Study of fossils shows prehistoric fish had sex

BANGKOK, Thailand -- The fossilized remains of two pregnant fish indicate that sex as we know it — fertilization of eggs inside a female — took place as much as 30 million years earlier than previously thought, researchers said Thursday.

Scientists from Australia and Britain studying 380 million-year-old fossils of the armored placoderm fish, or Incisoscutum richiei, said they were initially confused when they realized that the two fish were carrying embryos. They originally thought the fish laid their eggs before fertilization.

"Once we found embryos in this group, we knew they had internal fertilization. But how the hell are they doing it?" said John Long, the head of sciences at the Museum Victoria in Melbourne who wrote a paper on the discovery that appeared in Thursday's issue of the journal Nature.

The answer came when the scientists re-examined the pelvis of the male placoderm, armed with the new information about fertilization. After looking at specimens at the Natural History Museum in London and the Museum Victoria, they realized the pelvis had a fin not seen on the female fish, and surmised it was likely used to grip its mate during fertilization, much as sharks do.

"These fish have an extra large bone that attaches to the pelvic bone," he said. "It had been overlooked and hadn't been identified. So in a nutshell, we have reinterpreted the structure of the pelvic bone in these placoderms to show they had a method for copulation."

Zerina Johanson, a paleontologist at the Natural History Museum who also took part in the study along with the University of Western Australia's Kate Trinajstic, said findings of internal fertilization showed that "sex started a lot sooner than we thought."

"We expected these early fishes would show a more primitive type of reproduction, where sperm and eggs combine in the water and embryos develop outside fish," Johanson said in a statement.

Per Ahlberg, a professor of evolutionary organismal biology at Uppsala University in Sweden who did not take part in the study, said the discovery "may prove to have far-reaching implications for our understanding of early vertebrate evolution."