It has been hypothesized that teeth developed from exterior scales or that they were already present in the mouth at the time of their evolution. More support for the outside-in theory was discovered by researchers investigating a fossil of the extinct sawfish species Ischyrhiza mira, which existed in North America between 65 and 100 million years ago.
Its sharp spikes around its nose helped it avoid being eaten and protect itself from predators, much like modern sawsharks and sawfishes. These protrusions, known as rostral denticles, are believed to be evolved scales. Researchers looked at the hard enameloid external layer of the snout spikes to try to determine whether there was any connection between the rostral denticles and the scales, but their findings were quite different.
Enameloid on the fossil is identical to enameloid on current shark teeth in that it is composed of bundled bundles of fluorapatite microcrystals, organized in tight lines at the tooth’s surface and more randomly distributed farther down. Embedded microcrystals, aligned perpendicular to the tooth’s surface, run vertically through these layers. It seems that the I. mira is similar to other sharks in that its teeth have a variety of orientations that contribute to its strength and stress resistance.
It’s conceivable that the microstructure bundles in scales and teeth each developed independently, but the outside-in theory of tooth origin suggests that the latter emerged first. Remarkably, the scientists didn’t set out to investigate the evolutionary history of teeth when they started their investigation of rostral denticles, but their results might have far-reaching implications for future research in this area.
Teeth within our mouths are probably just highly developed fish scales, as more and more parallels are discovered between the exteriors of marine species and the gnashers inside our mouths.
