IT HAS long been assumed that sharks are one of the more primitive forms of fish.
Most people know that, unlike other fish, sharks have a distinctive internal skeleton made up of cartilage. Now a fossil from Western Australia reveals a surprising 'missing link' to an earlier, more bony form of the fish.
In research published today in the scientific journal PLOS One, Strategic Professor in Palaeontology at Flinders University Professor John Long strengthens the theory that modern sharks are less primitive than previously believed.
Professor John Long with the rare shark fossil found at the Gogo Formation.
The fossil remains were discovered by Professor Long in July 2005 at Gogo in Western Australia. Professor Long says the new fossil was so 3D perfect that he was able to study the cartilage structure with micro-CT scanning analysis, and found that the remnant skeleton had bone cells within its cartilage.
“This means sharks are not as primitive as previously thought, as their ancestors had more bone in their skeleton.”
Professor Long says the fossil, which dates from the Devonian Period (380 million years old), shows that sharks have lost bone through evolution as a specialised condition.
Rough restoration of the shark Gogoselachus based on fossil information.
Because sharks and rays have skeletons that are entirely connected by cartilage, it was traditionally thought that they were a part of a primitive evolutionary pathway, and that bone in other fish was the more advanced condition.
“In humans bone forms over a cartilage frame as we grow, so the absence of bone in sharks was always thought to be primitive, implying they never reached a stage in their evolution where they had bone.”
However, a series of discoveries in recent years have suggested that sharks have evolved, and are more likely to be descended from their bony ancestors.
"Our shark more or less nails that theory, because here we have a heavily mineralised type of cartilage in the skeleton, which contains remnant bone cells," Professor Long said.
"It's almost a missing link condition showing that early sharks had a lot more bone in their skeleton, and that just before modern sharks evolved they lost the bone, with only the soft cartilage remaining"
Cartilage histology showing the units or tesserae (left) and thin section showing bone cells in matrix (right, red line points to bone cell lacunae).
According to Professor Long, the research indicates a direction in their evolution which demonstrates that sharks are much more advanced than has been previously thought.
The Gogo Formation in the Kimberley region of Western Australia is the remains of a tropical reef now situated inland, and has proved to be one of the most important sources of Devonian fossil fish throughout the world.
Sharks are not well known from the Devonian period, as research is largely reliant on fossil teeth. Professor Long says the rarity is what makes the Gogo shark all the more remarkable.
Gogoselachus teeth including CT image on the far right.
"This is a partial articulated skeleton, with the jaws and shoulder and all the teeth and scales, but best of all, we have acid-etched the fossils out of the rock, so they are three-dimensional, and uncrushed and perfect," he said. "It's the first time a shark of that age has been prepared in that manner."
The discovery also represented a breakthrough as it was the first ever shark fossil found in 60 years of expeditions collecting from the Gogo site.
“I felt very triumphant the day I found it,”
“It means that we can go back and find more sharks with continued collecting,” says Professor Long, who will be returning to the site later in the year.
Above: The shark fossil as it was found. Below: Shark fossil cartilages coming out of the rock with acid preparation.