There are two theories about where teeth originated: they developed from external scales (extrinsic hypothesis) or from somewhere inside the mouth (internal hypothesis).
Researchers studying the fossils of Ischyrhiza mira, an extinct species of sawfish that lived in North America between 65 and 100 million years ago, found more evidence to support the outsider idea.
Like saw sharks and sawfish today, this creature had serrated spikes around its snout to help it avoid predators and find food. These spikes, called rostral denticles, are thought to be modified versions of the scales on the rest of the body.
In an attempt to test the connection between rostral denticles and scales, the team looked at the outer hard enamel layer of the snout quills, but what they found was quite different from what they expected.
There are two theories about where teeth originated: they developed from external scales (extrinsic hypothesis) or from somewhere inside the mouth (internal hypothesis).
Researchers studying the fossils of Ischyrhiza mira, an extinct species of sawfish that lived in North America between 65 and 100 million years ago, found more evidence to support the outsider idea.
Like saw sharks and sawfish today, this creature had serrated spikes around its snout to help it avoid predators and find food. These spikes, called rostral denticles, are thought to be modified versions of the scales on the rest of the body.
In an attempt to test the connection between rostral denticles and scales, the team looked at the outer hard enamel layer of the snout quills, but what they found was quite different from what they expected.
Although it is not impossible that these scales and teeth developed their clustered microstructures separately, it makes more sense if one came after the other—in other words, an external hypothesis of the origin of the teeth.
Interestingly, the researchers did not actually intend to investigate the evolutionary history of teeth when they began their analysis of the rostral denticles, but their findings could have a significant impact on future studies in the field.
As there are more and more similarities between the exteriors of sea creatures and the crustaceans inside our mouths, it seems that the teeth inside our mouths are actually developed from fish scales.
“This finding provides direct evidence supporting the extrinsic hypothesis, as it shows that scales have the ability to develop a complex tooth-like enamel outside the mouth,” says Cook.
