Ancient shark ancestor may have been bony, rare fossil reveals

A 380-million-year-old fossil shark discovered in Western Australia reveals that, contrary to earlier understanding of the ancient sea predators, sharks' ancestors may have had bones.

John Long, a professor of paleontology at the University of Flinders, found the fossil in July of 2005. He describes the moment as "euphoric," citing the rarity of finding a shark fossil in the region. 

"Finding a shark at Gogo has been a bit of a holy grail for fish paleontologists as we all expected a shark from this site would have extraordinarily good preservation. It should reveal something new about early shark evolution, as nearly all other sharks of this age were flattened and poorly preserved," he writes in a blog post. 

Long's findings were recently published on PLOS One, an online scientific journal.

According to the study, the fossil contained the complete lower jaw as well as some teeth, scales, and pieces of the shoulder girdles, found embedded in sedimentary rock, a rarity, as there are very few known shark remains from this period

Scientists removed the specimen from its original limestone with acetic acid. Eventually, they were able to reassemble the lower jaw. 

 Most notably, Long found that the fossil contained vestigial bone cells amidst its cartilage skeleton.

The fossil, say the researchers, presents a species in transition. Like modern sharks, the ancient specimen had a cartilaginous skeleton, but researchers were surprised to find that the matrix holding together  cartilage units contained bone cells, suggesting that a transition was underway from a bony structure to a completely cartilaginous skeleton. 

"This implied that sharks most likely evolved from ancestors that had much more bone in the skeleton. The evolution of modern sharks was driven by their loss of bone, which suggested they are not as primitive as previously thought," Long writes. The transition to a lighter cartilage structure most likely allowed the fish to swim more quickly.

As Long told Discovery News, “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.”