Sydney - The discovery of an embryo in a fish that lived 375 million years ago pushes back the first known live birth by 200 million years, an Australian scientist said Thursday.

The fossil find is also the earliest example of vertebrate sex ever discovered.

It's the biggest ever breakthrough in palaeontology, Museum Victoria's John Long told national broadcaster ABC.

"This is one of these real eureka moments in science you have once in your lifetime," he said. "When you look down the microscope and there it was - an embryo inside a 380-million-year-old fish - and I was blown away by the very thought of this fish giving birth to live young almost 400 million years ago."

The 25-centimetre fossil of a fish species called placoderms, now extinct, was found three years ago in the Kimberley region on Australia's north-west coast.

It has been named Masterpricis attenboroughi after celebrated British nature documentary maker Sir David Attenborough and its science published in the journal Nature.

"The discovery is certainly one of the most extraordinary fossil finds ever made and changes our understanding of the evolution of vertebrates," Melbourne-based Long said.

"The existence of the embryo and umbilical cord within the specimen provides scientists with the first every example of internal fertilization - that is, sex - confirming that some placoderms had remarkably advanced reproductive biology."

Long and his team of Australian scientists were initially unaware of the significance of their find. The breakthrough only came to light when the fossil was dipped in an acid bath to reveal more of its features.

"When I pulled it out of the acid, my jaw dropped," Long recalled. "We were really over the moon."

Putting the fossil under a powerful scanner in Canberra in November showed the yolk sac and a major blood vessel inside the umbilical cord.

The University of Western Australia's Kate Trinajstic has worked with Long on the Gogo fossil deposit that was brought to the world's attention by Attenborough 20 years ago.

She said the find probably altered the evolutionary timeline for backboned animals like humans.

"It's telling us that the basic body plan that makes us a vertebrate was already present 380 million years ago and we are now looking at how these animals grew and reproduced," she said.

Trinajstic said that more fossils on the site were giving up the secrets of how the transformation occurred and fish moved out of the water and onto the land.

"We've also found fossilized muscle tissue from these fish and it's the same sort of muscle tissue that we use for running - these fish were using it for swimming." (dpa)