By examining a 415-million-year-old fish skull from Siberia, scientists have found evidence suggesting that the last common ancestor of jawed vertebrates, such as reptiles and humans, were not shark-like.
For the study, which was published in the journal Nature on Jan. 12, Sam Giles from Oxford University and colleagues used a micro-CT scan to examine the 0.8-inch-long skull of a bony fish that was originally found in the 1970s.
The researchers were then able to produce a 3D model of the remains of the ancient animal dubbed Janusiscus schultzei, which was named after the Roman god Janus, who was often depicted as having two faces. It was also named after Hans-Peter Schultze, who first described the fossils nearly 40 years ago.
Present-day jawed vertebrates are either bony or cartilaginous but the examination of the prehistoric brain case of the fish indicates that the fossil has the characteristics of both bony and cartilaginous fish.
The scan shows the presence of sensory line canals on the primitive fish's skull. These canals, which are located outside their body, allow bony fish to monitor pressure changes in their surroundings. This could help them avoid predators.
Interestingly, the skull also has features similar to those of fish made of cartilage. Scans reveal that blood vessels inside the primitive skull of the fish can also be seen in cartilaginous fish.
Giles said that it has long been thought that the common ancestor of the cartilaginous and bony fishes had less bones than cartilage, but what the fossil suggests is that the last common ancestor of both branches of jawed vertebrates was characterized by lots of bones.
The researchers said that both main types of fish had a common ancestor ages ago, but they split about 420 million years ago and that it is possible that the Siberian fossil happens to be actually one of the common ancestors of both the cartilaginous and bony fishes.
"Losing your bony skeleton sounds like a pretty extreme adaptation," said study researcher Matt Friedman from the Oxford University's Department of Earth Science. "But with remarkable discoveries from China, Janusiscus strongly suggests that that the ancient ancestors of modern sharks and their kin started out just as 'bony' as our own ancestors.'"
