Posts Tagged ‘extinction’

The Extinct Pleistocene Giant Tortoise (Hesperotestudo crassicutata) Must Have Been Able To Survive Light Frosts

April 15, 2011

Illustration of the extinct giant tortoise that lived in the southern parts of North America.  It grew as large as the Galapagos Island tortoises but was more closely related to the much smaller extant gopher tortoise.

Scientists often use the presence of giant tortoise fossils as a proxy for past temperatures.  They conclude that because giant tortoises can not survive freezing temperatures than they must have lived during a time when the region was completely frost free.

Hesperotestudo crassicutata scute

Photo of part of a tortoise shell or scute from a specimen found in Texas.

Three species of closely related land tortoises lived in southeastern North America: a giant species (Hesperotestudo crassicutata) that grew as big as modern day Galapagos Island tortoises, an intermediate-sized species (Hesperotestudo incisa), and the gopher tortoise (Gopherus polyphemus) which is still extant.  It has occurred to me that the two larger species must have been able to survive light frosts, otherwise they would have become extinct when Ice Ages began.  Here are 5 reasons why I have come to this conclusion and disagree with the scientific consensus that the presence of tortoise fossils indicates warmer winters in this region than those of today.

1. The giant Pleistocene tortoise existed for at least 2 million years.  Within this vast time span, there must have been climatic phases, or at least events of crazy weather, that led to frosts in the deep south.  Today, frosts occur as far south as

Look at how much average temperatures fluctuated before the Holocene (~11,000 BP) when it’s assumed once a decade frosts began occurring in south Florida.  Notice also how much lower average temperatures were previous to the Holocene.  It doesn’t make sense the frosts in the deep south just began occuring 11,000 years ago.  They must have occurred before then.

south Florida at least once a decade.  It doesn’t make sense that these once a decade frosts just began to occur ~11,000 years ago and were absent for the previous 2 million years.  It just seems improbable that frosts began to occur in the deep south during the Holocene, a time of relative climatic stability, but didn’t occur during the Ice Ages which were times of dramatic climatic fluctuations (as the above chart shows) and generally of cooler climates.  If it’s true that giant tortoises couldn’t survive in an environment of light frosts, than that means they were extirpated in the southeast every time there was a frost.  They could only recolonize the south from enclaves in central America or what’s now Mexico, but that would mean a geographical corridor in the deep south must have remained frost free for thousands of years at a time–an unlikely climatic scenario, even during warm interglacials.

2. Scientists believe giant tortoises couldn’t escape the cold because they didn’t dig burrows.  This is a shaky assumption.  The only surviving species of giant tortoise lives on islands near the equator where there are no frosts.  As I discussed with my first point, Hesperotestudo did evolve in a region that must have had occasional light frosts, and therefore to survive, it must have evolved adapatations to escape the cold.  Moreover, Hesperotestudo is not the same species as extant giant tortoises, and we have no knowledge of its behavior patterns.  It’s closest living relative, the gopher tortoise, has a deeply innate instinct to dig burrows, and I see no reason for the assumption that giant tortoises didn’t also dig burrows.  Sea turtles dig deep pits to lay their eggs, proving that size is no obstacle to digging deep holes.

Gopher tortoises dig extensive burrow systems. The giant Pleistocene tortoise was closely related to the gopher tortoise.  There is no reason for the assumption that they did not also dig burrows which would have helped them survive frosts.

3. There is no evidence of tropical plants or pollen in the Pleistocene fossil record of the deep south.  If winters were warmer than those of today, and frost free, there should be fossils of tropical species of plants.  Instead, for example, a study of fossil plants from a site in the Aucilla River in north Florida, dating to the Pleistocene, found almost the exact same species that exist in the region today.  No tropical species were found.  Only 3 species outside their present day region were discovered here–osage orange, wild squash, and hazlenut. All three are temperate species, and the latter prefers cooler temperatures than exist today here.

4. Fossils of extant mammal species tend to be on average of individuals larger than those of the same species found in the region today.  According to Bergmann’s Rule, this indicates cooler climates and precludes warmer winters.

5. The prolonged freeze of 2009/2010 in south Florida caused a high mortality rate of the invasive Burmese python but did not cause their complete extirpation.  It seems reasonable to suppose that eventually, large reptiles that are maladapted to occasional frosts, would through selective pressure evolve to have an adapatation that enables them to seek thermal refuges.  And in fact, there are 2 clades of Burmese pythons with differing behavior patterns in their responses to frosts: the majority of the ones imported for the pet trade come from southeast Asia, and they’re naive to frost; but another population of this species occurs in temperate regions, and they’ve learned to seek refuge and hibernate during colder times of the year.

Like the northern population of Burmese pythons, and the American alligator, the giant Pleistocene tortoise was likely an animal of the subtropics that extended its range into southern temperate regions during warmer climatic stages.  And like pythons and alligators, selective pressures chose those individuals that took action to escape frost.  Alligators know to escape frost by moving into deep water, while caimans and crocodiles and southern Burmese pythons continue basking in subfreezing temperatures which leads to their deaths.  Like the alligator, Pleistocene giant tortoises must have survived frosts by moving to thermal enclaves such as burrows they dug themselves, the dens of other species, caves, hot springs, or under upturned tree roots.  How they survived frost is a subject for conjecture, but I have no doubt that somehow they must have.

