The era when sex became a popular form of reproduction has been fixed by the discovery of a fossilised pregnant fish and her embryo.

Remains of the primitive fish, Incisoscutum ritchiei, have provided the earliest known evidence of copulation and live births in the animal kingdom.

Until the evolution of the armoured fish, sex is thought to have been limited to external fertilisation techniques in which sperm and eggs were squirted into the water to mix.

The species, with the fossil dated at 350-380 million years old, is the same age as another closely related fossilised fish, Materpiscis attenboroughi, which was found last year with a newly born offspring still attached by the umbilical cord.

Researchers said the discovery of two types of fish living at the same time shows copulation among vertebrates was a common means of reproduction some 200 million years earlier than had been thought.

“Seen in one fossil, it could have been a one-off. With our new discovery we are beginning to think sex is characteristic,” said Dr Zerina Johanson, of the Natural History Museum in London. “We now have to rethink how animals reproduced way back then.

“We would have expected before this, that this very primitive fish had an external form of fertilisation. We are having to rethink that now. It’s challenging how we think about reproduction at this early evolutionary stage.”

Analysis of fossilised males revealed that the species had developed claspers on its pelvic fins which would have enabled the male to insert a package of sperm into the female.

“This is what I understand as copulation - a transfer of the sperm inside the female, then fertilisation takes place inside the female,” Dr Johanson added. “Sex started a lot sooner than we thought.”

By evolving internal fertilisation and giving birth the fish had opted to invest a lot of energy in a few offspring which were well-developed and capable of avoiding predators by the time they were born.

The alternative was for the female to lay huge numbers of eggs and for the male to squirt sperm over them. Such external fertilisation creates large numbers of tiny larvae but few survive to adulthood.

The embryo found inside I. ritchiei was originally thought to have been the mother fish’s last meal but was reassessed after the discovery of M. attenboroughi.

Both species were found in Western Australia’s Gogo formation which is thought to be the remnants of a reef in tropical inland sea. I. ritchiei was first described in the 1980s and the new interpretation is reported in the journal Nature.

The two species of copulating animals were examples of placoderms which were a class of fish that had armoured plates on their heads and thorax and dominated the seas during the Devonian.

They are the most primitive form of jawed vertebrates yet found and most were predators. Placoderm fossils have been found in Europe, North America, North Africa, Australasia and Antarctica.

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