Teeth hurt? It could be because of a 500-million-year-old fish
The exact origin of teeth -- and what they were for -- has long proved elusive to scientists.
Their evolutionary precursors are thought to be hard structures called odontodes which first appeared not in mouths but on the external armour of the earliest fish around 500 million years ago.
Even today, sharks, stingrays and catfish are covered in microscopic teeth that make their skin rough like sandpaper.
There are several theories for why these odontodes first appeared, including that they protected against predators, helped with movement through the water or stored minerals.
But the new study published in the journal Nature supports the hypothesis that they were originally used as sensory organs which transmitted sensations to nerves.
At first, the study's lead author Yara Haridy was not even trying to hunt down the origins of teeth.
Instead the postdoctoral researcher at the University of Chicago was probing another major question puzzling the field of palaeontology: what is the oldest fossil of an animal with a backbone?
Haridy asked museums across the United States to send her hundreds of vertebrate specimens -- some so small they could fit on the tip of a toothpick -- so she could analyse them using a CT scanner.
She began focusing on dentine, the inner layer of teeth that sends sensory information to nerves in the pulp.
- Things get fishy -
A fossil from the Cambrian period called Anatolepis seemed to be the answer she was looking for. Its exoskeleton has pores underneath the odontodes called tubules that could indicate they once contained dentine.
This has previously led paleontologists to believe that Anatolepis was the first known fish in history.
But when Haridy compared it to the other specimens she had scanned, she found that the tubules looked much more like sensory organs called sensilla of arthropods, a group of animals that includes crustaceans and insects.
The mighty Anatolepis was therefore demoted to the rank of an invertebrate.
For modern arthropods such as crabs, scorpions and spiders, sensilla are used to perceive temperature, vibration and even smell.
How little these features have changed over time suggests they have been serving these same functions for half a billion years.
The researchers said they found "striking" similarities between these features in Anatolepis and vertebrate fish from around 465 million years ago -- as well as some better-known fish.
"We performed experiments on modern fish that confirmed the presence of nerves in the outside teeth of catfish, sharks and skates," Haridy told AFP.
This shows that "tooth tissues of odontodes outside the mouth can be sensitive -- and perhaps the very first odontodes were as well," she added.
"Arthropods and early vertebrates independently evolved similar sensory solutions to the same biological and ecological problem."
Senior study author Neil Shubin, also from the University of Chicago, said that these primitive animals evolved in "a pretty intense predatory environment".
"Being able to sense the properties of the water around them would have been very important," Shubin said in a statement.
Haridy explained that over time, fish evolved jaws and "it became advantageous to have pointy structures" near their mouth.
"Little by little some fish with jaws had pointy odontodes at the edge of the mouth and then eventually some were directly in the mouth," she said.
"A toothache is actually an ancient sensory feature that may have helped our fishy ancestors survive!" —AFP
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