Deep below the surface of a water-filled cave in Madagascar, divers and paleontologists have uncovered a boneyard full of extinct giant lemurs.

Hundreds of bones dot the silty bottom of Aven Cave in Tsimanampetsotse National Park. The remains include exotic species such as the extinct elephant bird, a flightless giant similar to an ostrich, but the most numerous bones are from long-lost giant lemurs.

The largest of the extinct lemurs were as big as gorillas, and paleontologists sometimes refer to the different types as sloth lemurs, koala lemurs, and monkey lemurs to describe their different lifestyles and the living animals they most closely resemble. Sometime between 2,000 and 500 years ago, all these giants disappeared, possibly at the hands of humans.

The underwater caves offer an unprecedented look at these lost species. "The preservation is really incredible," says Brooklyn College anthropologist Alfred Rosenberger, a National Geographic grantee who is leading the project.

Puzzling Burial

How all these animals found their way into Aven Cave is a mystery. The diving and scientific teams have only just begun to catalog what's on the surface, much less puzzle out how the bone bed was assembled. But from what they've uncovered so far, Rosenberger suspects that many of the bones washed into the cave over time, both before and after humans arrived on the island.

In addition to individual bones, the cave contains entire skeletons that provide a detailed look at the anatomy of these extinct animals. The remains show hardly any sign of post-death damage (from, say, being eaten). From the look of them, Rosenberger says, some of the lemurs "got defleshed in place," with decomposition slowly revealing the bones, and have rested in peace ever since.

While the giant extinct lemurs Pachylemur and Mesopropithecus are among the most tantalizing finds in Aven Cave, they're hardly the only animals there. The list includes birds, turtles, crocodiles, rodents, carnivores, and more.

"We have a real cross-section," Rosenberger says, of both "tiny things and big things."

And these are just the finds scientists can see on the cave floor. "Who knows what's under there?" Rosenberger wonders.

Searching for Answers

The team, led by Phillip Lehman of the Dominican Republic Speleological Society, has also explored two other caves in Madagascar that preserve different animals from the same time as those in Aven Cave. One of these, Mitoho Cave, seems to be a den where the predator Cryptoprocta spelea, an extinct carnivore known as the giant fossa, once dwelled. (Watch a National Geographic video about the mysterious fossa of today's Madagascar.)

It's unusual for paleontologists to see so many well-preserved, relatively complete skeletons in one place. "The abundance of the Malagasy remains is extraordinary," says Rutgers University anthropologist Susan Cachel.

The skeletons will also give paleontologists an unprecedented look at lemur species that have previously been cobbled together from isolated parts, adds Stony Brook University anthropologist William Jungers.

"I hope the fossils will yield dates and perhaps [ancient] DNA that will bear upon the extinction process that took place," Jungers says.

The promise of the caves is "the possibility of doing things that couldn't be done before," says Rosenberger. Bone by bone, he hopes, a new view of Madagascar's lost world will come into view.