New fossil discovery turns the evolution of jaws on its head

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fish

It would seem strange for us to try and find common ground with a tuna sandwich, but a recent fossil finding suggests that our deeply rooted evolutionary similarities with bony fish go back much further than previously thought.

The study, published in Nature last month by a collaborative team of palaeontologists from China, Sweden and the USA describes a new species of fossil fish which may be the first animal with a modern jaw. The 419 million year-old Entelognathus primordialis was found in southwest China and is being heralded as one of the most significant fossil discoveries in recent years.

For the first 60 million years after the emergence of the first animals with backbones, the prehistoric seas were inhabited by small armoured fish with no jaws. The first jaws appeared with a group called placoderms, who had a bony skull and a very primitive jaw comprised of simple plates with no teeth. They existed for around 70 million years before suddenly going extinct 360 million years ago, being replaced by modern bony fish.

The classic view in palaeontology is that modern fish evolved from a shark-like creature with a skeleton made of cartilage, a view supported by the discovery of a group of fossil fish called acanthodians whose bodies were a mishmash of both cartilage and bone. This suggests that being shark-like is more primitive than being bony, and acanthodians were a step toward the evolution of bony fish. The placoderms represented the end of the line for armoured fish – or so we thought.

Entelognathus turns this whole idea on its head because at first glance it ticks all the boxes that would label it as a placoderm, but the characteristic simple plates of a placoderm jaw are absent and it possesses instead what looks extraordinarily like the jawbones found in modern bony fish and land vertebrates – including humans. The research team interpret this as evidence that modern fish evolved from the placoderms and that modern jawbones evolved much earlier than previously believed. A new analysis of the fishy family tree places Entelognathus as the closest relative of all jawed vertebrates, and the research team believe this and the modern jawbone both prove that placoderms are the ancestors of modern fish, and by extension our ancestors as well.

So what do these findings mean for palaeontology? This family tree reshuffle has displaced acanthodians as our ancestors and instead now implicates them as the ancestors of sharks, which would mean having a skeleton made of cartilage is not primitive at all but is instead a highly specialised adaptation. We haven’t been giving sharks enough credit. For its part, Entelognathus lived in a warm tropical sea in what would later become Yunnan Province, was only about 20cm long, and may be the first creature with a face. This resolves a long standing discrepancy in the early vertebrate family tree and adds another piece to the complex puzzle of human evolution – though I imagine it will be some time before genealogy websites catch up.

Photo: Wikimedia Commons

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