THE ORIGIN OF OUR TEETH IS AMONG THE MOST PRIMITIVE JAWED FISH
The most primitive jawed fish fossils with teeth, found near Prague more than 100 years ago, have been studied at beamline ID19 for the first time. The results show that their dentitions have surprisingly modern features, directly ancestral to our own.
STRUCTURE OF MATERIALS
Teeth in the two main groups of extant jawed vertebrates bony fish (and their descendants, the land animals) and cartilaginous fish (sharks and their relatives) usually develop on the inner side of the old ones (in humans this pattern has been modified so that new teeth develop below the old ones, deep inside the jawbone). There are, however, several differences: sharks have no bones and their teeth do not attach to the cartilaginous jaws, whereas in bony fish and land animals the teeth are always attached to jaw bones. In addition, while sharks shed their worn-out teeth in their entirety, simply by detaching them from the skin, bony fish and land animals shed theirs by dissolving away the tooth bases. This diversity raises many questions about the origin of teeth. Until now, researchers have focused on fossils of a group of ancient fish that lived about 430 to 360 million years ago the arthrodires which were the only stem jawed vertebrates in which teeth were known. However, they struggled to understand how they could have evolved into the teeth of modern vertebrates, as arthrodire teeth are very different in position and mode of tooth addition in comparison to bony fish and sharks .
This work aimed to determine whether this peculiar type of dentition was really ancestral to ours, or just a specialised offshoot of the lineage leading towards modern jawed vertebrates, by studying the acanthothoracids, an early fish group closely related to the common ancestor of jawed vertebrates. The very finest collection of them comes from the Prague Basin in the Czech Republic, from rocks that are just over 400 million years old. They have proved difficult to study by conventional techniques because the bones cannot be freed from the enclosing rock, and have therefore
never been investigated in detail. Propagation phase-contrast synchrotron microtomography was used at beamline ID19, enabling the visualisation of the internal structure of the fossils in 3D without damaging them. The results were truly remarkable, including unexpected well-preserved dentitions. Follow-up scans at higher resolution (Figure 130) made it possible to visualise the growth pattern and even the perfectly preserved cell spaces inside the dentine of these ancient teeth.
Like in arthrodires, the acanthothoracid dentitions are attached to bones. This indicates that bony fish and land animals retain the ancestral condition in this regard, whereas sharks are specialised in having teeth that are only attached to the skin in contrast to the common perception that sharks are primitive living vertebrates. Again, like arthrodires, the teeth of acanthothoracids were not shed. In other ways, however, acanthothoracid dentitions are fundamentally different from those of arthrodires. Like sharks, bony fish and
Fig. 130: Fossil skull of Radotina mounted for precise, high-resolution spot scans at beamline ID19. Image
courtesy of Boris Ekrt, National Museum in Prague.