Tooth enamel -- the hardest substance produced by vertebrates -- isn't just found on teeth. Lots of vertebrates have enamel-covered teeth, but some of them also have enamel-covered scales. Sharks are famous for having so-called dermal denticles -- skin teeth -- which decrease drag to help them swim more efficiently.
In some bony fishes -- like the North American gar (Lepisosteus) as well as many extinct species, which were the precursors of land vertebrates -- these scales are covered with ganoine, a substance that appears very much like human tooth enamel.
"People have been studying the origin and early evolution of vertebrate hard tissues such as enamel for many decades, ever since the middle years of the 20th century," study author Per Ahlberg, Professor of Evolutionary Organismal Biology at Uppsala University, told The Post.
In the new paper, he and his colleagues combined this long history of paleontological study with a relatively new science: genomics. Genetic analysis of Lepisosteus showed that the fish has the genes for two of our three enamel matrix proteins, and that these genes are expressed in the skin.
"In our study we have been able to show on the basis of genomic data that ganoine, an enamel-like surface tissue of primitive ray-finned fishes, is indeed enamel," Ahlberg said. "Mature enamel consists almost entirely of a mineral, hydroxyapatite, but it is initially laid down on an organic matrix composed of three enamel matrix proteins that are not used anywhere else in the body."
The team then turned to the fossil record, examining two fish -- Psarolepis from China and Andreolepis from Sweden -- that are over 400 million years old. In Andreolepis, only the scales showed signs of enamel. In Psarolepsis, even though scales on the face carried enamel, the teeth themselves did not.
"Andreolepis and Lophosteus are among the earliest known bony fishes, and both belong to the most basal part of the family tree," Ahlberg said. "The fact that they both lack tooth enamel is thus highly significant and suggests that enamel originated somewhere on the outside of the body, most probably on the scales, before colonizing the teeth."
Ahlberg and his colleagues plan to continue combining genomic study and the fossil record to investigate the origin of other vertebrate tissues.
"[The findings] show us the strange paths that evolution can take, co-opting a tissue from one part of the body to play a quite different role in another, and then abandoning the original role altogether," he said. "Looking at our own teeth, and the important role that enamel plays in their function, you would swear that the tissue had evolved there; and yet it did not."