Tracing the evolution of vertebrate teeth

The evolutionary roots of teeth and dermal jawbones (cheekbones), the precursor to vertebrate jaws as we know them today, may be older than previously thought.

That information comes thanks to some very modern technology and some very old fossils with surprisingly modern-looking teeth.

Using synchrotron microtomography, a team of Swedish, Czech, French and UK researchers led by Sweden’s Uppsala University took a detailed look at a collection of 400-million-year-old fossils of acanthothoracids – an early fish group closely related to the very first jawed vertebrates – found near the Prague Basin in the Czech Republic a century ago.

These have been difficult to study by conventional means because the bones cannot be freed from the enclosing rock, but the new imaging technology, which uses intensely bright and laser-like X-rays given off by electrons accelerated to near the speed of light, was able to visualise their internal structure in 3D without harming them.

Follow-up scans at higher resolution taken at the European Synchrotron Radiation Facility in France allowed the researchers to visualise the growth pattern and even the perfectly preserved cell spaces inside the dentine.

“The results were truly remarkable, including well-preserved dentitions that nobody expected to be there,” says Valéria Vaškaninová, lead author of a paper in the journal Science.

The first vertebrates were jawless, but vertebrates now exhibit a variety of teeth and jaws that differ greatly across species in form and function. How and when this transition happened is poorly understood, however.

The work by Vaškaninová and colleagues suggests the jaws and teeth of acanthothoracids, and the way their teeth grew, shared more similarities with bony fish, sharks and even land animals than another early fish group, the arthrodires. Acanthothoracid dentitions are attached to jaw bones, as in bony fish and land animals.

“These findings change our whole understanding of the origin of teeth,” says co-author Per Ahlberg.

“Even though acanthothoracids are among the most primitive of all jawed vertebrates, their teeth are in some ways far more like modern ones than arthrodire dentitions. Their jawbones resemble those of bony fish and seem to be directly ancestral to our own.”