Outside-In Origin For Your Teeth

Our skeletons are time capsules. Even though the whole marks us as a distinct species, we’re also a collection of elements that hearken back to our deep history. Teeth are among the oldest of these evolutionary curiosities. The enamel-covered structures were supposed to be the first hard parts our squirmy, swimming ancestors developed, later covering the bodies of pseudo-fish and setting the stage for the evolution of the vertebrate skeleton. As it turns out, however, the story of the first teeth isn’t so much inside-out as outside-in.

Enigmatic creatures called conodonts have been key to ideas about how teeth and skeletons evolved. Found in strata between 500 and 200 million years old, these early animals were initially known only as spike-like teeth. Later finds of intricately-preserved specimens showed that conodonts looked like miniature lampreys – long, thin bodies with a jawless orifice full of nasty-looking spikes. But even finding the bodies of these animals didn’t quell uncertainty about what they were and their relationship to us. Ideas about conodont identity have run  the gamut from oddballs unrelated to our direct ancestors to pioneers of the vertebrate skeleton we inherited.

[Video: A reconstructed vision of a 250 million year old conodont’s mouth, showing how all those spurs may have worked.]

In those scenarios that have drawn our past back to conodonts, though, the connection has been through the mouth. Conodont teeth were hard structures that seemed to grow in a similar manner to ours and be made up of the same stuff. But a new Nature study by University of Bristol paleontologist Duncan Murdock and colleagues has discovered that this isn’t so. Conodonts don’t actually represent the early days of our skeleton.

By analyzing X-rays of conodont spikes, Murdock and coauthors found that these animals did not grow teeth in the same way as our ancestors. As vertebrate teeth develop, they are capped by a hard coating of enamel. But this didn’t happen in conodonts. Early conodonts didn’t add layers to their spikes, and later species of conodont grew layers around the sides of their spurs that never covered the tip. This difference in development, and apparent differences in composition, indicate that conodonts did not have true teeth at all, but unique, tooth-like structures.

This changes what paleontologists expect about the pattern in which the skeletons of our deep ancestors came together. Conodonts may or may not have been vertebrates – that point is still disputed – but, regardless, they evolved hard mouthparts independently of the early vertebrates that were our forebears. So with conodonts removed from our story, the origin of our teeth goes back to a different sort of creature.

Instead of conodonts, Murdock and colleagues suggest, fish-like creatures with primitive jaws were probably the first vertebrates with true teeth. Our teeth probably got their start as hard external scales made of dentine that were embedded in the skin of these prehistoric swimmers. From there, teeth moved into the mouth just about the time that vertebrates evolved the ability to bite. Stop and think about that for a moment – the teeth in your mouth started as body armor.

References:

Murdock, D., Dong, X., Repetski, J., Marone, F., Stampanoni, M., Donoghue, P. 2013.The origin of conodonts and of vertebrate mineralized skeletons. Nature. doi:10.1038/nature12645

Turner, S., Burrow, C., Schultze, H., Blieck, A., Reif, W., Rexroad, C., Bultynck, P., Nowlan, G. 2010. False teeth: conodont-vertebrate phylogenetic relationships revisited. Geodiversitas. 32 (4): 545-594