Fossil reveals origin of chimaeroid fishes


A High-definition CT scans of the fossilised skull of a 280 million-year-old fish has revealed the origin of chimaeras, a group of cartilaginous fish related to sharks.

JOHANNESBURG – High-definition CT scans of the fossilised skull of a 280 million-year-old fish reveal the origin of chimaeras, a group of cartilaginous fish related to sharks, a study has revealed.

Analysis of the brain case of Dwykaselachus Oosthuizeni, a shark-like fossil from South Africa, shows telltale structures of the brain, major cranial nerves, nostrils and inner ear belonging to modern-day chimaeras.

This discovery allows scientists to firmly anchor chimaeroids — the last major surviving vertebrate group to be properly situated on the tree of life and sheds light on the early development of these fish as they diverged from their deep, shared ancestry with sharks.

The paper was co-authored by Dr Rob Gess, a partner of the South African Centre of Excellence (CoE) in Palaeosciences, who is based at the Albany Museum in Grahamstown. The CoE is hosted at Wits University as part of the Evolutionary Studies Institute.

“The origin of holocephalan fish (chimaeras) has long puzzled scientists, who had worked out the evolutionary relationships of all other major living vertebrate groups. Chimaeras were clearly somehow related to sharks and rays but they are very different, with a very distinctive skull. All fossil chimaeras were already completely chimaera so there were no recognised in-between forms to bridge the gap between their ancestors and the ancestors of modern sharks,” said Gess.

“Our research demonstrates that Dwykaselachus provides the missing piece of the puzzle, as it has features of certain primitive ‘sharks’ as well as of chimaeras. We can now anchor chimaeras firmly on the tree of vertebrate life.”

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There are about 50 living species of chimaeras, known in South Africa as the Josef, St. Joseph sharks or elephant sharks.

St Joseph sharks generally live very deep in the ocean, but come into shallower waters to breed in summer. As a result they are often caught in summer as bicatch and are also sometimes deliberately netted on the west coast of the Western Cape. Their meat is however rather oily and they are not a popular dish.

The Dwykaselachus fossil was discovered by amateur paleontologist and farmer Roy Oosthuizen in the 1980s.

The most important features of the fossil weren’t accessible at the time. After that it was archived in a small cardboard box in the strongroom reserved for type specimens at the South African Museum in Cape Town.

There it remained until 2013 when Wits’ Evolutionary Studies Institute obtained a micro-CT scanner.    

CT scans showed that the Dwykaselachus skull was remarkably intact, one of very few early cartilaginous skulls that had not been crushed during fossilisation. The scans also provide an unprecedented view of the interior of the brain case.

The scans show a series of telltale anatomical structures that mark the specimen as an early chimaera, not a shark.