Posts Tagged ‘Cuban crocodile’

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.


Olson, Starrs

“Fossil Vertebrates from the Bahamas”

Smithsonian Contributions to Paleobiology #48 1982