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Origins of teeth turned outside-in

17 October 2013, by Alex Peel

Teeth may have evolved from scales on the outside of animal bodies, before being trapped in the mouth by the evolution of the jaw, say scientists.

Conodont and paraconodont tooth

For decades, it's been thought that bony materials originated as teeth in the mouths of predators, before spreading out over the body surface to form skeletal scales. But a new study, published in Nature, turns the inside-out theory, outside-in.

The team, led by Duncan Murdock of the University of Bristol, looked at the 500 million-year-old fossils of extinct eel-like creatures called conodonts.

Conodonts had no jaw and were entirely soft-bodied except for a tiny set of teeth-like elements in their mouths, just a few millimetres in size.

For 40 years, these were thought to be the first traces in the fossil record of the bony materials that went on to form our skeletons and teeth.

But the team, working with physicists at the Paul Scherrer Institut in Switzerland, used high-energy X-rays to examine the development and structure of the tiny teeth.

'This removes a key piece of evidence from the hypothesis that teeth evolved before the skeletal armour'
Professor Philip Donoghue,
University of Bristol

Comparing them with the teeth of ancestors and modern-day animals, they found that the conodonts had evolved separately from the ancestors of today's bony creatures.

The discovery removes a crucial plank of evidence from the idea that skeletons evolved from the oral weaponry of predators. Instead, it now seems more likely that teeth developed from the defensive armour of our more sedate, mud-slurping ancestors.

'We were able to visualise every tissue, cell and growth line within the bony teeth, allowing us to study their development,' says Murdock.

'We compared the tooth-like skeleton of conodonts to that of their 'paraconodont' ancestors and to teeth in living vertebrates, demonstrating that the tooth-like structure of conodonts was assembled through evolutionary time independently of other vertebrates.'

'This removes a key piece of evidence from the hypothesis that teeth evolved before the skeletal armour,' says colleague Professor Philip Donoghue, also of Bristol.

'It suggests that the common ancestors of conodonts and other vertebrates likely lacked a mineralized skeleton. Rather, it seems that teeth evolved from the armour of our meek filter-feeding ancestors.'

The work was funded by NERC and the Paul Scherrer Institut.

Murdock DJE, Xi-Ping Dong, Repetski JE, Marone F, Stampanoni M, Donoghue PCJ, 'The origin of conodonts and of vertebrate mineralized skeletons,' 2013, Nature, DOI: 10.1038/nature12645

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