An animation shows how the prehistoric fish might have looked
A fossil fish from Australia was one of the earliest known vertebrates to reproduce by fertilising eggs inside the female, a study suggests.
Nature journal says the ancient fish was carrying a 5cm-long embryo.
The fertilisation of eggs by sperm outside the mother's body - external fertilisation - is thought to have evolved before copulation.
The fossil suggests the fertilisation of eggs inside the female's body evolved sooner than previously thought.
"These (fish) show some of the earliest evidence for internal reproduction," Zerina Johanson, curator of fossil fish at London's Natural History Museum (NHM), told BBC News.
Evidence of reproductive biology is extremely rare in the fossil record
Zerina Johanson, Natural History Museum
"We expected that these early fishes would show a more primitive type of reproduction, where sperm and eggs combine in the water and embryos develop outside the fish."
According to Dr Johanson, the 365 million-year-old specimen shows that "the type of advanced fertilisation, taking place inside the mother, was more common among early fishes than previously thought.
"This discovery is incredibly important because evidence of reproductive biology is extremely rare in the fossil record," she said.
Dating to the Upper Devonian Period, the specimen has been bestowed with the scientific name Incisoscutum ritchiei.
It belongs to a group of early fish known as placoderms, which were covered in tough armour.
This specimen shows a modification of the pelvic fin on its belly. The authors of the Nature paper believe this structure, called a clasper, would have been used by the male to grip the female during mating. A similar organ is seen in modern sharks.
"The clasper is an intermittent erectile organ that is inserted inside the female to transfer sperm," said co-author Dr John Long, a palaeontologist at Museum Victoria in Australia.
In one type of placoderm called the ptyctodonts, this organ is covered in bone and hooked.
"This new group... has more flexible claspers. In the Nature paper, we're suggesting this is the beginning of erectile male fertilisation, because part of that organ has been taken over by soft cartilage," Dr Long explained.
The fossil comes from the Gogo formation of western Australia
The process of internal fertilisation and giving birth to live young distinguishes some fish and mammals from other animals such as reptiles and amphibians.
Dr Johanson thinks this was the main reproductive method for early fish such as placoderms and could have evolved in other fish groups too.
"'Sex' was far more common in these primitive prehistoric animals. We used to think that external fertilisation was the earliest form of reproduction but copulation appears to be the main way they reproduced," said Dr Johanson.
'Age of fish'
The fish, which has been in the collections of the NHM since the 1980s, was first thought to have died soon after its final meal. The bones were thought to have been those of a smaller fish it had eaten.
But research on related fish prompted Dr Johanson and her colleagues to reinterpret the fossil. They came to the conclusion that the "last meal" was in fact a young fish developing in the womb of its mother.
The find is very early evidence of internal reproduction
Placoderms are thought to be among the most primitive jawed vertebrates. They were extinct by the end of the Devonian, which is often described as "The Age of Fish".
"Placoderms are right at the base of the vertebrate tree. So they are distantly ancestral to us in a way," said Dr Long.
After the placoderms died out, another type of jawed vertebrate, the bony fish, evolved into tetrapods - the four-limbed animal group which includes mammals, birds, reptiles and amphibians.
Last year, scientists reported the earliest instance of a live birth in another placoderm fossil from the Gogo formation of western Australia.
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