Skull completes the picture
A Grahamstown scientist has helped provide the link with the last major vertebrate group to the tree of life. Steven Lang reports
oosthuizeni 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 lightlimited 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 Oost- huizen, 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 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 of 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 bone. 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.
Reconstruction of a symmorid shark, with brain case.
Right: Rock nodule containing the Dwyka fossil.
3D printed models of the interior and exterior of the Dwykaselachus brain case.