Posts Tagged ‘Pleistocene giant tortoise’

Banana Hole Fossil Sites

November 11, 2011

The Pleistocene Bahamas epitomized the romantic image of deserted islands.  Lowered sea levels in response to glacier expansion over Canada (known as a eustatic fluctuation) caused the land area of the islands to expand to more than 10 times their present size.  This consolidated the present day 29 islands and 661 sand spits into 5 major islands, plus 1 major island (the Cay Sal Bank), now completely inundated, rose above sea level.

Map of the Bahamas.  The light purple represents the area that rose above sea level during the Last Glacial Maximum. 

Despite being surrounded by ocean, the climate in the Bahamas during the LGM (~28,000-~15,000) was arid.  Temperatures were on average about 8-10 degrees F cooler than those of today but still never subfreezing.  The dry climate fostered an environment consisting of extensive grasslands dotted with occasional scrub oak thickets or hardwood hammocks growing near scarce freshwater springs.  The island hosted no large mammal species giving them a deserted feel that many a writer imagines when casting a fictional ship-wrecked crew on islands.  Other than bats, the only mammal known to have lived on the Pleistocene Bahamas was the hutia (Geocapromys ingrahami).

Hutias were the only mammal besides bats that are known to have lived on the Pleistocene Bahamas.  They’re large rodents weighing up to 15 pounds.

Mammal life on Pleistocene Cuba just to the south of the Bahamas was more diverse–dwarf ground sloths and many insectivores and rodents lived there–but none ever managed to colonize the Bahamas.  The dearth of mammals meant birds dominated the environment.

Extinct giant barn owls were a top predator on the Pleistocene Bahamas.

The most spectacular predator on the Pleistocene Bahamas was the extinct giant barn owl (Tytos pollen) which was 3 times the size of a modern barn owl (Tytos alba).  Today, barn owls are rare visitors to the Bahamas, but evidentally they were as common as their extinct larger cousins here during the Pleistocene.  Instead of barns they must have roosted in the abundant caves on the islands because there weren’t many hollow trees either.  Burrowing owls were also common on the Bahamas.  A relic population of burrowing owls still lives on one corner of a Bahamian island today.  The extinct 3 foot tall burrowing owl that dominated Pleistocene Cuba never made it to the Bahamas.  Another interesting predatory bird on the island was an extinct giant hawk (Titanohierex gloverallani) that was larger than most species of eagle.  It formerly ranged throughout the Caribbean during the Pleistocene.  Large extinct subspecies of sharp-shinned hawks and red-shouldered hawks were common on the islands then but don’t live there now.

Scientists assume grasslands covered most of the Bahamas during the Ice Age based on the kinds of birds found in the fossil record.  Almost all are species that require an open landscape–burrowing owls, eastern meadowlarks, thick-knees (long-legged plovers), Key West quail-doves, Cuban crows, nighthawks, and caracaras.  The presence of some species suggests some scrub oak thickets existed.  There were Cuban parrotts, white crowned pigeons, red-necked pigeons, scaled pigeons, Bahamian mockingbirds, great lizard cuckoos, red-bellied woodpeckers, red-legged thrushes, tanagers, hummingbirds, and others.  Of course, sea birds must have been abundant.

One megaherbivore did trudge across the deserted landscape.  An undetermined species of giant tortoise (Hesperotestudo sp.) thrived on the dry vegetation.  Rock iguanas shared the dry land plant foods with their distant reptilian cousins.  Smaller lizards–several kinds of anoles and at least 1 species of gecko–abounded on the deserted paradise as well.  Dwarf boa constrictors terrorized the hutias and along with raptors and owls kept their populations in check.  Crocodiles patrolled the marshes.  Scientists aren’t sure if the species was the same as the extant freshwater (Crocodylus rhombifer) or saltwater (Crododylus acuta) crocodiles still found in Cuba or whether it was a unique extinct species because the fossil material is too meager.  Perhaps both lived on the islands then.  There are no crocodiles on the Bahamas today because the existing mangrove swamps are too saline for juvenile saltwater crocodiles which prefer brackish swamps.

If we could take a time trip back to the Pleistocene Bahamas, crocodiles would be the only dangerous animal to be wary of.  This is a Cuban freshwater crocodile.  From google images.

The extinction of most of these species was caused by environmental change.  Caribbean pine trees spread over the islands beginning about 13,000 years ago.  Rising sea levels inundated much of the habitat, and a humid climate allowed pine forests to replace the grassland, eliminating any suitable remaining habitat for many of these species.

Fossil sites on the Bahamas are known collectively as banana holes, and one site is known specifically as the Banana Hole Fossil Site.  Originally, they were caves or sinkholes formed from coral reefs that became covered in sediment.  The fossil coral reefs became limestone, and rainwater dissolved the stone, creating caverns underground.  Some remain as caves, others collapse and form sinkholes.  Large animals such as tortoises and iguanas fall into these caverns and die, but the majority of fossils here are deposited by roosting predatory birds.  Over time fertile soil fills the caverns.  Farmers mine this rich soil and many fossils are actually found in farmer’s fields.  Thanks to the productive soil, banana farmers often choose these sites to plant their trees, hence the name–banana hole fossil sites.

Photo from google images of Caribbean bananas.  I love these little bananas which are far tastier than the big ones most often found in American supermarkets.  Bill Clinton took a $600,000 bribe from Chiquita banana and in exchange agreed to help crush the Caribbean banana industry with international trade agreements that punish small farmers.  Bill Clinton is a crooked shmuck.

Reference:

Olson, Starrs

“Fossil Vertebrates from the Bahamas”

Smithsonian Contributions to Paleobiology #48 1982

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.