Chuck Bednar for redOrbit.com – Your Universe Online

The next time you call your spouse a “two-faced son-of-a-bitch,” just know that estimation is actually kind of realistic. Even for yourself.

A two-faced fish more than 400 million years old is likely the last common ancestor of all jawed vertebrates–including us–and provides additional evidence that sharks are not primitive creatures, as had long been assumed.

The skull of the creature, which was named Janusiscus after the two-faced Roman god Janus, possesses external features that led experts to believe that it had belonged in osteichthyans, the group that includes both bony fishes and all land-based creatures with backbones.

However, when scientists from Oxford University and Imperial College London used X-ray CT scanning to take a closer look inside the skull, they found that the structure surrounding its brain was actually closer to that of cartilaginous fishes (chondrichthyans) such as sharks and rays.

“This 415 million year-old fossil gives us an intriguing glimpse of the ‘Age of Fishes’, when modern groups of vertebrates were really beginning to take off in an evolutionary sense,” study author Dr. Matt Friedman of Oxford’s Department of Earth Sciences said in a statement. “It tells us that the ancestral jawed vertebrate probably doesn’t fit into our existing categories.”

Dr. Friedman and his colleagues, who have published their findings in the journal Nature, note that chondrichthyans have often been viewed as primitive and treated as proxies for what ancient jawed vertebrates might have looked like. One reason for this is their lack of a bony skeleton.

“The results from our analysis help to turn this view on its head: the earliest jawed vertebrates would have looked somewhat more like bony fishes, at least externally, with large dermal plates covering their skulls,” said first author Sam Giles, also of Oxford University.

“In fact,” he explained, these creatures would have actually had “a mix of what are now viewed as cartilaginous- and bony fish-like features,” and that this discovery supports the idea that both groups “became independently specialized later in their separate evolutionary histories.”

“This mix of features, some reminiscent of bony fishes and others cartilaginous fishes, suggests that humans may have just as many features that you might call ‘primitive’ as sharks,” added Dr. Friedman.

The fossilized skull was originally discovered in 1972 near the Sida River in Siberia, and it is currently in the possession of the Institute of Geology at the Tallinn University of Technology in Estonia. Study author Martin Brazeau of Imperial College London first found the specimen in an online catalogue, and he and his colleagues decided to take a closer look at it.

The study authors used X-ray CT to virtually dissect the fossil. Just like in hospital X-rays, different materials attenuate X-rays to different amounts, and bones appear to be brighter than either skin or muscles. These same principles can be applies to fossils, as fossilized bone and rock attenuate X-rays to different degrees, the researchers explained.

Using this technique, the team was able to construct a 3D virtual model of the fossil, allowing them to examine both its internal and external features in great detail. Trace amounts left behind by networks of nerves and blood vessels could then be compared to sharks, bony fishes and other jawed vertebrate groups.

“Losing your bony skeleton sounds like a pretty extreme adaptation, but with remarkable discoveries from China, Janusiscus strongly suggests that that the ancient ancestors of modern sharks and their kin started out just as ‘bony’ as our own ancestors,” said Dr Friedman.

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