Researchers Find Evidence that Teeth Evolved from The Scales of Sharks
When it comes to determining the origin of various body parts and systems in mammals and other creatures, evolutionary scientists are confronted with an endless number of complex puzzles to solve. One of the more interesting and hotly contested of the questions they must address has to do with the origin of teeth. It is fair to consider the debate hotly contested, since two primary theories of the origins of teeth have emerged and both are supported by highly credentialed experts.
The first theory, which is known as the “outside-in” hypothesis, suggests that teeth evolved from scales on the body that over time moved closer and closer to creatures’ mouths. At some point they moved directly into animals’ mouths and became adapted to eating sometime after that. The second theory, which unsurprisingly is referred to as the “inside-out” hypothesis, asserts that proto-teeth developed deep within the oral cavity in ancient animals, migrated to the jaws, and ultimately became fixed in that location.
Naturally, scientists interested in discovering the true origin of teeth have tried to devise research methodologies that would allow them to find better evolutionary models. In science, however, some of the most fascinating and revealing discoveries occur by accident, when scientists are researching one question but stumble onto information that helps to solve another problem.
This is what happened to a team of paleontology experts from Penn State University, who’ve just published a study in the Journal of Anatomy. Their outside-in scenario for the evolution of teeth is remarkable since their original research goals were quite different.
Rostral spines of the Ischyrhiza mira leidy, a Late Cretaceous sclerorhynchid sawfish, which was used in the recently proposed “outside-in” theory of teeth and the evolution of teeth in general. (Historical Biology)
Fish Scale Modifications Lead To Creeping Teeth
As study author and vertebrate paleontologist Todd Cook tells it, he and his colleagues were not searching for answers about how teeth evolved. Instead, they were studying the tissue structure of rostral denticles, which are jagged spikes that line the long snouts of sawfish (also known as carpenter sharks) and sawsharks. Despite a similar shape and appearance these spikes aren’t teeth, but they are used for foraging and self-defense, nonetheless.
Looking deep into the fossil record, the Penn State scientists analyzed fossilized denticles attached to the skeletal remains of an ancient and now-extinct form of sawfish known as Ischyrihiza mira. This species swam in the great lakes of North America during the Cretaceous period, between 100 million and 65 million years ago. The fossil samples the scientists obtained all came from a rock formation in New Jersey, and they offered a unique opportunity to study high-quality specimens from the unimaginably distant past.
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Notably, rostral denticles are thought to have evolved from scales before moving inward toward the mouth (but not all the way into the mouth).
"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," Cook explained in a Penn State press release covering the new study. “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."
To see if they could confirm a connection between the outer enameloid layers of denticles and sawfish scales, they used an electron scanning microscope to examine sections of the fossilized spikes. What they discovered during this procedure was quite unexpected.
"Surprisingly, Ischyrhiza mira's rostral denticle enameloid was anything but simple; it was considerably more complex than the enameloid of body scales," Cook said. "In fact, the overall organization of the enameloid in this ancient sawfish resembled that of modern shark tooth enameloid, which has been well-characterized." This discovery was significant, since sawfish and sharks are closely related species.
A more thorough examination of the Ischyrhiza mira denticles found clear parallels between the structure, design, and function of its enameloid coverings and those of sharks’ teeth. What Cook described as “bundles of microcrystals” were arranged in layers in both instances, and in the case of shark teeth it is known that these hard microcrystal layers help offset mechanical stresses on the teeth that are experienced when prey is being eaten.
"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," Cook noted, as he explained the sense of excitement the scientists felt when they realized that sawfish denticles and sharks’ teeth may have followed a common evolutionary path.
Since scientists are fairly certain that rostral denticles evolved from modified scales, this new discovery has some incredibly important implications.
How strange that the dreaded teeth of killer sharks and our own teeth began as something quite different. (ARTYuSTUDIO / Adobe Stock)
‘Outside-In’ is the Winner?
The scientists weren’t looking to solve the riddle of the origin of teeth. But inadvertently they collected evidence that suggests one of the two alternatives is definitely viable.
"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,” Cook said. “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."
Scientists know they are on firm ground when tracing an evolutionary relationship between scales and rostral denticles on sawfish. Scales that evolved to become harder while migrating toward the snout of these sea creatures may have completed the journey into the mouth in the case of sharks, from where their ultimate conversion into teeth was completed.
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Given the obvious evolutionary advantages of such a new development, it would have been preserved through generations and continued to persist as new species developed both inside the sea and out over the course of tens of millions of years. Species that could trace their lineage back to the original toothy shark would have kept the teeth, and if other species living in the sea developed teeth independently, through the same mechanism of scale-to-tooth evolution, they too would have passed that adaptation on into the future.
It should be emphasized that this discovery does not prove that the “outside-in” theory is true. But the evidence collected by the Penn State scientists is consistent with that theory, and as a result it has definitely tilted the playing field in favor of the “outside-in” scenario.
Top image: A recent study has likely proven that shark's teeth began as sawshark denticles that evolved forward to the snout to become fish teeth. This is a sawfish or sawshark or carpenter shark swimming in the sea. Source: tsuyoshi kaminaga / EyeEm / Adobe Stock
By Nathan Falde