Scientists have found the most primitive creature to have had a face like humans, and there’s definitely something fishy about it.

This first face belongs to the newfound species Entelognathus primordialis, “primordial complete-jaw,” an armored fish that plied the seas nearly 420 million years ago.

Related fish from that period had simple jaws made mostly of cartilage.

But Entelognathus had a complex jaw, knit together from many bony plates like those found in the jaws of humans and dogs and thousands of other living animals with backbones.

That strangely modern jaw — stuck on an otherwise primitive body — gives it what could be called the earliest known modern face.

“This is like finding the nose of a space shuttle in a hay wagon from the Middle Ages,” paleontologist Xiaobo Yu of Kean University in New Jersey, one of the researchers responsible for the new find, says via e-mail.

The new fish is also contributing to a major upheaval in scientists’ understanding of the base of the family tree that spawned rattlesnakes and guppies and penguins and, eventually, Homo sapiens.

The first Entelognathus fossil was unearthed in China in 2010, but it was not until scientists had chipped away at the specimen in the lab that they realized they were onto something very weird.

Their new fish looked like a placoderm, an ancient swimmer girded in homegrown armor made of bony plates.

The fish, described in this week’s issue of Nature, had small, almost immobile eyes and a flat forehead. And then there was its lower face: a jigsaw puzzle of interlocking bones a lot like humans. It’s a homely ancient fish with a supermodel’s bone structure.

The find is “a little bombshell,” says University of Chicago paleontologist Michael Coates. “It’s important … because it’s unexpected. It does present something we had no hint of before.”

Next Yu and his colleagues turned to a computer program to draw a new family tree for the jawed vertebrates, animals with jaws and backbones.

Early in its history, this group included only fish. But from these humble beginnings arose two important branches of animals.

One group was the sharks and rays, whose skeletons are built mostly of cartilage.

The second was the bony fishes and their descendants, including amphibians, reptiles, birds and mammals.

Scientists say the new fish shows that the fossil record still harbors surprises that will rock researchers’ understanding of our origins.

“There are probably still more animals like Entelognathus waiting to be found, that offer a slight or more dramatic variation on the same theme,” says paleontologist Martin Brazeau of Imperial College London. “It’s an exciting prospect.”

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