First hard evidence for outside-in theory of origin of teeth

Some researchers believe that teeth evolved from dermal scales called odontodes around the mouth region in primitive fishes. According to lead researcher Dr Donglei Chen, the study’s approach to understanding the origin of teeth is a different one.

“People have tended to search for dermal odontodes that look like teeth. However, even if the extra-oral ‘teeth’ have all the features thought to be unique to true teeth, this may only represent convergent evolution based on a flexible developmental tool kit shared by all dermal teeth,” she explained.

Based on their work on one of the earliest known bony fishes, Lophosteus superbus, from the late Silurian period of Estonia, the Uppsala University researchers looked for clues into the origin of teeth. This fish had many skull ornamentations that were similar to that of earlier groups of fishes called arthrodires. Some of these ornamentations on and around the mouth had a dome-shaped appearance that the researchers referred to as “tooth cushions”. These appear to represent the most primitive form of a tooth battery within the mouth.

By using synchrotron microtomography, Chen and her colleagues were able to reconstruct 3-D images of specimens of different ages in order to compare the growth history and development of the teeth. Chen said, “By modelling the successive resorption surfaces in three dimensions it allows us to visualize the entire developmental trajectory of the dentition.”

As a result, the dental development of Lophosteus may cast light on the possible origin of teeth from dermal odontodes and on the evolutionary relationship between dentitions of all jawed animals.

“We can watch how fishes initiated and replaced teeth one by one, and how the blood vessels of these teeth were formed, 400 million years ago. It is as if we have traveled through a space-time portal to a living, microscopic world inside the fossil bones,” she added.

This research was presented in August at the 77^th annual meeting of the Society of Vertebrate Paleontology in Calgary.

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