This supports an idea that’s gaining in popularity — the idea that sharks aren’t “primitive,” Science Daily reports.
Because of the fossil skull’s external features, it was always thought to be a type of bony fish (osteichthyan), and this group includes several fishes that we are familiar with, such as cod and tuna, along with all land-dwelling creatures with backbones.
Inside the fossil, a surprise awaited.
Scientists from Oxford University and Imperial College London decided to take a closer look and used X-ray CT scanning to view the inside of the skull, and what they found was fascinating. The structure surrounding the brain was quite similar to cartilaginous fishes (chondrichthyans) such as the sharks and rays.
So this means the skull looked much like a bony fish, but the structures inside were similar to sharks. Hence the name ‘Janusiscus’ named after the Roman two-faced god Janus.
But there was also another surprise:
Dr. Martin Brazeau from the Department of Life Sciences at Imperial College could tell by looking at the fossil that it might provide new insights into the period of evolution.
“I knew straight away that this was a fossil that merited further investigation,” he told The Daily Mail. “Although Janusiscus does have bony skull – which led to it being first classified as a bony fish – it lacked many other features you’d expect in that group.”
Perhaps the most surprising feature is what the skull doesn’t have. In early bony fishes and sharks, there’s a division across the braincase. That’s not the case for Janusiscus.
“This clearly places it at an early stage of evolution, before the two branches split,” he said.
The research has been published in the journal Nature.
“This 415 million-year-old fossil gives us an intriguing glimpse of the ‘Age of Fishes,’ when modern groups of vertebrates were really beginning to take off in an evolutionary sense,’ Dr. Matt Friedman of Oxford University’s Department of Earth Sciences told Science Daily. Dr. Friedman is an author of the report. “It tells us that the ancestral jawed vertebrate probably doesn’t fit into our existing categories.”
Chondrichthyans have long been considered primitive, and viewed as proxies for what these ancestral jawed vertebrates might have looked like. Cartilaginous fishes lack a bony skeleton and this fueled that view, Science Daily reports.
“The results from our analysis help to turn this view on its head: the earliest jawed vertebrates would have looked somewhat more like bony fishes, at least externally, with large dermal plates covering their skulls,” Sam Giles of Oxford University’s Department of Earth Sciences, and the first author of the report, told Science Daily. “In fact, they would have had a mix of what are now viewed as cartilaginous and bony fish-like features, supporting the idea that both groups became independently specialized later in their separate evolutionary histories.”
Dr. Friedman added:
“This mix of features, some reminiscent of bony fishes and other cartilaginous fishes, suggests that humans may have just as many features that you might call ‘primitive’ as sharks.”
Found near the Sida River in Siberia in 1972, the fossil skull is currently at the Institute of Geology at the Tallinn University of Technology, Estonia.
The team used X-ray CT (computed tomography) to ‘virtually’ slice through the fossil, Science Daily reports. Just like an X-ray at a hospital, bones show up brighter than muscles and skin. This process can also be used for fossils, because fossilized bone and rock will also show up differently. Using this technique, the scientists were able to build a 3D model of the fossil, making it easier to examine the skull’s internal and external features in great detail.
Amazingly, even traces left by tiny networks of blood vessels and nerves, which are frequently less than 1/100 of a centimeter in diameter, can be compared to the structure in many jawed vertebrate groups. Including, of course, sharks and bony fishes, Science Daily reports. While the skull was comprised of large, bony plates similar to modern bony fish, these nerves and blood vessels that surrounded this creature’s brain more closely resembled those found in cartilaginous fish, Science AAAS reports.
Based on the findings, it appears that the common ancestor of both branches of these jawed vertebrates had features usually found in bony fish that were, over time, lost in the lineage of cartilaginous fish — like the bony plates of the skull.
The findings back up a study conducted in 2013 that showed several traits previously thought to be unique to bony fish, such as the aforementioned bony plates, were also found in placoderms, an extinct group of jawed fish related to the ancestor of bony and cartilaginous fishes. It also lends credence to a 2014 study that showed that a 325-million-year-old fossil shark had many features typical of bony fishes, and this suggests that the ancestor also had these features and that sharks may be more specialized than we previously knew, Science AAAS reports. It’s these findings as a whole that could correct the erroneous idea that cartilaginous fish are more primitive than bony fish, Dr. Giles said.
“The idea was that the last common ancestor of the two groups would not have had bone, then the bony fish evolved and went on to be more sophisticated,” she said, in an interview with International Business Times.
“This animal shows the last common ancestor of the two groups would have had bone, so if anything, the cartilaginous fishes have become more specialist in losing that bone, whereas the bony fishes retained it from their ancestors.”
There’s a misconception that sharks haven’t done much in the last 400 million years, but the fossil evidence increasingly shows that this isn’t true.
“If you have a look at the latest fossil and even the living things, they’ve developed a huge range of ways of living in the sea and adapting to their environment,” Dr. Giles said, “so they’re no more unevolved or primitive than we are. There’s a huge range of them and just because they’ve stayed in the sea and not moved to land does not mean that they’re primitive in any way.”
She noted that the lineage leading up to humans can be viewed as more primitive than that of the cartilaginous fishes. Both groups are very well evolved, she said.
“Both have taken the body plan of their last common ancestor and specialized it in a multitude of different ways to suit their own environment. [But] sharks lost their skeleton and we kept it from our ancestors.”
“Losing your bony skeleton sounds like a pretty extreme adaptation,” Dr. Friedman said, “But with remarkable discoveries from China, Janusiscus strongly suggests that the ancient ancestors of modern sharks and their kin started out just as ‘bony’ as our own ancestors.”
