Remains of embryos entombed in their fish mothers' wombs for 380 million years have been found in fossils from an ancient rock outcrop in Western Australia. The finding is a big deal because it suggests that sex goes way back.

The prehistoric fish, called placoderms, are found at the base of the vertebrate evolutionary tree (in a large group we humans also belong to), so it now looks like sexual intercourse, and the mating behaviors that go along with it, were more widespread in these ancient animals than previously thought, said the scientists who made the discovery.

The embryos were found in the body cavities of Incisoscutum ritchiei, an extinct jawed fish that lived from about 430 million years ago to 360 million years ago. Placoderms' heads and parts of their bodies were covered with bony armor. They were also some of the first jawed animals.

The specimens were found in the Gogo Formation of Western Australia by John Long of the Museum Victoria in Melbourne, Australia. At first, he thought that animal remains found in the placoderm fossils were the animals' last meals, but after finding embryos in another group of placoderms, Long and his colleagues took a closer look and found that the new specimens were also the remains of unborn embryos.

"We could see that the new specimens had the same bone structure as the previous embryos, were the same species as the adult, they did not have any broken or stomach-etched features (from digestive acids or from being chomped) and that they were at the same stage of growth as the previous embryos," Long wrote in an email. "All of these facts proved they were embryos, not prey items."

The scientists' work was funded by the Australian Research Council.

Sex evolution

Because the embryos were found inside their mothers, these placoderms must have copulated to produce offspring, instead of laying eggs and fertilizing them outside the mothers, as some species of fish and others animals do.

And since these placoderms sit near the beginnings of the vertebrate tree of life, "it means that complex forms of mating evolved probably about the same time as jaws evolved," Long told LiveScience. (Source: livescience.com)

Story Date: March 2, 2009