Fish ‘invented’ sex — 365 million years ago

A PREGNANT fossil fish may have been one of the first animals to have “full sex” 365 million years ago.

The placoderm, a long extinct armour-plated fish, contains a five-centimetre long embryo that was first thought to be the remains of a meal.

Scientists now realise that the creature must have reproduced by means of internal fertilisation.

Previously it was assumed the earliest fish had to make babies by mixing sperm and eggs in water.

But placoderms appear to have engaged in a more modern form of sex, with fertilisation taking place inside the mother.

The fossil, originally found in western Australia, has been housed at London’s Natural History Museum since the 1980s.

It was thought to have died soon after its final meal, whose fossil bones could still be seen. But further examination of the fish, from the species Incisoscutum ritchiei, revealed the “meal” was in fact a young fish developing in the womb of an adult.

“The embryo in the fish was in its final stages of development, inside the mother, waiting to be born,” said Dr Zerina Johanson, who led the study in the journal Nature.

Large claspers on the fish’s pelvic fins may have been used by the male to grip the female during mating.

“We used to think external fertilisation was the earliest form of reproduction, but copulation appears to be the main way these prehistoric animals reproduced, demonstrating that ‘sex’ started a lot sooner than we thought,” said Dr Johanson.