Piercing teeth in vertebrates didn’t evolve from their scales

A new study has questioned one of the most important theories in vertebrate history after scientists discovered the formidable array of teeth found in the mouth of sharks did not evolve from their scales.

Scientists from the University of Sheffield, in collaboration with the Natural History Museum, King’s College London, Birkbeck University of London, and the University of Vienna, used X-ray computed tomography to closely examine remains of the fossil ray, Schizorhiza stromeri, which has a near identical jaw formation to sharks, and an extended cartilaginous rostrum, like saw sharks today.

mainThe exquisite detail identified clear differences between the organisation and replacement of the animal’s tooth-like external scales along its saw-rostrum and teeth structure.

This contradicts classical theories that suggested teeth in vertebrates directly evolved from external skin scales.

Dr Gareth Fraser, from the University of Sheffield’s Department of Animal and Plant Sciences, said: “Shark teeth are so distinctive and the creature’s most fascinating feature.

“Like the poster for the 1970s movie Jaws, whenever we think of sharks we immediately picture their piercing teeth. However as scientists we know very little about how they were formed – especially across different species.

“The rostral saw-teeth in this unique fossil ray highlights that skin teeth or denticles in sharks and Rays have the potential to regenerate in a similar way to oral teeth."

These new fossils from the Cretaceous of Morocco identified important differences between the structure, organisation and replacement of saw-teeth and oral teeth, suggesting teeth and scales had independent evolutionary histories.

Dr Zerina Johanson, a researcher at the Natural History Museum and lead author of the study said: “The data generated from the X-ray computed tomography has provided us with an exceptional insight into the evolution and adaptation of the teeth of some of the most fearsome creatures on Earth. The ‘saw-teeth’ of Schizorhiza stromeri, despite being from a Cretaceous-aged fossil, preserved important development detail and showed an unusual series of rotations before they became functional, very different from the development of typical teeth in the mouth.

“These differences contradict classical theories of tooth evolution; saw-teeth are simply modified external scales.

“We can now use these methods to research a range of fossil and living vertebrates and find out more about the evolution and diversity of the teeth on some of the most dangerous creatures on our planet”.

The research is published in the Proceedings of Royal Society B and is sponsored by Natural Environmental Research Council (NERC).

Notes for editors

National History Museum
The Natural History Museum welcomes more than five million visitors a year and is a world leading science research centre. Through its unique collection and unrivalled expertise it is tackling the biggest challenges facing the world today. It helps enable food security, eradicate disease and manage resource scarcity. It is studying the diversity of life and the delicate balance of ecosystems to ensure the survival of our planet. For more information go to www.nhm.ac.uk

University of Sheffield
With almost 26,000 of the brightest students from around 120 countries, learning alongside over 1,200 of the best academics from across the globe, the University of Sheffield is one of the world’s leading universities. A member of the UK’s prestigious Russell Group of leading research-led institutions, Sheffield offers world-class teaching and research excellence across a wide range of disciplines. Unified by the power of discovery and understanding, staff and students at the university are committed to finding new ways to transform the world we live in. In 2014 it was voted the number one university in the UK for Student Experience by Times Higher Education and in the last decade has won four Queen’s Anniversary Prizes in recognition of the outstanding contribution to the United Kingdom’s intellectual, economic, cultural and social life. Sheffield has five Nobel Prize winners among former staff and students and its alumni go on to hold positions of great responsibility and influence all over the world, making significant contributions in their chosen fields. Global research partners and clients include Boeing, Rolls-Royce, Unilever, AstraZeneca, Glaxo SmithKline, Siemens and Airbus, as well as many UK and overseas government agencies and charitable foundations.


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