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Archive for December, 2007

New Galapagos

12 12th, 2007

Wearing elaborate and expensive diving equipment, cave diver Brian Kakuk sinks gently below the potentially poisonous hydrogen-sulfide zone of an Abaco blue hole. Below this level, where fresh and salt water meet, darkness closes in and only the beam from his powerful torchlight illuminates the mysteries of a long ago world. Kakuk has spent innumerable hours in this dark, delicate underwater sphere known as the Sawmill Sink, a blue hole south of Marsh Harbour, where extraordinary discoveries have been made of remarkably intact and diverse animal fossils. They include specimens of an extinct and previously unknown tortoise; 38 skeletons of Cuban crocodiles; an unknown flightless shorebird; an extinct falcon, Bahamas Caracara; and remains of present-day species, many of which are no longer found on Abaco, including the hutia, a rodent still living on a few cays in the central and southern Bahamas.

Most of these creatures are thought to have arrived in the Abacos more than 10,000 years ago, when the last ice age dramatically lowered global sea levels by hundreds of feet, exposing the banks of the Bahamas and allowing animals to disperse to the now- separated islands. During this period, the Sawmill Sink was a dry cave, inhabited by owls whose discarded meals Blue Holes of Abaco
created a bed of bones. Later, when sea levels rose to near their present level, the sink became aquatic. During this phase, crocodiles, hutia and birds became entrapped and died. The slow-moving, three-foot-long herbivorous tortoises most likely fell into the sink and drowned there. Sediment soon covered their remains, so that neither scavengers nor weather destroyed them.
About 2,000-4,000 years ago, the sink may have been surrounded by brackish swamp, a habitat more suitable to the crocodiles whose skeletons Kakuk found preserved in a 70-foot, cone-shaped pile of peat on top of terrestrial animal fossils on the cave floor. Each layer of this undisturbed “talus cone” tells a compelling story of geology and biology, one that is fast revising what is currently known of Abaco’s prehistory, and that may in fact further humanity’s understanding of how and why species change.
Vertebrate paleontologist Gary Morgan, one of several scientists currently identifying specimens from the Sawmill Sink, has surveyed fossils from various Caribbean islands for more than 30 years. Curator of paleontology at the New Mexico Museum of Natural History, Morgan was the first to discover a fossilized crocodile in the Cayman Islands, a fresh-water species of crocodile, which now lives only in the Zapata swamp on the south coast of Cuba. As a biologist of extinct animals, Morgan usually works with “disassociated parts: a tooth, tailbone, part of a jaw.” By comparison to most fossil sites, the Sawmill Sink is a paleontologist’s mother lode. “To find a whole skeleton is vanishingly rare.” Morgan says. “Abaco has produced, hands down, the best fossils in all of the Bahamas.”

So “exquisitely preserved” are the fossils that Morgan is confident they will render precise radiocarbon dating to determine their age, as well as DNA information to determine their genetic relationship with other species in the Caribbean. He says it’s normally impossible to recover DNA data from bones that are between two and 10,000 years old, but some fossils from this site actually include tissues, stomach content and excrement.

“It’s the first time that complete and well-preserved specimens of any type of fossil have been found in the Bahamas,” says Dick Franz, an associate scientist at the Florida Museum of Natural History at the University of Florida in Gainesville. Franz is a museum herpetologist who specializes in amphibians and reptiles of the Bahamas and West Indies. Why the Sawmill Sink specimens are whole seems to be lucky chemistry, as neighboring blue holes have not yielded such well-preserved fossils. As Franz explains, “The Sawmill Sink bones appear not to be mineralized like so many fossils, but maintain their original boney nature. This requires very special environments, such as water where little or no oxygen is present. The salt water in the zone of the best preserved fossils probably added to the preservation.”

At its surface, the sink appears to be a fresh-water pond, 55 feet across, in the midst of the pine barrens. Underwater, however, its deep vertical entrance widens into a cavern 550 feet in circumference. Here, the water is saline, mixing with in-coming tidal seawater via a complex subterranean cave system.

As Brian Kakuk submerges, he passes through the blue hole’s red and potentially poisonous acidic layer that often occurs where fresh and salt water meet, where sunlight is kept from penetrating the cave’s depths. Below this layer, Kakuk and other expert divers collecting specimens must rely on torchlight. Their diving apparatus includes closed-circuit re-breathers which do not release air bubbles, essential for maintaining the low-oxygen chemistry of the water in the blue hole. A re-breather scrubs carbon dioxide from the diver’s used air, recycling it and extending, by hours, the time a diver can remain underwater.

Kakuk and the other project divers take precautions not to disturb the deep deposit of fine silt on the cave floor, 95 feet down. Failing to do so makes recovering fossils more difficult, and more risky, as raised silt quickly reduces visibility. “It’s a really dangerous hole,” says Nancy Albury, project coordinator of the Sawmill Sink Fossil Site.

