Researchers say that part of an armored fish that swam in the oceans 400 m years ago may turn the evolutionary history of skull sharks on its head.
Bone fish such as salmon and tuna, as well as almost all terrestrial vertebrates from birds to humans, have skeletal skeletons. However, the skeletons of sharks are made from a soft material called cartilage – even in adults.
Researchers have long explained that the last common ancestor of all jaw vertebrae is the inner skeleton of cartilage, and that skeletal skeletons are formed after sharks have already formed. The resulting living vertebrae are divided into “bone vertebrae” and “cartilaginous vertebrae”.
Among other sources for the theory, the remains of early fish show that blackcoderm – creatures with skeletal plates that formed part of the jaw – also contained internal skeletons made of cartilage.
But the startling new discovery raises the theory: Researchers have discovered part of a blockchain made up of skull-roof and brain case.
About 410 m old fossils are reported in the magazine Natural Ecology & Evolution, Discovered in 2012 in western Mongolia and belongs to a blockchain dubbed Minginia turgensensis And would have been about 20-40 cm long.
“This fossil is the most amazing thing I have ever worked on in my life. I never expected to find it,” said Dr. Martin Braso of Imperial College London.
“We know a lot [placoderm] Anatomy and we have hundreds of different species – none of which show this type of bone. ”
The new discovery casts doubt on the idea that sharks branched off from the evolutionary tree of the jawbone before the skeletal skeleton was formed.
“This kind of flips its head because in the evolutionary history of jaw-made vertebrae we never expected it to be a skeletal inner skeleton,” Brace said. “It simply came to our notice then [that suggests] We need to rethink how all of these different groups came to be. “
One possibility is that skeletal skeletons may have formed twice – once for the newly discovered Blackcoderm species and once for the ancestors of all living skeletal vertebrates – the ancestors of sharks and skeletal vertebrae actually having skeletal skeletons, but at some point in their evolutionary history of skeletal formation.
Braso said the new findings add weight to the notion that the last common ancestor of all modern jaw vertebrates did not resemble “some kind of weird shark”, which is often depicted in textbooks. Instead, he said, such an ancestor may resemble a placoderm or primitive bony fish.
Dr. Daniel Field, a backbone archaeologist at the University of Cambridge who did not engage in this work, welcomed the findings. “Evolutionary biologists have been guided for a long time, and the simple explanation – which reduces the number of predicted evolutionary changes – is often correct. With more information from the fossil record, we often find that evolution has progressed in more complex directions than we previously thought,” he said..
“The new creation of Braso and colleagues suggests that the evolution of the cartilage skeleton of sharks and their relatives arose surprisingly from a bone ancestor – adding further evolution explains why previous hypotheses are so simple.”
