and precipitation due to the region’s vicinity to the ocean.  The greater amount of precipitation and cloud cover allowed a shrubby maritime habitat to flourish, and it was quite different from the grassy steppe that covered so much of the northern hemisphere.  The Beringian Buckle served as a refuge for wet tundra plants that later recolonized Alaska and Siberia and unlike the interior of the continents then, it was studded with lakes.

Woolly rhinos weighed on average 7000 pounds, making them the 2nd largest Ice Age mammal in Eurasia.  They originally evolved 3.7 million years ago on the grassy Tibetan Plateau, long before Ice Ages began to occur.  When Ice Ages began to occur on a cyclical basis, woolly rhinos were able to expand their range across most of Eurasia.  Some scientists have tied their extinction to the end of the Ice Age when the Mammoth Steppe habitat contracted.  However, I disagree with this assessment because they originally evolved before Ice Ages began to occur, and they survived previous interglacial conditions.  I do agree that their range contracted following the end of the last Ice Age but some steppe habitat remained as happened in previous interglacials. (Areas of Mongolia where wild and domestic horses and nomadic herders still thrive is an example of suitable steppe habitat capable of supporting woolly rhinos.)  I propose the population of woolly rhinos living on relic steppe habitat after the end of the Ice Age were wiped out by men.  If not for men, I believe woolly rhinos would still exist, ready to expand their range again upon commencement of the next Ice Age.

I hypothesize a similar scenario for 2 other Eurasian species of Pleistocene rhinos.

Merck’s rhino (Stephanorhinus kirchenbergensis).  The background setting of the illustration is inaccurate.  This species preferred temperate forest habitats.

The narrow nosed rhino (S. hemitoechus) also lived in temperate regions of Eurasia but preferred meadows and prairies.

Merck’s rhino lived in temperate forests from what’s now England east to Korea and from Germany and Poland south to Israel.  It was adapted to eat forest vegetation.  The narrow-nosed rhino lived over much of the same geographic range but was adapted to open grassland habitats, eating mostly grass.  Both evolved from and replaced a common ancestor (S. hudsheimensis) that was adapted to eat both forest and grassland vegetation.  The extinction of both species coincides with the beginning of the Last Glacial Maximum when forest and meadow were replaced by the arid Mammoth Steppe habitat.  Relic habitat suitable for both temperate species of rhinos may have remained in southern Europe but relic populations of rhinos then were more vulnerable to human hunters.  If not for man, I believe both of these species would have survived on these relic habitats and recolonized Europe following the end of the Ice Age.

Climate change did cause the complete extinction of rhinos in North America before the Pleistocene began.  North America was home to several species of rhinos during the Miocene.  The hippo-like rhino (Teloeceras major) and the hornless rhino (Aphelops) were the most common large herbivores in America other than horses for about 20 million years.  Their extinction coincides with the first Ice Ages that occurred at the beginning of the Pliocene ~5 million years ago.  They may have been incapable of surviving frosts or changes in vegetation.  So it is possible that Pleistocene Eurasian rhinos succumbed to changing climate, but man is a strong suspect in my opinion.

References:

Elias, Scott; and Barnaby Crocker

“The Bering Landbridge: a moisture barrier to the dispersal of steppe tundra biota”

Quaternary Science Review 27 (December 2008)

Guthrie, R. Dale

“Origin and Causes of the Mammoth Steppe, a story of cloud cover, woolly mammal tooth pits, buckles, and inside-out Beringia”

Quaternary Science Review 20 (2001)