For years, Albury has maintained a database on Abaco’s caves and blue holes, taking scientists to these sites and recording their tidal fluctuations, geology, flora, fauna, and other data. A caver since 1970 and a long-time Man-O-War resident, Albury is presently a graduate student of geosciences at Mississippi State University. She says the sink project is a work in progress, adding that “Brian has a knack for seeing anomalies and finds something new almost every dive” He discovered the first tortoise in December 2004, and last summer, Albury says, divers identified five new crocodiles in one week.

The crocodile skeletons found in Abaco, including skulls complete with lower jaw and teeth, present a tantalizing case for Morgan, who says they are unlike any others found thus far in the Bahamas. Although bat fossils are his specialty, Morgan became a “self-taught” crocodile bone expert after finding Grand Cayman’s fresh-water Cuban crocodile fossils. And his preliminary examinations of the Abaco skulls point to the same fresh-water Cuban crocodile species, Crocodylus rhombifer. The bones of a crocodile collected in 1993 from another Abaco blue hole proved to be of the Cuban fresh water species and dated 2,840 years Before Present.

Morgan has not ruled out the possi- bility that the Sawmill Sink crocodile fossils may belong to a new species or subspecies, but he is sure they are all of the same species as one another. The skulls bear features similar to those of the Cuban species: a short, alligator-like head, and a short and broad snout. Fossils of the American salt-water crocodile, which still exists in Florida, Central and South America, and the Greater Antilles, have been identified on Andros, but not on Abaco. The American salt-water crocodile can grow over 15 feet long and has a wide girth, whereas the Cuban fresh-water species is no longer than eight-to-10 feet, with a narrower width.

The curious thing is that C. rhombifer fossils have not been found elsewhere in the Bahamas, nor even on any of the Greater Antillean islands near Cayman, excluding the still-living population in Cuba. Morgan speculates that during the last ice age, the Cuban crocodiles simply crossed the 10-to-30-mile gap between northern Cuba and Great Bahama Bank and spread out in the northern Bahamas, a wet climate zone. However, accessing Cayman from Cuba would require a 200-mile open-water swim, which leads Morgan to think that C. rhombifer must be salt-water tolerant, or maybe once was, before becoming trapped inland and adapting to its current brackish swamp habitat. Because Cuba is not open to scientific research, Morgan has not had access to the living population. “More skulls have been collected from the Sawmill Sink than from Cuba,” he says.

Early naturalists in the Bahamas describe an abundance of “alligators,” a term they often used synonymously with “crocodiles,” although alligators have no tolerance for salt water and are unlikely to have ever been present in the Bahamas. In a book published in 1725, Mark Catesby writes, “In shallow salt water, these impenetrable woods of mangroves are frequented by great numbers of alligators. … [I]n no place have I ever seen such remarkable scenes of devastation as amongst these mangroves in Andros… where the fragments of half-devoured carcasses were usually floating on the water.” By the late 19th century, these reptiles had vanished from the Bahamas, presumably hunted into local extinction.

As coordinator of scientific research for the Sawmill Sink fossil project, Franz is working together with Morgan and other University of Florida scientists, as well as fossil flora experts and geologists, to reconstruct past ecosystems of Abaco and the northern Bahamas, perhaps venturing as far back as the Pleistocene Epoch, which began almost two million years ago. Franz speaks expertly on the sink’s tortoise fossils, which are all members of an extinct and as yet “undescribed” species that appears to belong to a group of South American tortoises in the genus Chelonoidis, which includes the red-footed tortoise, yellow-footed tortoise, Chaco tortoise and giant Galapagos tortoise.

“Six shells of the extinct tortoise have been collected to date…. A large male, with a shell length of nearly 50 cm, includes an associated skull, limb bones, pectoral and pelvic girdles, and neck and tail vertebrae. Large seeds, probably part of a last meal, were found inside the shell of a large female.” While tortoise fossils have been reported from Andros, New Providence, Turks and Caicos Islands, Cuba and Hispaniola, Franz says the Sawmill Sink tortoise “is not like any other fossil tortoise from the Bahamas or the West Indies.”

Based on what he knows about similar tortoises, Franz says the Abaco species would have eaten fruits and leaves, and likely were important seed dispersers. “Since we are dealing with an extinct species, we can only speculate about how they lived. They could have been here [on Abaco] in high densities, such as we see on other islands today, because of the lack of large mammal predators. I suspect they lived in lightly forested habitats, probably pine forests, where there was enough forage for food. They probably knew when a particular shrub or tree was dropping fruit, as tortoises use their noses to smell ripe fruits.” Tortoises dig nest chambers in sand or loose soil and lay “hard-shelled, golf-ball-sized eggs,” Franz says. Based on the body size of the Abaco tortoise, he suspects they laid between five and 20 eggs.

