Where did predatory fish get their teeth? A prehistoric creature helped scientists unravel the mystery

Just like today sharks and sawflies, the ancient fish-like vertebrate Ischyrhiza mira also had jagged spikes around its rump, which helped it ward off predators and obtain food. These spiky projections, called rostral denticles, are thought to be a modified version of the scales that covered the rest of its body.

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In an attempt to investigate the relationship between the rostral denticles and the scales, the team analyzed the hard outer layer of the tadpole’s spikes—but what they found was significantly different from what they expected.

The enamel resembled shark enamel

“Surprisingly, the enamel of the rostral denticle of the vertebrate Ischyrhiza mira was anything but simple. It was actually significantly more complex than the enamel of the body scales,” said vertebrate paleontologist Todd Cook of Pennsylvania State University, according to Science Alert.

“In fact, the overall organization of the enamel of this prehistoric pilon resembled that of modern shark tooth enamel,” he added. Specifically, it was mainly that both the enamel on the fossil and the enamel on modern shark teeth are made up of bundles of fluorapatite microcrystals, arranged in neat lines near the tooth surface and more randomly deeper in the tooth.

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Microcrystals located perpendicular to the tooth surface pass through these layers. These different orientations give shark teeth their strength and resistance to pressure. And apparently, the teeth of the sawfish Ischyrhiza mira were exactly the same.

“It is also likely that the bundle-like arrangement of the enamel microcrystals of the rostral denticles of Ischyrhiza mira served as a way to resist mechanical forces,” says Cook.

The fact that the jagged protrusions in the groin may have originated in scales is well illustrated by the possible form of another extinct fish, Onchopristis numidaThe fact that the jagged protrusions in the groin may have originated in scales is well illustrated by the possible form of another extinct fish, Onchopristis numidaSource: Wikimedia Commons, Gasmasque, CC BY-SA 4.0

It confirms the hypothesis about the external origin of the teeth

While it is not impossible that scales and teeth developed their bundled microstructures separately, it seems more logical to hypothesize that one microstructure followed the other—in other words, that it supports the “outside-in” hypothesis of the origin of teeth from scales.

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This finding could have significant implications for future studies in this area. “It provides direct evidence supporting the outside-in hypothesis because it shows that scales have the ability to develop complex tooth-like enamel outside the mouth,” says Cook. His team’s research was published in the Journal of Anatomy.

“And as we find more and more similarities between the exteriors of sea creatures and the crunching in our mouths as well, it seems more likely that our teeth too are actually highly evolved fish scales,” Science Alert concludes.

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