Ancient fish skull tells a shark’s tale

2017-01-06 08:24
A reconstruction on the symmoriid with the dwyka skull superimposed. (Kirsten Tietjen)

A reconstruction on the symmoriid with the dwyka skull superimposed. (Kirsten Tietjen)

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Johannesburg - It wasn’t just the jaws but the full fossilised skull of a 280 million-year-old fish a farmer found in the 1980s – that has now made waves in revealing how chimaera fish share their ancestry with sharks, the University of Witwatersrand said on Thursday.

“Analysis of the brain case of Dwykaselachus oosthuizeni, a shark-like fossil from South Africa, shows the tell-tale structures of the brain, major cranial nerves, nostrils and inner ear belonging to modern-day chimaeras,” the university said in a statement.

Dr Rob Gess of the Centre of Excellence in Palaeosciences was one of the co-authors of an academic paper published on the discovery this week.

He explained that the origin of holocephalan fish (chimaeras) had for long left scientists high and dry.  They could identify that the chimaeras – a group of cartilaginous fish – were related to sharks and rays, but yet they also had differences, including a very distinct skull. 

Little did amateur palaeontologist and farmer Roy Oosthuizen know just what a sea change in science he would stir, when he discovered the fossil in a rock on his farm in Prince Albert in the 1980s.

“He found the nodule of rock that had weathered out of 280 million-year-old mudstone, just 30cm above the top of the Dwyka glacial deposits.”

The farmer asked his son to hold the rock while he hit it with a hammer. When it split into three pieces the fossil skull was revealed.

A side-view of the nodule. (Rob Gess)

Fossil scanned

The fossil was kept in a cardboard box in an archive of a Cape Town museum until 2013, when the Wits Evolutionary Studies Institute obtained a micro-CT scanner.

Soon, Gess was granted permission to begin scanning the fossil. Being remarkably intact, it allowed for a clear view of the interior of the braincase.

“We spent days getting the best possible scans. It scanned incredibly well, so our team were able to create a perfect 3D digital image of the skull, inside and out.”    

Their research showed that indeed the Dwykaselachus was not just one of plenty of fish in the sea. It proved to be the first specimen to reveal both features of certain primitive “sharks” and of chimaeras.

The scans allowed Gess to trace the early development of these fish as they diverged from their deep, shared ancestry with sharks.

“We can now anchor chimaeras firmly on the tree of vertebrate life,” explained Gess.

The Dwykaselachus shares features of symmoriid sharks, “a bizarre group of 300+ million-year-old sharks, known for their unusual dorsal fin spines, with some resembling boom-like prongs and others surreal ironing boards”. The scans, however, revealed that they also carried a series of tell-tale anatomical structures that marked the specimen as an early chimaera.

Remarkable specimen

And the Dwykaselachus has netted interest across international seas. Lead author of the study, Dr Mike Coates, serves as a professor of organismal biology and anatomy at the University of Chicago.

“When I saw it for the first time, I was stunned. The specimen is remarkable,” Coates said.

The 50 living species of chimaera found in the world today have, in fact, now evolved very differently to sharks. They have large eyes and tooth plates, adapted for grinding prey.

Known in South Africa as Josef, St Joseph, or elephant sharks, they generally live deep in the ocean but come to shallow waters to breed in summer.

(Coates et al 2016)

Read more on:    wits university  |  sharks  |  paleontology

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