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Fun Facts about Teeth across the Animal Kingdom

Two competing theories about the evolutionary origins of teeth have been battling back and forth for decades, vacillating with the latest supporting discoveries in developmental biology or the fossil record. The “outside-in” hypothesis suggests that toothlike dermal scales with pulplike centers covered in hardened mineral—similar to denticles found today—gradually migrated across the body’s exterior surface over successive generations of fish before moving inward to take up residence in our ancestors’ jawbones. The “inside-out” hypothesis suggests that teeth originated internally before migrating forward in the oral cavity to become oral teeth.

An investigation of a fossilized sawtooth shark’s rostral denticles (the “teeth” on the fish’s sawlike bill) showed complex internal structures incredibly similar to those found in shark teeth. This discovery suggests that the developmental gap between dermal scales and teeth is smaller than originally thought, edging the outside-in hypothesis ahead of inside-out once more.