The remains of the 47-million-yearold 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.