This ancient fish had four sets of teeth instead of two

By Stephen Beech

A newly discovered 310-million-year-old fossilised fish is the earliest known example of one with extra teeth deep inside its mouth.

The ray-finned fish found in Staffordshire evolved a "unique" way of devouring prey, say scientists.

They explained that Platysomus parvulus used a "tongue bite" to devour its prey - a special set of teeth on the floor and roof of its mouth to help it crush and chew tough food, such as shells or insects.

The fossil find, described in the journal Biology Letters, sheds new light on the evolution of extra teeth.

Most fish today use their jaws to bite and chew, but some also have tongue bites, which work like a second set of jaws.

Until now, the oldest known fish with such a dental arrangement lived around 150 million years later than Platysomus parvulus.

The international research team used high-resolution CT scanning to reconstruct the internal anatomy of the fossil, which was discovered in Carboniferous rock formations.

They discovered a sophisticated arrangement of tooth plates on the roof of the fish’s mouth and the gill skeleton.

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Lead author Professor Sam Giles, from the University of Birmingham, said: “Our discovery helps us understand how fish evolved after the End-Devonian Mass Extinction, which wiped out many species.

"After this extinction event, fish started to change and develop new body shapes and ways of feeding.

“Tongue bites have evolved many times in different fish groups - including in modern ones such as trout and bonefish, demonstrating that it is a useful tool that helps fish eat a wider variety of food and survive in different environments.”

Giles explained that the tongue bite mechanism involves opposing sets of teeth - one on the roof of the mouth and another on the gill skeleton - that work together to grip and crush prey.

The Platysomus fossil studied is uniquely preserved in 3D, allowing researchers to peer inside its mouth and digitally dissect its anatomy.

That revealed a multi-part lower tooth plate and narrow upper plate, both bearing a single layer of pointed teeth - suggesting a transitional stage in the evolution of more advanced tongue bite systems seen in later fish such as Bobasatrania.

Co-author Dr. Matthew Kolmann, from the University of Louisville, said: “Later fish, like the Bobasatrania group, had more advanced tongue bites and did not use their jaws at all, relying on their tongue bite to crush hard food.

"Platysomus parvulus is like a missing link between simple jawed fish and more advanced tongue-biters.”

Co-author Professor Matt Friedman, from the University of Michigan, added: “Tongue bites are just one of many feeding innovations that emerged during this time.

"This fish represents a key evolutionary step and helps us understand how ancient ecosystems functioned and how modern fish lineages came to be."