Paleontologists are using 3D printed copies of a 400 million-year-old fossil to make their case that placoderms, an important class of early jawed fish, lacked true teeth. The argument goes to the heart of the development of one vertebrates' most enduring features.
The discovery of the 400 million-year-old fossil Buchanosteus, a member of the placoderm armored fishes, near Canberra in Australia, has presented Dr Gavin Young of the Australian National University with an opportunity to get a better picture of how teeth first appeared.
Teeth, such a common feature of fish and land-based vertebrates, appeared in the Devonian era (416 million to 358 million years ago). But there is plenty of debate about when and how this process occurred.
It interests biologists in part because it may not have been inevitable. Many different animal lineages have evolved eyes, some of them fairly similar to our own, but true teeth may have been a one-off development.
Placoderms were among the first species to develop a jaw, making them central to the debate as to whether teeth preceded or followed the development of hinged jaws. They certainly possessed hard mouthparts with which to bite their prey, but paleontologists continue to disagree over whether these were true teeth, or teeth-like plates of bone.
“All these early animals had what we call dermal bones,” Young told IFLScience. “These have tubercles, but they can only grow inside skin.” Some bumps on placoderm fossils have been interpreted by other paleontologists as teeth, but Young disputes this.
The model Young and Hu made of the Buchanosteus shows it had hard protuberances, but they think they were bone under skin, not teeth. Australian National University
The capacity of most animal species to discard worn down teeth and replace them with sharper ones probably represented a major advantage for the first evolvers, but we don't know how the process occurred. Despite the fact that injuries to teeth don't heal the way other parts of the body can, the superiority of teeth is so great they have seldom been discarded, other than by birds needing to lighten their baggage for flight.
Distinguishing dermal bones from true teeth in 400 million-year-old fossils is hard enough, harder still when the fossils are too delicate to examine as we might wish. Young and PhD student Yuzhim Hu took CT scans of the Buchanosteus fossil and fed them into a 3D printer to create a more robust model to determine whether the "tooth-like denticles" represented a transitional stage in the evolution of teeth. He says this provides evidence that what others have interpreted as placoderm teeth were bony lumps that were not even inside the fish's mouth.
Young and Hu claim their model of Buchanosteus as evidence that placoderm fishes lacked true teeth, a case they make in a paper published in Biology Letters, disputing the findings of an article the journal published last year suggesting that the extinct placoderms had real teeth.
“We know some other fishes at the time had teeth,” Young said to IFLScience. “We're trying to work out when the common ancestor with teeth occurred.”
If placoderms did not have true teeth, that ancestor was probably more recent than others have argued. Since placoderms certainly had modern-type jaws, confirming their lack of teeth would settle the question of which evolved first.