Image caption The turtles were found in male-female pairs

Turtles killed as they were having sex and then fossilised in position have been described by scientists.

The remains of the 47-million-year old animals were unearthed in the famous Messel Pit near Darmstadt, Germany.

They were found as male-female pairs. In two cases, the males even had their tails tucked under their partners' as would be expected from the coital position.

Details are carried in the Royal Society journal Biology Letters .

Researchers think the turtles had initiated sex in the surface waters of the lake that once existed on the site, and were then overcome as they sank through deeper layers made toxic by the release of volcanic gases.

The animals, still in embrace, were then buried in the lakebed sediments and locked away in geological time.

"We see this in some volcanic lakes in East African today," explained Dr Walter Joyce of the University of Tübingen.

"Every few hundred years, these lakes can have a sudden outburst of carbon dioxide, like the opening of a champagne bottle, and it will poison everything around them."

The turtles described in Biology Letters are of the extinct species Allaeochelys crassesculpta.

They are about 20cm in length; the females are slightly bigger than the males.

Their nearest living relatives are probably the pig-nosed turtle ( Carettochelys insculpta), a much bigger species that swims in waters around Australia and Papua New Guinea.

A. crassesculpta is just one of thousands of exquisitely preserved fossil creatures pulled from Messel Pit, which has Unesco World Heritage status because of its palaeontological significance.

Nine pairs of turtles have been unearthed at the site over the past 30 years.

In most of the couples, the individuals were discovered in contact with each other. For the pairs that were not, the individuals were no more than 30cm apart.

"People had long speculated they might have died while mating, but that's quite different from actually showing it," said Dr Joyce.

"We've demonstrated quite clearly that each pair is a male and a female, and not, for example, just two males that might have died in combat.

"This fact combined with the observation that their back ends are always orientated toward one another, and the two pairs with tails in the position of mating - that's a smoking gun in our view."

It is said to be the only example in the fossil record of vertebrates being preserved in the act of having sex.

For invertebrates, there are numerous examples in the scientific literature of copulating insects being caught in amber, or fossilised tree resin.

Image caption The closest living relative is probably the distinctive pig-nosed turtle

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter