Fossils of sardine-size fish that swam in ancient oceans are the earliest examples of vertebrates with teeth that grow from their jawbones, according to new a new study.
The fish, which lived 420 million years ago, are a "very modest" beginning for the jaw-and-tooth pattern widespread in nature today, said study co-author Philippe Janvier, a paleontologist at the National Museum of Natural History in Paris, France.
"It's really the first evidence that we have of the earliest bony fishes—the earliest ancestors of all the fishes that have the[ir] bones and teeth implanted in the bones of the jaw," Janvier said.
Modern bony fish such as cod, herring, and coelacanths have this tooth arrangement. So do tetrapods—four-limbed creatures such as frogs, crocodiles, and humans, which are all descendants of bony fishes. (Related: "Fossil Fish With 'Limbs' Is Missing Link, Study Says" [April 5, 2006].)
When a bony fish or a tetrapod loses a tooth, a new one grows from the bone below the void, whereas other jawed vertebrates, such as sharks, have teeth that grow from inside their gums. Sharks have skeletons of cartilage instead of bone.
Shark teeth are lined up in "families." New teeth grow at the inner end of their respective tooth family, and old teeth fall off at the end of an inside-out progression—similar to a conveyor belt.
Though fossil representatives of the earliest members of each of these living groups are well known, the earliest stages of jawed vertebrate evolution presents a fuzzier picture.
The new fossils help clarify these questions, Janvier said.
Transition Fossils
The researchers discovered the telltale bony fish fossils among fragments collected on the Swedish island of Gotland in the Baltic Sea. Other fragments came from boulders carried to Germany by glaciers a few million years ago.
Some of the fossils belong to the species Andreolepis hedei and others to Lophosteus superbus, fish previously identified by scale and head bone specimens.
Whether they were truly bony fish or more like sharks was an open question, however.
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