Seals and Walruses off Southeastern North America’s Pleistocene Coast

January 28, 2011

Scientists expressing concern about anthropogenic global warming always seem to ignore paleontological evidence.  There is no better example of their alarmist approach than the oft-stated fear that global warming might cause the extinction of arctic marine mammals.  Yet, during the Sangamonian Interglacial (~132,000-~118,000 BP) the north polar ice cap completely melted, and there are no known extinctions of arctic marine mammals when this occurred.  Pleistocene ranges of seals and walruses were greatly expanded compared to the extent of where they live now.  And during both cold and warm climate cycles marine mammals occurred much farther south than they do today.  It’s more likely that their ranges are more limited today due to anthropogenic overhunting rather than changes in climate.  Evidence of my above-stated observation comes from fossils of seals and walruses found off the coast of South Carolina.

Gray Seal–Halichoerus grypus

Photo of gray seal from google images.  This species occurred at least as far south as South Carolina during the Pleistocene.  Now that they’re protected, it’s believed their range will expand farther south than New Jersey once again.

Fossils of this species have been found on Edisto Beach, South Carolina.  Until the Marine Mammal Protection Act of 1972, bounties kept the population of gray seals low, reducing them to 2,000 individuals on the North American side of the north Atlantic.  Since then, the population has expanded south to New Jersey where they frequently resort during winter.  They’re expected to spread even further south and may some day recolonize South Carolina.  They eat fish, lobster, and octopus.

Hooded Seal–Erignathus barbatus

Hooded seal from a picture at google images.  Man are they ugly.  The balloon on the males’ nose is used to attract mates.

Hooded seal flipper bone fossil found off the South Carolina coast.  All the pictures of fossils in this blog entry are from Additions to the Pleistocene Mammal Fossils of South Carolina, North Carolina, and Georgia by Albert Sanders.

The above fossil flipper bone was recovered in Horry County, South Carolina and is believed to be Sangamonian in age.  This solitary species mostly occurs in the arctic but is known to wander widely and has been reported in modern times as far south as the Carribbean.  Perhaps this specimen belonged to one such straggler.

Walurs–Odobenus romarus

Photo of a walrus from google images.

Fragments of walrus tusk fossils  discovered off the coast of South Carolina.

At least 6 fragments of fossil walrus tusks have been recovered from sites near Charleston, South Carolina, proving this species lived in the southeast long ago.  Moreover, one amateur fossil collector reports finding a walrus fossil in Florida.  Other walrus fossils turned up in North Carolina and Virginia, so at one time they must have been frequent visitors to southern beaches.  Most of the walrus fossils are assumed to be from a warm interglacial age because they’re found near the present day coast.  During glacial times, the Atlantic coast was many miles to the east of the present day coast due to the drop in sea level during the Ice Age.  I have no doubt walruses are solely limited to arctic regions today because of the remote geography where they’re difficult for human hunters to access.

Walruses mostly eat marine worms and molluscs which they find on the ocean floor with their whiskers.  Male walruses occasionally kill or scavenge and eat seals.  This behavior is probably related to high testosterone levels, much like male elephants that go on a rampage when they’re in the mood to mate.

Monk Seal–Monachus tropicalis

It’s sad to think that this is the only evidence left of a species once frequenting Georgia’s coast.  It’s a vertebrate of a monk seal.

Man hunted this warm climate seal into extinction by 1952.  It’s last reported sighting was off the coast of Jamaica.  It was last reported off the coast of Texas in the 1930’s.  One fossil collector  found bones of a monk seal on Andrews Island near Brunswick, Georgia, and others have found them at many sites off the coast of South Carolina.  What a shame the species couldn’t make it to 1972 when the Marine Mammal Protection Act passed.

Reference

Sanders, Albert

Additions to the Pleistocene Mammal Faunas of South Carolina, North Carolina, and Georgia

American Philosophical Society 2002

Georgia’s Pleistocene Horses

August 20, 2010

A herd of handsome, reddish-brown horses graze on a small beautiful prairie at the bottom of a lightly forested foothill.  A stallion pauses and raises his head from a clump of grass and white asters, while his harem of six with three foals continue tearing at the tawny and green broomsedge.  He detects the distinct odor of dire wolf nearby.  He whinnies in alarm and begins high-stepping as if signaling for his mates that they must gallop away from this place and now.  The mares get ready to follow his lead, but one seems reluctant to leave.  He nips her on her side to get her going, and they stampede 400 yards to the other side of the prairie where he leads them on a well-used trail through a young stand of oaks and pines.  A flock of grouse explode into the air, startling the horses and sending them in a different direction.  They hurdle bushes, vines, and fallen trees and reach a small creek.  Here they encounter a lone bull mastodon which they perceive as no threat.  They stop and drink, the wolves no longer close enough to pose an immediate threat.

