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."
"Every once in a while, a discovery comes along that puts our biological understanding of some extinct group of organism on much firmer footing," Ahlberg wrote in Nature. "Long and colleagues present such a discovery."
Long first became enamored with the reproductive skills of this ancient fish last year, when his team identified the first placoderm containing embryos at the Gogo dig site in Western Australia.
The 380 million year old fossils were hailed as one of the most important discoveries in Australia and the fossil represents the world's oldest known vertebrate mother. The site, which Long has worked since 1986, is believed to have once been the home of an ancient tropical reef that would have rivaled the Great Barrier Reef.
Researchers hope to use the fossils to better understand where placoderms fit into the evolutionary history of jawed vertebrates and determine whether they are more closely linked to sharks and rays or other boney fish like tuna and swordfish.