Vertebrate Teeth Evolved from Ancient Fish Scales, Researchers Say

New findings from researchers at the University of Cambridge, UK, support the theory that teeth in the animal kingdom evolved from the jagged scales of ancient fish, the remnants of which can be seen today embedded in the skin of extant cartilaginous fish (sharks, skates and rays).

Dermal denticles on the tail of the little skate (Leucoraja erinacea). Image credit: Andrew Gillis, University of Cambridge.

While most fish in the sea have bones, cartilaginous fish possess skeletons made entirely of cartilage.

These fish retain some primitive characteristics that have been lost in their bony counterparts, including small spiky scales embedded in their skin called dermal denticles that bear a striking resemblance to jagged teeth.

In a new study, Dr. Andrew Gillis from the University of Cambridge’s Department of Zoology and co-author tracked cell development in the embryo of a cartilaginous fish, the little skate (Leucoraja erinacea).

They found that these thorny scales are in fact created from the same type of cells as teeth — neural crest cells.

The findings support the theory that, in the depths of early evolution, these ‘denticle’ scales were carried into the emerging mouths of jawed vertebrates to form teeth.

“The scales of most fish that live today are very different from the ancient scales of early vertebrates,” said Dr. Gillis, lead author of a paper published in the Proceedings of the National Academy of Sciences.

“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.”

“By labeling the different types of cells in the embryos of Leucoraja erinacea, we were able to trace their fates. We show that, unlike most fish, the denticle scales of sharks and skate develop from neural crest cells, just like teeth.”

“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,” he said.

“The very name of these scales, dermal denticles, alludes to the fact that they are formed of dentine: a hard calcified tissue that makes up the majority of a tooth, sitting underneath the enamel.”

“The jagged dermal denticles on sharks and skate are remnants of the earliest mineralized skeleton of vertebrates: superficial armor plating.”

“This armor 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.”

Dr. Gillis and colleagues hypothesize that these early armor plates were multi-layered: consisting of a foundation of bone and an outer layer of dentine — with the different layers deriving from different types of cells in unborn embryos.

These layers were then variously retained, reduced or lost in different vertebrate lineages over the course of evolution.

“This ancient dermal skeleton has undergone considerable reductions and modifications through time,” Dr. Gillis said.

“The sharks and skate have lost the bony under-layer, while most fish have lost the tooth-like dentine outer layer. A few species, such as the bichir, a popular fish in home aquariums, have retained aspects of both layers of this ancient external skeleton.”

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J. Andrew Gillis et al. Trunk neural crest origin of dermal denticles in a cartilaginous fish. PNAS, published online November 20, 2017; doi: 10.1073/pnas.1713827114