Franz suspects the tortoise lived on Abaco for thousands of years, but this duration is just one of many facts still to be determined. “Where they came from originally is easy…South America. How they got to Abaco is an important question that is not easily answered,” Franz says. “I suspect they lasted until the native peoples arrived and ate them into extinction. They were easy prey.”

Albury has also coordinated analyses of seeds, leaves, pollen and other plant remains from Sawmill Sink. She says, “The forest was really different than it is today to have been crocodile habitat.” When botanists examined core sediment samples from different levels in the blue hole, they found that 50 feet down, samples revealed the presence of pine, but 85 feet down – a layer thought to represent the ecology of approximately 10,000 years ago – there was no evidence of pine; instead, samples revealed hardwood or coppice. This reinforces studies conducted by scientists from the University of Tennessee, who analyzed pond sediment on Great Abaco and saw a shift from predominantly coppice woods to pine forest beginning about 800 years ago, in sync with human colonization and related fires.

No one knows exactly how many blue holes there are in the Abacos; local explorers guess about 50. It is commonly said that if we were to cut the Bahama Islands in half, they would look like Swiss cheese. Unlike other Caribbean islands, which are the tops of volcanoes, the Bahamas formed from an accumulation of dead organisms, such as corals, mollusks and algae, which created masses of calcium carbonate over tens of millions of years. When the sea level dropped during ice ages, the calcium carbonate was exposed to air and rainwater, causing a chemical reaction that hardened it into limestone. Since limestone is soluble, over the ages, fresh water percolated through it, creating caves and blue holes.

Scientists on Abaco are studying such diverse areas as flora, bats, birds, geology, sea life and tree rings. Their work is important locally because it will provide a clearer picture of the region’s natural history and will inform future management of Abaco’s natural resources, such as the Abaco National Park, the pine barrens and coppice forests, and endemic species like the Abaco parrot. But their work is also important on a grander scale, as the Bahama Islands offer science something rare.

“I think the Bahamas are the young Galapagos,” explains Dr. James Hickey, Professor of Botany at Miami University, Ohio. “The Galapagos Islands were the group that showed how evolution occurred [in the past]. In the Bahamas, we’re actually seeing evolution occur. It’s the most dynamic archipelago possibly anywhere. It’s a tremendous science resource.” As Hickey explains, the Bahamas are “young islands; most weren’t here during the Pleistocene.” The islands are immature ecologically and still evolving. Add to this their unique history of having emerged as large land masses when sea levels dropped, then having separated into many islands, and you have a live classroom for speciation, the metamorphosis of one species into another.
“Geological processes make for differentiation in a species,” Hickey says. “The fluctuating water level has changed population dynamics on all of the islands. On the various islands, Hickey says, “We see differentiations between parrots, iguanas, anoles, boas,” offering as an example the Abaco parrot’s anomalous ground-nesting characteristic versus the Inagua parrot’s tree-nesting.

With the help of Friends of the Environment members, Albury has undertaken the crucial task of preserving, labeling, and categorizing the fossils. It’s delicate work. “Any time anything is moved, it has the potential for falling apart,” Albury says. Prepping a fossil takes up to six weeks, she explains. “They require a series of fresh water washes, without which the bones would totally deteriorate.” Then the fossils are coated in a polymer resin. “If they’re not prepped properly, the salt air causes them to fracture and crumble.” The Antiquities, Monuments and Museums Corporation, a private government entity based out of Nassau, has been the primary financial and logistical support for the project, and FRIENDS, the Bahamas National Trust, and the Abaco Department of Agriculture have provided local support.

In the early 20th Century, the Sawmill Sink site was a fresh-water resource for a lumber camp and provided water for the steam locomotive that hauled lumber to the dock for shipping to Nassau, Cuba, and the United States. Like many blue holes, it also served as a dumpsite. “The sink is now closed to all divers, unless permitted with the Antiquities, Monuments and Museums Corp., and Bahamians involved are serious about protecting it,” Albury says. The Sawmill Sink Fossil Site is part of Bahamians’ heritage, she says, and she wants locals to become involved in the project and to have a sense of ownership, while taking care to preserve it.

Once she has documented all the fossils, Albury hopes they will be displayed in both Nassau and Abaco. Some fossils will also be housed within the collection of the Florida Museum of Natural History at the University of Florida in Gainesville, which has a large West Indian research program and employs most of the scientists working on the Sawmill Sink project. Of particular importance to the Florida museum is a holotype of the new species of tortoise for future research.

When Charles Darwin visited the Galapagos Islands in 1835, he described the archipelago as a world in which “…we seem to be brought somewhat near to that great fact – that mystery of mysteries – the first appearance of new beings on this earth.” As the Abacos reveal their unique biological story, one that is yet in flux, shifting perpetually toward ecological maturity, we too come closer to “that mystery of mysteries,” the origins of species.



       
 

©  Jim Kerr, Abaco Life Magazine
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