German palaeontologists have dug up the remains of nine turtle pairs that died while mating some 47 million years ago, sinking into poisonous waters while locked in a final embrace, a report said Wednesday.

The find represents the first-ever fossil record of copulating vertebrates (animals with a backbone), said a report in the Royal Society Journal Biology Letters.

"Millions of animals live and die every year and many enter the fossil record through serendipitous circumstances, but there really is no reason to enter the fossil record while you are mating," co-author Walter Joyce told AFP about the rarity of the find.

"The chances of both partners dying at the same time is highly unlikely and the chances of both partners being preserved afterwards even less likely."

The discovery at the Messel Pit fossil site between Darmstadt and Frankfurt in Germany allowed the team to deduct that the waters of the Messel Lake had been hospitable enough to allow turtles to live and mate.

But the animals died accidentally when they sank while locked together into deeper, poisonous layers, their skins absorbing the noxious substances in the volcanic lake.

"There is no doubt that this lake killed many unsuspecting animals," said Joyce of the University of Tuebingen.

The paper said animals do not typically die while undertaking daily routines like eating, brooding their nests or mating, leaving scientists to rely on conjecture to determine what their behaviour would have been like.

The paper said it was common in fresh water turtles for the couple to freeze into a mating position.

"If mounting occurs in the open water, the mating couple is likely to thereby sink to considerable depths," it said -- possibly explaining why so many individuals fell into the same death trap.