New research from the University of Cambridge supports the theory that the jagged scales of ancient fish evolved into teeth. Those scales can still be seen today embedded in the rough skins of fish, such as ray, shark and skate.

The findings reignite the long-running debate in biology about whether the tooth followed its own evolutionary path or whether fish scales moved into the mouth as jaws developed. Recent research on species such as zebrafish revealed scales and teeth developing from distinctly different clusters of cells in fish embryos. This seemed to reject theories of ‘teeth from scales’.

But scientists at Cambridge say that while most sea fish have bones, one ancient lineage – sharks, skates and rays – have skeletons made entirely of cartilage. And these fish retain small spiky scales embedded in their skin called ‘dermal denticles’ that bear a striking resemblance to jagged teeth.

The jagged dermal denticles are most likely to be the remnants of superficial armour plating from the earliest mineralised skeleton of vertebrates. The researchers say this armour would have perhaps peaked some 400 million years ago in now-extinct jawless vertebrate species, as protection against predation by ferocious sea scorpions, or even their early jawed kin.

By tracking cell development in the embryo of a little skate, the researchers found that the fish’s thorny scales are created from neural crest cells, exactly the same type of cells as teeth. Their findings are published in the journal PNAS.

“The scales of most fish that live today are very different from the ancient scales of early vertebrates,” says study author Dr Andrew Gillis from Cambridge’s Department of Zoology and the Marine Biological Laboratory in Woods Hole.

“Primitive scales were much more tooth-like in structure, but have been retained in only a few living lineages, including that of cartilaginous fishes such as skates and sharks.

“Stroke a shark and you’ll find it feels rougher than other fish, as shark skin is covered entirely in dermal denticles. There’s evidence that shark skin was actually used as sandpaper as early as the Bronze Age.”

Gillis added, “Neural crest cells are central to the process of tooth development in mammals. Our findings suggest a deep evolutionary relationship between these primitive fish scales and the teeth of vertebrates.

“Early jawless vertebrates were filter feeders – sucking in small prey items from the water. It was the advent of both jaws and teeth that allowed vertebrates to begin processing larger and more complex prey.”