Shark-like Skull Completes the Picture
Thursday, 5th January 2017
A Grahamstown scientist is a partner in a significant scientific breakthrough that provides the link with the last major vertebrate group to the tree of life.
A small rock found in the Karoo is key to solving a puzzle that has intrigued scientists for more than a century. Scans of the rock containing a fossilised skull of a 280 million-year-old shark-like fish have revealed the origin of chimaeras, a group of cartilaginous fish related to sharks and rays.
Grahamstown palaeontologist and co-author of the study, Dr Robert Gess*, borrowed the rock from the South African Museum in Cape Town and had it scanned using high-definition computer tomography at Wits University. The scan allowed Gess and his colleagues to see not only the outer surface of the Dwykaselachus oosthuizeni fossil in perfect three dimensions, but also the inside of its skull.
The discovery, published online in Nature magazine this week, has finally made it possible for scientists to link the last major vertebrate group to the tree of life. It also provides insights on the early development of these fish at a crucial time in their evolution as they were diverging from sharks. They represent one of four fundamental divisions of modern vertebrate biodiversity.
There are currently about 50 species of chimaeras known in various oceans around the world.
They are strange-looking fish with names that reflect their peculiar appearances: ratfish, rabbit fish, ghost sharks, St Joseph sharks or elephant sharks.
These deep-water-dwelling fish have large eyes which help them forage in dimly lit waters, suggesting that they may have adapted to light-limited conditions.
The new evidence suggests that this adaptation occurred earlier than previously thought. Instead of teeth, chimaeras have tooth plates adapted for grinding their meals.
Dwykaselachus oosthuizeni is named after Roy Oosthuizen, an amateur palaeontologist who found the fossil on his farm near Prince Albert in the 1980s. Suspecting that a smooth stone in the veld was a nodule (the hard case around a fossil, that tends to weather out as a pebble) he cracked it open.
The exposed part of the broken rock revealed to scientists that Oosthuizen had found a fossil skull belonging to an extinct shark-like cartilaginous fish. It was named in 1986.
The new study, that reveals the whole skull for the first time, allows the authors to identify it as coming from an extinct order of ‘sharks’, the Symmoriidae.
Analysis of the scans also however revealed telltale structures of the brain, major cranial nerves, nostrils and inner ear showing that internally Dwykaselachus is very similar to modern-day chimaeras. This combination of shark-like and chimaeroid features has allowed the researchers to solve the riddle as to where chimaeroids branched from sharks.
Lead author of the study, Professor Michael Coates of the University of Chicago, said that chimaeras are unusual throughout the long span of their fossil record.
“Because of this, it's been difficult to understand how they got to be the way they are in the first place. This discovery sheds new light not only on the early evolution of shark-like fishes, but also on jawed vertebrates as a whole.”
Chimaeras rarely fossilise because, like sharks, their skeletons are made of cartilage and not bones. Until now, the chimaeroid evolutionary record consisted mostly of isolated specimens of their hyper-mineralised tooth plates.
*Dr Robert Gess of the Albany Museum is a South African Centre of Excellence in Palaeontology partner who was based at the Rhodes Geology Department at the time.
Article by Steven Lang, Grocotts Mail