Shark 'Missing Link' Reveals Bony Past

Modern sharks are known for their cartilaginous skeletons, but a new study has revealed a surprising "missing link" to more bony ancestors.

The team from Flinders University describes in the journal PLOS ONE a rare fossil discovered in 2005 in Western Australia, dating back to the Devonian Period about 380 million years ago. Detailed CT scanning has now shown that the 3-D skeleton contains trace amounts of calcified bone as well as cartilage.

The fossil is believed to represent a shark caught in a transitional evolutionary stage between a more bony frame and modern-day cartilaginous skeleton. This adds to growing evidence that sharks are less primitive creatures than scientists previously believed.

"Our shark more or less nails that theory, because here we have a heavily mineralized type of cartilage in the skeleton, which contains remnant bone cells," Professor John Long, a paleontologist at Flinders University, said in a blog post. "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."

It was long thought that sharks and rays, because they have a cartilaginous skeleton, were part of a primitive evolutionary pathway, and that more bony fish were the advanced species. But it appears that the opposite may be true.

According to Long, sharks from the Devonian Period are not well known and most research is based on fossilized teeth. Their shark (found in the Gogo Formation in Western Australia, which was once a tropical reef), on the other hand, is rare because of how complete it is, and therefore paints a better picture of modern sharks' evolutionary past.

"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, uncrushed and perfect. It's the first time a shark of that age has been prepared in that manner," Long said.

The authors conclude from their remarkable discovery that sharks may be "more evolutionarily derived," and are likely to be descended from bony ancestors.

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