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Fish with oldest known 'face' may reshuffle evolutionary family tree

University of Oxford

Scientists say they have discovered a fossil of the oldest known vertebrate animal with a jaw -- a strange chimera of a fish that could unseat the shark as a representative of extremely "primitive" jawed fishes and turn our evolutionary understanding of humans’ ocean-dwelling ancestors on its head.

The new fossil described in the journal Nature, called Entelognathus primordialis, is a 419-million-year-old armored fish from the end of the Silurian period, right before the start of the Devonian, known as the Age of Fishes for their remarkable diversity during that period.

Scientists have long thought that the ancestor of jawed vertebrate fish -- a group that includes the extinct armored fishes known as placoderms, cartilaginous fish called chondrichthyans (which includes sharks and rays), and bony fishes called osteichthyans (including humans’ ancestors) -- must have looked something like a shark.

"It’s kind of a classic view … that of the living jawed vertebrates, that sharks are especially primitive, that they’re living fossils, they’ve been unchanged for hundreds of millions of years," Friedman said. "And that [idea] has some pretty deep pre-Darwinian roots."

But the mix of traits found in the new fossil from China throws that theory for a loop.

"It’s kind of a mishmash," said Matt Friedman, a paleobiologist at Oxford University who was not involved in the study. "It looks like a placoderm with a bony fish grin."

Here’s a brief history of the early fish family tree and its human connection:

Scientists think that some sea creatures grew backbones, earning the name vertebrates, and then some of those vertebrates developed jaws. (A few jawless fish, like lampreys and hagfish, live on today.)

A subset of these jawed fish developed bony plates on their heads and shoulders, earning the name osteichthyans. Descendants of a subset of those bony fish, called lobe-finned fishes, eventually crawled out of the water to become the first tetrapods, the ancestors of all terrestrial vertebrate life on Earth, including humans.

“We’ve got bony plates too,” Friedman said. “We’re a special kind of bony fish that’s gone onto land and lost its fins.”

While scientists are pretty clear that humans came from osteichthyans, they don't know what those bony fishes' ancestors looked like. Osteichthyans, chondrichthyans, placoderms and another extinct class called the acanthodians must have descended from a mysterious jawed ancestor -- which many thought must have looked something like a shark.

But the Entelognathus fossil seems to contradict that idea. The skull has bone plates like one of the extinct placoderms. But a side view of the skull reveals patterns in the jaw that look much more like those in osteichthyans, our ancestral group of bony fish.

“It’s a bit of Rosetta stone, if you will, that allows us to begin to understand the relationships between these two kinds of platey skeletons,” Friedman said.

The researchers’ analysis shows that the early ancestor, which was probably not Entelognathus but something like it, would have probably had bony plates like osteichthyans, not scales like sharks. This would make humans, not sharks, the inheritors of a more ancient, "primitive" trait.

The study could cause scientists to rethink the way they’ve interpreted fossils in the past, Friedman said. 

"I think it’ll encourage people to go out and look for other creatures  that might help fill in the story a little bit," he said.

Copyright © 2014, The Baltimore Sun
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