Messel pit
Nine mating pairs of fossil turtles have been found. The turtles, Allaeochelys crassesculpta, were in coitus (in the act of having sex). They are male-female pairs, with the male's tail tucked under the female, which is how they have sex. Their death must have been rapid. It is supposed that the turtles had started mating in the aerated surface waters of the ancient lake. As they sank into deeper water, they were overcome by the release of toxic volcanic gas. They were then buried in the lakebed sediment. Turtles belonging to this clade are still living. They have lost their reptilian scales, and their skin absorbs oxygen from the water. Normally, this is an advantage: it helps them stay submerged for long periods. However, under anoxic conditions it is a disadvantage, because carbon dioxide and dissolved poisons are absorbed as well.