German palaeontologists have unearthed nine pairs of fossilized turtles that perished while having sex some 47 million years ago, unveiling the first-ever fossil record of copulating vertebrates.
Dug up at the Messel Fossil Pit, a UNESCO world heritage site south of Frankfurt in western Germany, the turtles belonged to an extinct species known as Allaeochelys crassesculpta. It is believed the copulating turtles succumbed to poisonous gas at the bottom of a volcanic crater.
Detailed analysis of the fossil material, published in the latest edition of the Royal Society Journal Biology Letters, revealed that each pair consisted of a female and male individual.
More importantly, even though the males typically face away from the females, the tail of some male individuals can be found wrapped under the shell of the female.
“There is no doubt in my mind. These animals died some 47 million years ago in the act of mating. No other vertebrates are known to have died during this important biological process and then been fossilized ,” Walter Joyce, a geoscientist from the University of Tübingen, said.
Most scientists agree that the Messel Pit Fossil Site originated as a deep volcanic crater lake that preserved animals and plants that sank to its bottom. But some questions remain, such as whether the lake had poisonous surface or only subsurface waters.
Modern relatives of the fossil turtles found at Messel have permeable skin that allows them to breathe and stay under water for a long time. However, this adaptation can become lethal if these turtles enter poisonous waters.
According to the researchers, the very fact that turtles were seeking to reproduce at Messel reveals that the surface waters of the volcanic lake supported a thriving biotope.
“In our opinion, it is implausible that the A. crassesculpta couples found at Messel would actively swim, court and finally mate in poisonous surface waters or ingest poisonous surface waters only while mating,” Joyce and colleagues wrote.
“We propose, instead, that the turtles initiated copulation in habitable surface waters, perished when their skin started to absorb poisons while sinking during their embrace into deeper portions of the lake made toxic from the build up of volcanic gases or decay of organic matter, and fully or partially separated once they reached the bottom of the lake.”
They added: “The mating pairs from Messel are therefore more consistent with a stratified, volcanic maar lake with inhabitable surface waters and a deadly abyss.”
W. G. Joyce, N. Micklich, S. F. K. Schaal, T. M. Scheyer. Caught in the act: the first record of copulating fossil vertebrates. Biology Letters, 2012; DOI: 10.1098/rsbl.2012.0361