Dec. 3, 2007 -- Divers exploring a water-filled sinkhole in the Bahama Islands recently recovered one of the world's largest and most pristinely preserved collections of animal and plant fossils from a tropical island.
Like a time machine, the fossils reveal in stages what ecosystems were like on the island of Abaco from periods between 12,000 to 1,000 years ago.
"Their ultra-high quality of preservation puts the fossils in a category all their own," David Steadman, who led the project and is curator of ornithology at the Florida Museum of Natural History, told Discovery News.
"The potential for future analysis involves physical as well as chemical analysis," he added before explaining that stable isotopes, or atomic particles, can show what certain species ate, allowing scientists to reconstruct entire ecosystems.
The "blue hole," called Sawmill Sink, is a water-filled void in limestone bedrock that's open at the surface. The water, depleted of oxygen, necessitated special diving equipment and methods.
The divers wore "mixed gas rebreathers," closed-circuit devices that don't release exhaled air bubbles. This prevents bubbles from disturbing the site's unique water chemistry, while keeping the bubbles from whipping up clouds of bacterial mats, which could obscure visibility.
The fossils included two extinct species of tortoise. One specimen had three sets of healed bite marks from a Cuban crocodile that's now locally extinct in the Bahamas.
A particularly large group of fossils came from a part of the site known as "the owl roost."
"Owls cough up bony pellets and are extremely efficient accumulators of small vertebrates," Steadman explained.
Although no ancient owl was found at the roost, this part of the site yielded one species of lizard, three types of snakes, 25 species of birds and four bat species. Among the birds, one was a never before described extinct flightless rail. Four other locally extinct birds -- the Cooper's Gundlach 's hawk, the flicker, the cave swallow, and an eastern meadowlark -- were also recovered from the roost.
Aside from the bats, the only other mammal fossils belonged to a hutia, which is a large rat, and a 10 to 13-year old human found buried in the sinkhole. The individual, who lived on the island around 1,050 years ago, represents the earliest known human occupant of the northern Bahamas.
"What's also notable is that, aside from the humans in the later periods, here is an ecosystem where reptiles, rather than mammals, were the big carnivores," Steadman said, adding that preliminary chemical studies reveal the crocodiles on the island were mostly terrestrial, as opposed to aquatic, feeders.
Fossils of pine trees, Acacia, Ficus and other woody species indicate the island once consisted of grassy pineland that changed to more of a tropical, dry evergreen forest.
The findings appear in this week's Proceedings of the National Academy of Sciences.
Steadman hopes the new knowledge about locally extinct species may lead to their reintroduction to Abaco in future. Already, the University of Exeter's Peter Mumby and his colleagues have found that a once near-extinct predator, the Nassau grouper, is returning by itself to a coral reef Bahaman ecosystem.
"Caribbean reefs are still trying to recover from the devastating effects of an El Nino bleaching event in 1998, which caused widespread damage to coral around the world," Mumby said.
In addition to aiding island conservation efforts, Steadman hopes the study will inspire further exploration of accessible limestone blue holes, mostly in tropical areas around the world.
He said, "I expect there are other 'treasure chests' full of exquisite fossils that have yet to be discovered and opened."
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