Biologists have debated whether ancient fish scales moved into the mouth once jaws emerged or if the tooth had its own evolutionary inception. The scales and teeth of zebrafish and other species developed from distinctly different clusters of cells in fish embryos. However, sharks, skates, and rays have skeletons made entirely of cartilage and small, spiky scales in their skin known as dermal denticles that resemble jagged teeth.
Researchers at the University of Cambridge have used fluorescent markers to track cell development in the embryo of a skate and found that dermal denticles are created from the same type of cells as teeth, neural crest cells. These findings support the theory that denticle scales were carried into the emerging mouths of jawed vertebrates to form teeth. Jawed vertebrates now make up 99% of all living vertebrates, from fish to mammals.
“The scales of most fish that live today are very different from the ancient scales of early vertebrates. 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,” said study author J. Andrew Gillis, PhD, MSc, of the Department of Zoology at Cambridge University and the Marine Biological Laboratory in Woods Hole, Mass.
“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,” said Gillis.
“By labelling the different types of cells in the embryos of skate, 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,” said Gillis.
“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,” said Gillis. “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.”
Dermal denticles are made of dentine. The jagged dermal denticles on sharks and skate—and, possibly, vertebrate teeth, the researchers note—are remnants of the earliest mineralized skeleton of vertebrates: superficial armor plating. This armor would have peaked 400 million years ago in now extinct jawless vertebrate species as protection against predation by sea scorpions or early jawed animals.
The researchers hypothesize that these 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 then were 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,” said Gillis. “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.”
The study, “Trunk Neural Crest Origin of Dermal Denticles in a Cartilaginous Fish,” was published by PNAS.