Researchers say the partial skull of an armored fish that swam in the oceans more than 400 million years ago could turn the evolutionary history of sharks on its head.
Bony fish, such as salmon and tuna, as well as nearly all terrestrial vertebrates, from birds to humans, have skeletons that ultimately consist of bones. However, shark skeletons are made of a softer material called cartilage – even in adults.
Researchers have long explained the difference by suggesting that the last common ancestor of all jawed vertebrates had an internal skeleton of cartilage, with the skeletons emerging after sharks actually evolved. Evolution was thought to be so important that living vertebrates split into “bone vertebrates” and “chondrocytes” as a result.
Among other evidence for this theory, the remains of early fish called placoderms – creatures with bony armor plates that also form part of their jaws – show that they had internal skeletons made of cartilage.
But a startling new discovery has turned this theory on its head: Researchers have found a partial cap in the skull and a brain state of a blackhead made of bone.
The fossil, which is about 410 million years old and reported in Nature Ecology & Evolution, was discovered in western Mongolia in 2012 and belongs to the layer of skin that it is called Minjinia turgenensis Its length was about 20-40 cm.
“This fossil is perhaps the most surprising thing that I have worked on in my career.” Dr Martin Brazo of Imperial College London, first author of the research, said, “I never expected to find this.
“We know a lot about [placoderm] Anatomy and we have hundreds of different types of these things – none of them showed this type of bone. ”
He said the new discovery casts doubt on the notion that sharks branched out from the evolutionary tree of jawed vertebrates before the internal skeleton developed.
“This kind of flips it upside down,” said Brazio, “because we didn’t really expect there to be this large internal skeleton in the evolutionary history of jawed vertebrates.” “That’s the kind of thing [that suggests] We might need to rethink a lot about how all these different groups evolved. “
While the team says one possibility is that the skeletons could have evolved twice – once giving rise to newly discovered platelet species and once to the ancestors of all living bony vertebrates – the most likely possibility is that it was the ancestor of sharks and bony vertebrates. In fact, they had a skeleton, but at some point in their evolutionary history, sharks lost the ability to make bones.
Brazzo said the new findings add weight to the idea that the last common ancestor of all modern-jawed vertebrates does not resemble “some kind of exotic shark,” as it is described in the textbooks. Instead, he said, such an ancestor likely resembled a primitive tablet or bony fish.
Dr Daniel Field, a Cambridge University vertebrate paleontologist who was not involved in the work, welcomed the results. “Evolutionary biologists have long been guided by the assumption that the simplest explanation – the one that minimized the number of inferred evolutionary changes – was likely correct. With more information from the fossil record, we discover repeatedly that evolutionary change has gone in more complex directions than we had assumed. Previously , “.
“New work by Brazeau and colleagues indicates that the evolution of the cartilaginous skeleton of sharks and their relatives surprisingly arose from a bony ancestor – adding an additional evolutionary step and demonstrating that previous hypotheses were overly simplistic.”
