Scientists have long debated the origins of teeth. Did they evolve from body scales that migrated into the mouths of ancient vertebrates and became adapted for eating—an idea known as the “outside-in” hypothesis? Or did they evolve independent of scales, originating deep within the oral cavity and ultimately mounting onto the jaws—known as the “inside-out” hypothesis? A new study led by Dr. Todd Cook, associate professor of biology, provides evidence for the “outside-in” hypothesis.
According to Cook, the research team, which included students Jack Prothero and Michael Brudy, did not set out to contribute to the debate about the origins of teeth. Instead, he and his colleagues were studying the tissue structure of ancient rostral denticles, the jagged spikes that run along the elongated snouts of sawsharks and sawfishes and are used in foraging and self-defense.
The team examined 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. The samples had been recovered from a rock formation in New Jersey.
“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 as with scales found elsewhere on the body, for a new denticle to form, an old one must first fall off to make a space available.
The study makes an important contribution to the long-standing debate about the origin of teeth. The rostral denticles have a similar tissue organization as the teeth of modern sharks. 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.”
The team’s discovery was featured in a Scientific American article where you can learn much more.