The secret history of teeth revealed: How they originated inside a 465-million-year-old fish's body
From vertebrates to teeth
In a
published in the journal Nature on Wednesday, researchers showed how sensory tissue discovered on the exoskeletons of ancient fish is linked to the same "genetic toolkit" that produces human teeth.
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"This shows us that 'teeth' can also be sensory even when they're not in the mouth," said Yara Haridy, co-author of the study and palaeontologist and evolutionary biologist at the University of Chicago.
Initially, researchers set out to identify the earliest vertebrate in the fossil record seeking specimens from the Cambrian and Ordovician periods. An obvious sign of a vertebrate species is the presence of internal tubules for dentine, a calcified tissue that is found under the enamel in the human teeth, but also in the external bumps or odontodes in ancient fish armour.
While making the use of high-resolution CT scans to analyse the jawless "first fish" species Anatolepis heintzi, the researchers discovered pores filled with dentine. They also compared the fossil to a coterie of ancient fossils and modern marine life.
The shocking discovery
Upon closer investigations, researchers realised that the supposed dentine-lined pores were like the sensilla, the sensory organs on the shells of crabs, thus making A.
Heintz, an ancient invertebrate arthropod and not a vertebrate fish.
Vertebrates and invertebrates, both have sensory armours connected to nerves that help them sense their environment. To substantiate the discovery that the fish was an arthropod, the team scanned fossils and modern specimens from snails and barnacles to sharks and catfish. It was revealed that the suckermouth catfish raised in Hardy's own lab had small tooth-like scales on their skin, called denticles, that were connected to nerves.
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This discovery also led to the realisation that ancient vertebrates had the same mineralized tissue that evolved into dentine and then our sensitive teeth. These tissues helped them sense their environment like cold waters or pressure from nearby objects, just like our teeth.
Thus, this study also supports a key theory in evolutionary biology called the "outside-in" hypothesis where sensory structures evolved on exoskeletons at least 460 million years ago and were used by animals to make teeth.
"Viewed through this evolutionary lens, the fact that teeth in the mouth are extremely sensitive is less of a mystery, and more a reflection of their evolutionary origins within the sensory armour of early vertebrates," wrote the researchers in the study.
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