"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.