Paleontologists believe they are the most primitive jawed vertebrates, even predating sharks.
(Related: "Fossil Meat Found in 380-Million-Year-Old Fish" [February 12, 2007].)
The newfound mother fish measures 10 inches (25 centimeters) long, but other placoderms can grow to 20 feet (6 meters)—"some gargantuan in size," Long said.
Much of the fish's soft tissue has been preserved in a three-dimensional state, making the fossil "basically an exact replica of the living animal," said study co-author Kate Trinajstic, a paleontologist at the University of Western Australia.
"The material was so well preserved that we were able to pick up subtle details," Trinajstic said.
Such details helped the scientists determine that the prehistoric mother and baby are a new species of ptyctodont, a type of placoderm that has plates around the head and neck rather than the extensive body armor of its relatives.
They named the species Materpiscis attenboroughi—a combination of "mother fish" and a nod to world-renowned naturalist Sir David Attenborough.
Attenborough's 1979 TV series Life on Earth first brought to light the scientific value of the Gogo area in the Kimberley region of Western Australia.
The area is the site of an ancient barrier reef that once teemed with marine life.
Fossils in the Gogo are so immaculately preserved because the reef became devoid of oxygen, which quickly killed the fish and the scavengers that would otherwise devour them, Trinajstic said.
Rapid burial and a stable tectonic continent made for near-perfect fossil preservation conditions.
A description of the fossil is published in this week's issue of the journal Nature.
Michael Lee, an evolutionary biologist at the South Australian Museum, was not involved in the new research.
"Live-bearing and maternal nourishment of embryos is a very important evolutionary innovation, which we ourselves exhibit," Lee said.
"The evidence that the included individual is an embryo [rather than ingested prey] is very strong—it's the same species, the right size to be an embryo, in the correct location within the body, and has what appear to be umbilical structures."
Live birth "might be preserved more commonly than we thought. Now that we know what to look for, it might be noticed more often," he added.
In fact, a reevaluation of a fossil found in 1986 reveals that it is a second placoderm fossil with three embryos nestled inside the mother. Study author Long had found the second specimen, a Gogonasus fossil, on an expedition to Gogo funded by a National Geographic Society grant. (National Geographic News is part of the National Geographic Society).
At the time, Long thought the embyros were scales.
(Related: "Ancient Fish Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
"There are still lots of things to discover," Long said. "Gogo is giving us a picture not just of reproduction, but of the whole lifestyle of these creatures."
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