Fish Fossil Has Oldest Known Face, May Influence Evolution

The 419-million-year-old fossil has the same jawbones as vertebrates.

Is this the first face? The extinct fish swims in ancient seas in an illustration.  Illustration courtesy Brian Choo

Scientists have found the oldest face—and it's a fish. (Not a fishface, though.)

The 419-million-year-old fish fossil could help explain when and how vertebrates, including humans, acquired our faces—suggesting a far more primitive origin for this critical feature of our success, a new study says.

"Entelognathus primordialis is one of the earliest, and certainly the most primitive, fossil fish that has the same jawbones as modern bony fishes and land vertebrates including ourselves," said study co-author Min Zhu of the Chinese Academy of Sciences in Beijing.

"The human jaw is quite directly connected to the jaw of this fish, and that's what makes it so interesting."

The bones comprising the fish's cheek and jaws appear essentially the same as those found in modern bony vertebrates, including humans, Zhu added. Because it boasts maxilla and mandible much like our own, the fish may be the earliest known creature with what we'd recognize as a face. (Related: "Ancient Toothy Fish Found in Arctic—Giant Prowled Rivers.")

Key Evolutionary Step
The development of jaws and faces was a key step in vertebrate evolution, and probably appeared as a way for fish to catch bigger and/or more nimble prey, according to the study, published September 26 in the journal Nature.

There remains much to learn about how it happened, however.

University of Oxford paleobiologist Matt Friedman, who wasn't involved in the research but penned a commentary for Nature, said the fossil boasts a jaw and face structure that's nothing like those in any other known members of Entelognathus's extinct family of primitive armored fishes, the placoderms. These creatures had simple jaws and cheeks composed of just a few large bones, Friedman explained, rather than complex arrangements of smaller bones like those found in modern bony fishes.

But in the new fossil, found in China, has a distinctive three-bone system still used by chewing vertebrates today: 

• a lower jawbone called the dentary and two upper jaw bones called 
• the premaxilla (holding the front teeth) and 
• the maxilla (holding the canine and cheek teeth).

"The exciting thing about this fossil is that when you look at the top of it, it looks like a placoderm, but when you look at the side of the fish and the structure of the jaw, it doesn't look like any placoderm that we know of," Friedman said.

"This tends to suggest the exciting possibility that these jawbones evolved way deep down in the lineage, so these features we used to hold as being unique to bony fishes may not be so unique.” (Related: "Ancient Fish Downsized But Still Largest Ever.")

Understanding Our Origins
The ramifications of that theory, if confirmed, would extend far beyond fish into the deepest roots of our own family tree, Friedman said.

"Basically, as terrestrial vertebrates, we are a kind of very specialized, very bizarre fish that about 370 million years ago went on land and lost its fins. Understanding the origin of bony fishes is inextricably linked to understanding our own origins because we're bony fishes.

"These different bones in our skull, the ones that medical students learn the names of, where and when in our family tree did they arrive?” he asked. (Related: "Flat-Faced Early Humans Confirmed—Lived Among Other Human Species.")

If it's the case that the bones we see in Entelognathus are genuinely related to the ones in our own faces, Friedman explained, we can trace the origin of those features very deep down into our own family tree, even before the lineage of bony fishes (including terrestrial vertebrates like humans) split from that of the cartilaginous fishes (including sharks and rays).

"It suggests a real antiquity to some of the most prominent features of our own bony faces."


ORIGINAL: NationalGeographic

Brian Handwerk for National Geographic News
Published September 25, 2013