A scene such as I described was probably a common one in North America during the Pleistocene.  Fossils of horses are usually found in most Pleistocene sites.  In Georgia disarticulated horse bones and especially teeth have been recovered from Ladds Mountain and Kingston Saltpeter Cave in Bartow County, and from the Isle of Hope, the Mayfair site, Porter’s Pit, Savannah River Dredgings, and Turtle River Dredgings along the state’s coast.

I perused the scientific literature to determine how many Pleistocene species of horse there were in what’s now Georgia and have concluded there were at least two.  An early paleontologist (Leidy, 18th century) identified one horse fossil from Georgia as belonging to the complex-toothed horse (Equus complicatus), also found in many eastern sites from Kentucky to Mississippi.  Horse fossils from South Carolina have been identified as belonging to the brother horse (Equus fraternus).  And Dr. Clayton Ray wrote that horse teeth from Ladds compared favorably to those of the Mexican horse (Equus conversidens).  However, a genetic study conducted by scientists from the San Diego Zoo compared DNA from the modern horse (Equus caballus) with that from Pleistocene-age fossils of the Yukon horse (Equus lambei), and they determined the two were actually one and the same species.  This calls into question the supposed large number of species of North American horses living here during the Ice Age.  It’s likely the number of species is inflated and many of the supposed different species are also simply no different than the modern horse, so loved by many today.

The confusing classification of Pleistocene horses is no surprise.  Different scientists in different places looked at newly discovered fossil horse teeth and couldn’t find an exact match, so they declared them as belonging to new species when the differences from known teeth were probably interspecific and due to a wide range of natural variance because horses were so abundant and widespread.  Imagine if horses went extinct today, and 10,000 years later a scientist in one region found bones of a large Clydesdale horse, and a scientist in another region found bones of a Shetland pony.  Each would conclude they’d discovered a new species, not realizing the great variability within the species.

True horses, known as caballoids, inhabited Georgia, but wild asses or donkeys, known as hemionids, lived here as well.  In Florida fossils of the pygmy onager (Equus tau) are sometimes found.  The hemionids are also split into many different species but probably can be simplified into one.

Upper left photo is of a wild horse.  Upper right is a photo of a horse tooth identified as Equus complicatus, that was discovered near Natchez, Mississippi.  Bottom photo is of Asian wild asses.  The confusing number of Pleistocene species of horses can probably be simplified into two:  the true horses or caballoids, and the wild asses, or hemionids.  The horse photo is from www.rewilding.org.  The ass photo is from www.birding-southamerica.com. The tooth photo is from an interesting website www.backyardnature.net.  An essay on this latter site discusses the glacial loess found in northern Mississippi.

During the Pliocene the zebra was probably the most common horse species in what’s now Georgia.  The zebra may have been one of the first single-toed horses to  evolve from their 3-toed ancestors.  There’s no way of knowing whether it had stripes like modern zebras.  Photo from www.JamesWarwick.co.uk.

Farther back in time, during the Pliocene, a species of horse (Equus simplicidens) resembling Grevy’s zebra roamed over what’s now Georgia along with the last of the 3-toed gazelle horses which were a dominant herbivore throughout the Miocene.  Horses first evolved in North America, and scientists found evidence of the very oldest known species of horse (the dawn horse) at the Red Hot Fossil Site in Mississippi.  A Wal-Mart has been built next to the site, but the site is still protected.  Fossils from here date all the way back to the Eocene, some 50 million years ago.  The dawn horse is famous for being at the bottom of the horse family tree in biology textbook discussions of horse evolution.

What happened to North America’s horses?

They became extirpated from the continent about 12,000 years ago.  I think human hunting contributed to the destruction of the entire population.  Humans didn’t kill every last individual, but they increased the mortality enough so that combined with normal natural mortality due to disease and predation, it exceeded the horse’s ability to reproduce and maintain a viable population.  Climate change models of extinction for North America’s horses don’t make sense.  When Europeans re-introduced horses, the beasts thrived in feral populations everywhere from Georgia to Nevada.  It’s hard to imagine a climatic phase that occurred 12,000 years ago that for only a narrow window of time rendered the entire continents of both North and South America unfit for horses.  And there is even archaeological evidence of humans hunting North American horses.  Clovis arrowheads with horse blood on them have been discovered associated with horse bones at Wally’s Beach in Alberta, Canada.  The scientists who studied the site think human-hunting combined with climate change caused the extinction of horses on the continent, but I disagree.  It was either one or the other.  I favor human hunting because horses survived millions of years of sometimes drastic climate changes but didn’t become extinct here until man shows up in the archaeological record.

Unlike bison, wild horses don’t migrate long distances.  Once humans exterminated them from a region, they were gone.  The last wild horses and asses lived in only the most remote areas of Asia and north Africa, indicating these regions remained less populated with people than any region in North America.

It pleases me to think of wild horses galloping across the woods and savannahs of Georgia.  Today, a nearby neighbor keeps horses that I sometimes hear whinnying in the evening.  I relax and imagine myself living in a cabin 36,000 years ago.