Outgrowths on the rostrum of sawfish told about the origin of teeth

(ORDO NEWS) — Scientists studied the teeth on the rostrum of the fossil sawfish Ischyrhiza mira, which are modified scales.

It turned out that the surface of these teeth is much more complicated than the rest of the scales on the body of the fish and rather resembles the coating of a shark’s tooth. The results of the study confirmed the hypothesis about the origin of teeth from scales.

Until now, there is no unified theory about the origin of teeth. Some scientists believe that they were formed from the scales of ancient vertebrates that moved into the oral cavity.

Others suggest that the teeth developed independently of the scales, right at the back of the mouth. A study by scientists from Pennsylvania State University (USA) provided new evidence in favor of the first hypothesis.

Initially, scientists did not intend to participate in disputes about the origin of teeth: they simply examined the structure of the tooth tissue on the rostrum (snout outgrowth) of the fossil sawfish Ischyrhiza mira.

This species inhabited the waters of North America at the end of the Cretaceous period, 65-100 million years ago. The remains of I. mira were found in New Jersey. It is believed that the tooth-like outgrowths on its rostrum are modified scales.

This is evidenced by their location, morphology and character of development. It was known that in order for a new tooth to form, the old one must fall off and make room for it, but the organization of their tissue remained a mystery.

In particular, scientists did not know anything about the structure of the outer hard layer of rostral teeth – enameloid.

Since the rostral denticles are probably modified scales of the body, the authors of the work suggested that their enameloid would have a structure similar to that of the rest of the scales, which is distinguished by a simple microcrystalline organization.

To study the microstructure of the rostral teeth, the researchers made longitudinal and transverse sections, which were placed in a scanning electron microscope.

It turned out that the enameloid of the rostral tooth of I. mira is much more complicated than the enameloid of its scales. Its general organization resembles rather the enameloid of a modern shark tooth. They both have a coating consisting of fluorapatite microcrystals, which are organized into separate clusters.

Outside, they are oriented parallel to the surface of the tooth, and in depth they are arranged randomly. Through both of these layers pass radial rays, consisting of microcrystals oriented perpendicular to the surface of the tooth.

This structure allows the shark’s teeth to withstand the mechanical stress of biting. It is quite possible that the structure of the enameloid of the rostral denticles of I. mira was also designed to withstand mechanical stress.

Most importantly, however, the discovery provided direct evidence for the theory that teeth originated from scales.

Scientists have shown that scales are capable of forming a complex tooth-like enameloid outside the mouth. Therefore, it is more likely that the scales formed similar microstructures in the teeth in the mouth than that both of these structures appeared independently of each other.

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