Sawfish fossils may hold the secret to how teeth evolved

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.

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