If anything could be more embarrassing than dying while having sex, it might be being preserved in flagrante delicto for millions of years so that members of an advanced species could dig you up, gawk at you, and write a journal paper about your final romantic encounter. For a group of ancient turtles, this nightmare just came true.

Online Wednesday in Biology Letters, paleontologists describe nine couples of a species of aquatic turtle that perished while copulating and that were then preserved -- the first such record among vertebrates, the researchers say. Far from a mere salacious look at the unfortunate reptiles, the fossils provide critical clues about the environment in which they lived.

For decades the Messel pit, a fossil site in west-central Germany, has yielded extraordinarily well-preserved remains. The fossils include complete skeletons of creatures ranging in size from rodents to pygmy horses, as well as insects and feathers that still have hints of their original colors. The oily shale that entombs those fossils was laid down as lake sediments about 47 million years ago, says Walter Joyce, a vertebrate paleontologist at the University of Tübingen in Germany. Although the site has yielded tens of thousands of fossils, only the saucer-sized turtles have been found in pairs.

While some researchers have speculated that the animals died while copulating, the new analysis is the first to provide strong evidence -- revealing, for instance, that each pair includes a male and a female. Male turtles of this species, like many of their modern relatives, have longer tails than females, says Joyce. Also, males are typically smaller than females, a trend clearly seen in the fossils. Finally, in seven of the nine pairs, the turtles are in direct contact along the edge of their shells just above their tails -- and in two of those pairs, the male's tail wraps below the female's shell in mating position.

"I'd heard about this hypothesis at various workshops," says Tyler Lyson, a vertebrate paleontologist at Yale University. "When [Joyce and his team] found that each pair included a male and a female, I was sold on the idea."

That still leaves a couple of mysteries: How did the turtles die, and why at such an intimate moment? Previously, some scientists had suggested that toxic algal blooms may have tainted the lake. Yet that scenario doesn't make sense, says Joyce, because there are no sediment layers containing large numbers of remains that would suggest mass death in a single event. Also, he notes, researchers haven't found any fossils of cyanobacteria that might have caused such blooms.

Other teams have suggested that the surface waters of the lake were saturated with dissolved carbon dioxide or other naturally toxic substances, as some volcanic lakes are today in Africa. Creatures that drank from the lake, including bats and birds that grabbed a sip as they swooped across its surface, could have succumbed to the substances dissolved in the lake, those scientists suggest. But that scenario doesn't jibe either, says Joyce, because researchers have found thousands of fossils of fish at the Messel pit site -- a sign that at least some of the lake's waters supported life. "This lake was clearly a good place to live."

Joyce and his colleagues suggest instead that while surface waters of the ancient lake were oxygenated, deeper layers were oxygen-poor and possibly saturated with carbon dioxide or other toxic substances. The lack of oxygen in deep water would help explain the wonderful preservation of creatures whose remains fell to the lake bottom.

The disparity between deep waters and the surface would also explain how the turtles ended up dying in pairs, Joyce says. Modern-day relatives of this species can absorb dissolved oxygen from the water through their skin, a trick that helps them stay submerged for long periods of time. Plus, he notes, the largely aquatic turtles mate in open waters and often begin to sink when they're copulating -- which is no problem in most lakes but proved fatal in the Messel pit lake. "Mating in turtles is quite strenuous and can go on for long periods," says Lyson. It's easy to see how these pairs could have run out of oxygen if they descended into anoxic or otherwise toxic waters.

The scenario "is very speculative but plausible," says Donald Jackson, an animal physiologist at Brown University. It's possible that the ancient turtles, like some modern species, were relatively oblivious to their surroundings when they were copulating and, therefore, unable to escape the deadly depths. Also, he notes, if the waters were warm, the turtles' revved-up metabolisms and increased activity would likely have consumed the oxygen in their bloodstream at an exceedingly high rate. "This could have produced mortality very fast."

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This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org

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