Helicoprion was a type of shark-like cartilaginous fish that lived about 250 million years ago. Interestingly, the animals teeth were arranged in a fantastic “tooth-whorl” strongly reminiscent of a circular saw. Check it out:
While modern sharks have rows of serrated teeth ready to replace any that fall out, in the whorl shark’s lower jaw newer teeth would push older teeth further along the line. Read more here, here or here.