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Fossil find revises history of jaws: placoderms may be ancestors of bony fish, land vertebrates.

A freaky fish with a head like a dolphin and a body like a tank may be to thank for human jaws.

The discovery of a 423-million-year-old armored fish from China suggests that the jaws of all modern bony fish and land vertebrates originated in a bizarre group of fish called placoderms, researchers report in the Oct. 21 Science.

Along with a different placoderm fossil found in 2013, the new find, named Qilinyu rostrata, is helping rewrite the story of early vertebrate evolution, says paleontologist John Maisey of the American Museum of Natural History in New York City. "We've suddenly realized we had it all wrong."

The jaws of humans--and dogs, salmon, lizards and all other bony vertebrates--contain three key bones: the maxilla and premaxilla of the upper jaw, and the dentary of the lower jaw.

"Anything from a human being to a cod has recognizably the same set of bones in the head," says study coauthor Per Ahlberg, a paleontologist at Uppsala University in Sweden. The big question, he says, is "Where did these bony jaws come from?"

More than 100 million years before dinosaurs walked the Earth, placoderms thrived. Scientists knew that these armored fish were early jawed animals, but their jaws were unusual: "They look like sheet metal cutters," Ahlberg says. "They're these horrible bony blades that slice together."

The blades, called gnathal plates, looked so peculiar that most scientists thought that the three-part jaw originated in an early bony fish and that placoderms were just a side branch in the vertebrate family tree. "The established view is that placoderms had evolved independently and that our jaw bones must have a separate origin," Ahlberg says.

Placoderms are a highly debated group, says paleontologist Martin Brazeau of Imperial College London. No one quite knows where to place them.

In 2013, Ahlberg and colleagues found a new clue in a 419-million-year-old fossil that had the body of a placoderm but the three-part jaw of a bony fish. That animal, called Entelognathus primordialis, "could never have been predicted from the fossil record," says paleontologist Gavin Young of Australian National University in Canberra.

That work bolstered the idea that placoderms didn't dead-end hundreds of millions of years ago--some were actually the ancestors of bony fish (and thus humans). But it was just one fossil, Ahlberg notes. "You don't want to draw too big of conclusions from one animal."

Two animals is a different story. Qilinyu, the new fossil, had an armored skull and trunk and was probably about the length of a box of tissues. Like Entelognathus, Qilinyu had a three-part jaw, though the creature looked a bit more like a typical placoderm, Ahlberg says. The two fossils "form almost perfect intermediates" between placoderms and bony fish, he says. Ahlberg and colleagues suspect the key jaw elements of bony fish (and all land vertebrates) evolved from those bony blades of placoderms.

"This is part of our own early evolutionary history," Ahlberg says. "It shows where our own jaws came from."

Maisey puts it another way: "We are all fundamentally placoderms."

Caption: A new fish fossil has characteristic placoderm body armor, as well as three jaw bones similar to those found in humans (maxilla shown).

Caption: Qilinyu rostrata, a 423-million-year-old armored fish, had a jaw that resembles those of modern bony fish and land vertebrates.


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Title Annotation:LIFE & EVOLUTION
Author:Rosen, Meghan
Publication:Science News
Date:Nov 26, 2016
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