The Janus Fish – Why sharks may not be so primitive after all

Sharks are often said to be ‘living fossils’ – seemingly ‘primitive’ fish which have hardly changed since the days of the dinosaurs. They certainly have a long pedigree, stretching back to their common ancestor with bony fish, the osteichthyes, which represent almost all of the other fish you see today and which also gave rise to the first amphibians and, ultimately, to us. It had long been assumed that this ancestor, which lived around 400 million years ago during the Devonian Period, would have closely resembled a shark. However, new evidence has come to light suggesting that sharks might not be so primitive after all.

A new study, published earlier this week (January 12th), focuses on an ancient fossil specimen of a species called Janusiscus, dated to around 415 million years ago. First discovered in 1972 near the Sida River, Siberia, it externally resembles a bony fish with thick dermal plates over the skull. However, this new research carried out at the University of Oxford has taken a look inside the skull using a technique called x-ray computed tomography. This process involved taking CT scans of the fossil then using a computer programme to manipulate that image and examine parts of the skull not readily accessible in the physical specimen. What they discovered showed that the interior of the skull much more closely resembled a modern shark, even whilst the exterior resembles a bony fish. These two different faces led the team to officially name the fish Janusiscus after the two-faced Roman God and to conclude that the common ancestor of both groups most likely already had a bony skeleton.

Sharks are unusual. They have a skeleton that isn’t made of bone like ours but instead is made of cartilage. Humans still have cartilage in our ears and noses and elsewhere in our body where it forms connective tissue, and it is also a developmental precursor to bone within our own skeleton. Evolutionarily therefore, it seemed most likely that bony skeletons would evolve from cartilaginous ones. Now it seems that the opposite is true. Sharks are not primitive, instead they have actually evolved to lose their bony skeleton and enjoy the flexibility offered by cartilage instead, an innovation which has doubtless helped them to become the successful apex predators we see today.


Featured Image: Oxford University

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Emma Gregg

I have an MSci in Palaeontology and Evolution and a passion for all things extinct! I've always loved writing about the science that interests me and I have a particular fascination for palaeopathology.

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