Trapped in a Relationship

47-million-year-old-site in Germany finds first vertebrate fossils to directly document mating behavior.

Fossils of mating turtles

Naturmuseum Senckenberg in Frankfurt

Most of the time paleontologists must make inferences about the behavior of extinct creatures, but every now and then fossils provide a snapshot of what animals were up to at the moment they perished. That is the case for the turtles Allaeochelys crassesculpta, part of a trove of rare fossils to have emerged from the Messel Pit, a 47-million-year-old site near Frankfurt, Germany.

Paleontologist Walter G. Joyce of the University of Tübingen, Germany, and colleagues closely examined nine pairs of the turtles and confirmed that each pair comprised a male and a female. In seven cases, the carapaces were in direct contact with one another, and in two of those pairs, the characteristically elongate tail of the male was aligned with that of the female and wrapped below her carapace. Those observations, among others, confirm that the couples perished while mating.

The rocks containing the turtles are the legacy of an ancient volcanic lake, hypothesized to have had habitable surface waters, but toxic layers deeper down. The researchers propose that, as some modern-day turtles do, the pairs likely began copulating near the surface and then sank, locked in embrace, into the deadly depths, absorbing the toxins from the water below through their skin. Two competing hypotheses—that periodic belches of volcanic toxic gases, or those emitted by aquatic cyanobacteria, could have poisoned the lake’s surface waters—appear less likely. “The mating turtles tell us that the surface waters of Messel Lake were hospitable enough to allow turtles to live and mate,” explains Joyce.

The turtle tangles represent the first vertebrate fossils to directly document mating behavior. (Biology Letters)