The finding, reported in the journal PLOS one , challenges evolutionary models suggesting animals with skeletons built of cartilage evolved into those with more advanced bony skeletons.
“Fish-like sharks were thought to be primitive because they only had cartilage, and never evolved to develop the bones,” says the study’s lead author, palaeontologist John Long of Flinders University.
“So now we’re turning that idea on its head by saying that early fossil sharks actually had bone in their skeleton and now they’ve lost it.”
The 380-million-year old fossil was unearthed in the Western Australian Kimberley region’s Gogo formation in 2005 by Long and colleagues.
The fossil, named Gogoselachus Lynnbeazleyae, is the first shark to be found in this area.
It remains include both sides of the lower jaw, parts of the shoulder girdle that supported the pectoral fins, several gill arch bones, about 80 teeth, and several hundred scales.
Evolution of Tissue
While modern sharks have a miniscule amount of bone in the roots of their teeth, they do not have any bone in their main skeleton.
Instead, their skeletons are built out of cartilage comprising tiny structures called tesserae, a soft rubbery tissue that is the precursor to bones in other animals.
However, when the authors recently examined the microstructure of the cartilage of Gogoselachus using micro CT scanning, they discovered the matrix holding the cartilage together in the jaw contained bone cells.
“Our fossil shark is the first to show a true cellular matrix made of bone binding the tiny cartilage units together,” says Long.
“We’re looking at a shark which has actually evolved from something that previously had a lot more bone in the skeleton, and eventually modern sharks would lose that bone and just become entirely cartilaginous.”
“So our fossil has given us a window into the evolution of tissues, and the reason sharks are so successful today is because they’ve reduced the bone in their skeleton and become more lightweight with an entirely cartilaginous skeleton,” he says.
Unique in the World
Long says the revolutionary new insights into the early evolution of shark cartilage was only possible because of the degree of preservation achieved by the Gogo formation.
Most fossil fishes from the Devonian period between 360 to 400 million years ago, are squashed flat in rock.
However, fossils in the Gogo formation occur in limestone nodules that preserve them in three dimensional form.
“Gogo is an ancient tropical reef, not a coral reef because they’re relatively recent, but a reef made of algae and sponge-like creatures,” says Long.
“It had a great abundance of life, many kinds of fishes, armoured placoderms which are an extinct group, but also early bony fishes like lung fishes and ray fin fishes, which are the dominate fishes today.
“We expect there should be more sharks there, except for some reason they’re not common on this particular reef where other kinds of fishes dominate.”
Read more at Discovery News
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