Sawfish fossils suggest teeth likely evolved from body scales in ancient fish

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 provides evidence for the 'outside-in' hypothesis.