for National Geographic News
The remarkably well-preserved fossil—about 375 to 380 million years old—shows an embryo connected to its mother fish by an umbilical cord.
It is the earliest evidence of a vertebrate giving birth to live young, shifting back the date some 200 million years, said John Long, head of sciences at Museum Victoria in Melbourne, Australia, and lead author of a new study describing the find.
(See a prehistoric time line.)
The fossil is also the earliest record of vertebrate sex, since live birth occurs when an ovum, or egg, has been fertilized internally by male sex cells.
“Having such advanced reproduction for a fish that primitive is amazing,” Long said.
Evidence of live birth—as opposed to egg laying—is extremely rare and has only been found in a few fossils of dolphin-like reptiles called ichthyosaurs and marine lizards known as mosasaurs, Long said.
The new fossil captures a long-extinct placoderm, a primitive, shark-like armored fish.
(Related: “Shark Ate Amphibian Ate Fish: First ‘Food-Chain Fossil’” [November 8, 2007].)
Dinosaurs of the Sea
Often called the “dinosaurs of the sea,” placoderms were the ruling class of marine creatures for 70 million years—in the middle of the Paleozoic period—until their extinction about 360 million years ago.
Evolutionary Innovation
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).
More information can be found at: http://news.nationalgeographic.com/news/2008/05/080528-mother-fossil_2.html
