Sawfish fossils hold clues to origin of teeth

Analysis of sawfish fossils might assist make clear the evolution of enamel.

Did they evolve from physique scales that migrated into the mouths of historical vertebrates and have become tailored for consuming—an thought generally known as the “outside-in” speculation?

Or did they evolve unbiased of scales, originating deep inside the oral cavity and in the end mounting onto the jaws—generally known as the “inside-out” speculation? A brand new examine offers proof for the “outside-in” speculation.

According to vertebrate paleontologist Todd Cook, affiliate professor of biology at Penn State Behrend, the workforce didn’t got down to contribute to the controversy concerning the origins of enamel. Instead, he and his colleagues have been learning the tissue construction of rostral denticles, that are the jagged spikes that run alongside the elongated snouts of sawsharks and sawfishes. The animals use them in foraging and self-defense.

Cook, lead writer of the examine within the Journal of Anatomy, notes that sawfishes belong to the identical group as skates and rays and are carefully associated to sharks.

The workforce examined the fossilized rostral denticles of Ischyrhiza mira, a species belonging to an extinct group of sawfishes that lived in North American waters through the late Cretaceous interval, round 100 to 65 million years in the past. The samples had beforehand 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,” says Cook, who explains that, identical to with scales discovered elsewhere on the physique, for a brand new rostral denticle to kind, an outdated one should first fall off and make an area out there.

“Yet, little or no was recognized concerning the group of the tissues that make up rostral denticles, significantly the exhausting outermost layer generally known as enameloid.

A cross part of a rostral denticle displaying the bundled enameloid. (Credit: Todd Cook/Penn State/Wiley Publications)

“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.”

To look at the inner microstructure of the fossil rostral denticles, the researchers hand sectioned the samples, each transversely—throughout the width—and longitudinally—throughout the size. Next, they used a scanning electron microscope to review the histology—or microscopic anatomy—of the rostral denticles.

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

Specifically, he notes that each Ischyrhiza mira rostral denticles and fashionable shark enamel have an enameloid protecting that largely consists of fluorapatite microcrystals packed collectively into distinct bundles. Towards the outer area of the enameloid, these bundles run parallel to the floor of the tooth and are known as the “parallel bundled enameloid.” Deeper, the bundles turn into randomly organized, a area generally known as the “tangled bundled enameloid.” Finally, passing by way of these layers is the “radial bundled enameloid,” which consists of packed microcrystals oriented perpendicular to the tooth floor.

In phrases of perform, Cook explains that having bundles of microcrystals organized in varied orientations allows shark enamel to withstand the mechanical stresses related to feeding. Similarly, he notes, “It is likely that the bundled microcrystal arrangement of the enameloid of Ischyrhiza mira’s rostral denticles also served as a way to withstand mechanical forces.”

However, probably the most shocking and consequential final result of this examine is that it makes an essential contribution to the long-standing debate concerning the origin of enamel, says Cook. Specifically, he explains, “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.”

A Penn State Behrend Undergraduate Research Grant supported this analysis.

Source: Penn State

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