Sawfish fossils may hold the secret to how teeth evolved

Could teeth really have come from body scales?
Loukia Papadopoulos
A current sawfish.jpg
A current sawfish.

Adrian Wojcik/iStock 

You may not know this, but there has been a long standing-debate over how teeth evolved. Indeed, paleontologists may have come a long way in their scientific advancements though no one yet knows how teeth came to be.

A long-standing debate may finally be resolved

Now, vertebrate paleontologist Todd Cook, associate professor of biology, Penn State Behrend, may have stumbled on the answer to this question while studying the tissue structure of rostral denticles. This is according to a press release published by the institution on Tuesday.

Rostral denticles refer to the jagged spikes that run along the snouts of sawsharks and sawfishes. They are typically used in foraging and self-defense.

“Rostral denticles are believed to be modified scales because of their location on the elongated snout, and they have an external morphology and developmental pattern that’s similar to scales,” said Cook, explaining that, just like with scales found elsewhere on the body, for a new rostral denticle to form, an old one must first fall off and make a space available.

“Yet, very little was known about the organization of the tissues that make up rostral denticles, particularly the hard outermost layer known as enameloid. Given that rostral denticles are likely specialized body scales, we hypothesized that the enameloid of rostral denticles would exhibit a similar structure to the enameloid of body scales, which have simple microcrystal organization.”

Sawfish fossils may hold the secret to how teeth evolved
A sawfish's teeth.

Cook’s team used a scanning electron microscope to study the histology of the fossilized rostral denticles of Ischyrhiza mira, a species belonging to an extinct group of sawfishes that lived in North American waters during the late Cretaceous period, around 100 to 65 million years ago.

“Surprisingly, Ischyrhiza mira’s rostral denticle enameloid was anything but simple; it was considerably more complex than the enameloid of body scales,” said Cook.

“In fact, the overall organization of the enameloid in this ancient sawfish resembled that of modern shark tooth enameloid, which has been well-characterized.”

The “outside-in” hypothesis

What does all this mean for how teeth came to be? Cook believes the results of the study are an indication that teeth evolved from body scales that migrated into the mouths of ancient vertebrates and became adapted for eating, a theory called the “outside-in” hypothesis.

Cook further explained: “This finding provides direct evidence supporting the ‘outside-in’ hypothesis, as it shows that scales have the capacity to evolve a complex tooth-like enameloid outside of the mouth. It is more parsimonious to suggest that scales produced a similar bundled microstructure in teeth and rostral denticles than to conclude that both these structures evolved a similar enameloid independently.”

So does that mean all teeth were initially body scales? The study would indicate so. So the next time you are chomping down on an apple or a carrot, think about the tremendous evolution that had to take place for you to be able to eat.