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CT scans of the fossilized skull of Dwykaselachus oosthuizeni — a shark-like fish that lived during the Permian period, around 280 million years ago — reveal the origin of chimaeras, a group of cartilaginous fish related to sharks. The research appears today in the journal Nature.
Reconstruction of the early chimaera Dwykaselachus oosthuizeni. Image credit: Kristen Tietjen.
Chimaeras, or chimaeroid fishes (subclass Holocephali), include 47 living species, known in various parts of the world as ratfish, rabbit fish, ghost sharks, St. Joseph sharks or elephant sharks.
They represent one of four fundamental divisions of modern gnathostomes (jawed vertebrates).
With large eyes and tooth plates adapted for grinding prey, these deep-water dwelling fish are far from the killer sharks of Hollywood. Their anatomy comprises features reminiscent of sharks, ray-finned fishes and tetrapods, and their form is shaped by hardened bits of cartilage rather than bone.
Because chimaeras are found in deep water, they were long considered rare. But as marine biologists gained the technology to explore more of the ocean, they are now known to be widespread, but their numbers remain uncertain.
“Chimaeroids belong somewhere close to the sharks and rays, but there’s always been uncertainty when you search deeper in time for their evolutionary branching point,” said lead author Prof. Michael Coates, from the University of Chicago.
“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.”
“Our discovery sheds new light not only on the early evolution of shark-like fishes, but also on jawed vertebrates as a whole.”
Vertebrate family tree showing the new position of Dwykaselachus oosthuizeni. Image credit: Michael I. Coates et al.
After a 2014 study detailing their extremely slow-evolving genomes was published in the journal Nature, interest in chimaeras blossomed.
Of all living vertebrates with jaws, chimaeras seemed to offer the best promise of finding an archive of information about conditions close to the last common ancestor of Homo and the great white shark.
Like sharks, also reliant on cartilage, chimaeras rarely fossilize. The few known early chimaera fossils closely resemble their living descendants.
Until now, the chimaeroid evolutionary record consisted mostly of isolated specimens of their characteristic hyper-mineralized tooth plates.
The Dwykaselachus oosthuizeni fossil resolves this issue. It was originally discovered by amateur paleontologist Roy Oosthuizen when he split open a nodule of rock on his farm in South Africa in the 1980s. An initial description named it based on material visible at the broken surface of the nodule.
In 2013, co-author Dr. Robert Gess, a researcher in the Geology Department and Albany Museum at Rhodes University in South Africa, CT-scanned the Dwykaselachus skull.
At the surface, the specimen appeared to be a symmoriid shark, a group of 300 million-year-old sharks, known for their unusual dorsal fin spines, some resembling boom-like prongs and others surreal ironing boards.
CT scans showed that the skull was remarkably intact, one of a very few that had not been crushed during fossilization. The scans also provide an unprecedented view of the interior of the brain case.
“When I saw it for the first time, I was stunned. The specimen is remarkable,” Prof. Coates said.
The images show a series of telltale anatomical structures that mark the specimen as an early chimaera, not a shark. The braincase preserves details about the brain shape, the paths of major cranial nerves and the anatomy of the inner ear.
All of which indicate that Dwykaselachus oosthuizeni belongs to modern-day chimaeras.
The scans reveal clues about how these fish began to diverge from their common ancestry with sharks.
“A large extinction of vertebrates at the end of the Devonian period, about 360 million years ago, gave rise to an explosion of cartilaginous fishes,” Prof. Coates said.
“Instead of what became modern-day sharks, revelations from the study indicate that much of this new biodiversity was, instead, early chimaeras.”
“We can now say that the first radiation of cartilaginous fishes after the end Devonian extinction was chimaeras, in abundance.”
“It’s the inverse of what we’ve got today, where sharks are far more common.”
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Michael I. Coates et al. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature, published online January 4, 2017; doi: 10.1038/nature20806
This article is based on a press-release from the University of Chicago Medical Center.
