Scientists have long debated the origins of teeth. Did they evolve from body scales that migrated into the mouths of ancient vertebrates and became adapted for eating — an idea known as the "outside-in" hypothesis? Or did they evolve independent of scales, originating deep within the oral cavity and ultimately mounting onto the jaws — known as the "inside-out" hypothesis? A new study by scientists at Penn State provides evidence for the "outside-in" hypothesis.
According to vertebrate paleontologist Todd Cook, associate professor of biology, Penn State Behrend, the team did not set out to contribute to the debate about the origins of teeth. Instead, he and his colleagues were studying the tissue structure of rostral denticles, which are the jagged spikes that run along the elongated snouts of sawsharks and sawfishes and are used in foraging and self-defense. Cook, who is the lead author of the study, which appears in the September issue of Journal of Anatomy, noted that sawfishes belong to the same group as skates and rays and are closely related to sharks.
The team examined the fossilized rostral denticles of Ischyrhiza mira, a species belonging to an extinct group of sawfishes that lived in North American waters during the late Cretaceous period, around 100 to 65 million years ago. The samples had previously been recovered from a rock formation in New Jersey.
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