There are two theories about where teeth originated: they either evolved from external shells (the outside-in hypothesis) or somewhere in the mouth (the inside-out hypothesis).
Researchers study a fossil of the Ischyrhiza mira species — an extinct sawfish that lived in North America about 65 to 100 million years ago — have found more evidence supporting the outside-in idea.
Like today’s saw sharks and sawfish, the creature had sharp points around its snout to ward off predators and forage for food. These spikes, called rostral denticles, are thought to be modified versions of the scales on the rest of the body.
In an effort to investigate the relationship between rostral denticles and scales, the team analyzed the hard enamelled outer layer of the snout spikes — but what they found was significantly different from what they expected.
“Surprisingly, I. miraThe rostral tooth enamel was anything but simple,” says Pennsylvania State University vertebrate paleontologist Todd Cook. “It was significantly more complex than the enamel of scales.”
“In fact, the overall organization of the enamel in these ancient sawfish resembled that of modern shark tooth enamel, which has been well characterized.”
In particular, the enamel on the fossil resembles the enamel on modern shark teeth in that it consists of bundled packs of fluorapatite microcrystals, arranged in neat lines near the surface of the tooth and arranged more randomly lower down.
Through these layers run packed microcrystals placed perpendicular to the tooth surface. These different orientations give shark teeth their strength and resistance to stress, and it seems it’s the same story with the I. mira.
“It is likely that the bundled microcrystal arrangement of the enamel of I. miraThe rostral denticles also served as a way to resist mechanical forces,” Cook says.
While it’s not impossible that these scales and teeth evolved their bundled microstructures separately, it makes more sense if one came after the other — in other words, the outside-in hypothesis about where teeth came from.
Interestingly, the researchers didn’t really set out to look at the evolutionary history of teeth when they began their analysis of rostral denticles, but their findings could have a significant impact on future studies in this area.
As more and more similarities are found between the exterior of marine animals and the grinders in our mouths, it seems more likely that the teeth in our mouths are actually highly developed fish scales.
“This finding provides direct evidence that supports the outside-in hypothesis, as it shows that shells have the ability to develop a complex tooth-like enamel layer outside the mouth,” Cook says.
The research is published in the Journal of Anatomy.