In the glacier-laden world of the Late Pleistocene Epoch, the Caribbean region was much drier and cooler than it is today, with far more land exposed. Little is known about its ecology, as most fossils found in the West Indies have been from the current epoch, the Holocene.
A fossil trove in a flooded sinkhole on the Bahamian island of Great Abaco, found in 2004 and analyzed by a team of American and Bahamian researchers, has more than doubled the number of known Pleistocene species in the West Indies and offered great insight into their diversity. The study revealed that the island’s animal community changed dramatically in the Pleistocene-Holocene transition 15,000-9,000 years ago – and changed again since then, possibly after humans arrived. The findings were published in October 2015 in the journal Proceedings of the National Academy of Sciences.
The 112-foot-deep “blue hole,” one of many throughout the tropics, was largely high and dry during the Late Pleistocene. Barn owls and night-herons rested and nested on ledges in a high mound of rocky rubble at its base, regurgitating the bones and other undigested parts of their prey. Today it contains layers of fresh water, saltwater full of light-blocking hydrogen sulfide, and saltwater devoid of oxygen. Skilled divers have recovered more than 5,000 such bones of 95 species from the dangerous depths, protected from decay in the lifeless darkness.
These fossils come from an array of reptiles, marine fishes, bats, a rodent called a Bahamian hutia (Geocapromys ingrahami), and 63 species of birds. Along with snakes and lizards, the reptiles include an Alsbury’s tortoise (Chelanoidis alsburyum) and a Cuban crocodile (Crocodylis rhombifer), both of which probably fell into the sinkhole. Of the birds, mammals, and reptiles, 39 species are now gone from Great Abaco, either extirpated (locally wiped out) or extinct. When the findings were compared with fossils from Holocene deposits elsewhere on Great Abaco, the results were striking.
Of the 31 extirpated or extinct bird species from the Pleistocene deposit, 17 were absent from the Holocene ones. Five are now extinct, but many of the others remain widespread in North America, generally preferring grasslands or pine woodlands. Along with estimates of the epoch’s Caribbean climate; this suggests that such habitats covered the Late Pleistocene Bahamas. As the air grew hot and humid, tropical broadleaf forests took over and the rising sea shrank the island to less than a tenth of its former size, birds which no longer found it hospitable died out or moved away. Others moved in; the Holocene deposits contained 16 bird species absent from the Pleistocene – mostly residents of waterways or broadleaf forests.
But 9 bird species, 4 mammals, and 3 reptiles found in Pleistocene and Holocene deposits are now extinct or extirpated from Great Abaco, as are 5 birds found only in the Holocene. This was likely due to the arrival of humans within the past thousand years. Some were probably hunted out, such as the extinct tortoise and extirpated crocodile. Others may have succumbed to the loss of their remaining habitat, such as a bat species dependent on hot, humid caves which were always scarce on the island. The study concluded with a suggestion that for the island’s remaining native species, “direct human activities, such as habitat alteration and introduction of invasive species, threaten their future more than climate change.”
The trove offered other inspirations for further study. Pleistocene fossils of the tortoise, crocodile, and other tropical reptiles indicate that they could tolerate more diverse climates and habitats than those within their current ranges. The fishes were the first Pleistocene fish fossils to be found in the West Indies, a starting point for understanding the epoch’s marine ecology. And many Caribbean blue holes have yet to be explored for secrets of the region’s past – and suggestions of its future.
Steadman, D.W., Albury, N. A., Kakuk, B, Mead, J.I., Soto-Centeno, A., Singleton, H. M., & Franklin, J. (2015) Vertebrate community on an ice-age Caribbean island. Proceedings of the National Academy of Sciences 112:44, E5963–E5971