The oldest known sexual beings, a 400 million year old fish sex movie and the prehistoric turtles which were fossilised in the act of copulation. Discovery this week is strictly adults-only as we begin a three-part natural history of sex. Adam Rutherford talks to the scientists studying the world’s most revealing fossils.

(Image: Two carettocheylid turtles, fossilised in mating position. Credit: The Royal Belgian Institute of Natural Sciences)