Nine pairs of fossils - the females identified by their shorter tails - were found in a the Messel Pit, near Darmstadt in Germany. It is believed the copulating couples were overcome by poisonous gas at the bottom of a volcanic crater.
Dr Walter Joyce, of the University of Tuebingen, Germany, said the turtles belonged to an extinct species known as Allaeochelys crassesculpta and anatomical evidence revealed each pair had a male and female member.
Their upper shell would have reached almost two feet in length and been more than a foot wide. Males had much longer tails protruding beyond the margin of the shell, whereas the females' were short enough to remain inside.
Dr Joyce said: "We demonstrate for the first time all couples contain one male and one female individual and the tails of some males are aligned with those of the female."