Fossils have upheld the theory of how shark skeletons evolved, scientists say. Environment

A partial skull of an armored fish swimming in the ocean 400 meters ago could turn the evolutionary history of sharks on its head, researchers say.

Bone fish, such as salmon and tuna, as well as almost all permanent vertebrate skeletons, from birds to humans, are made of bone. However, the skeletons of sharks are made from a soft material called cartilage – even in adults.

Researchers have long suggested explaining this difference by saying that the last common ancestor of the spinal cord in all responses was the internal skeleton of the cartilage, the skeletons of sharks arising after the sharks had already developed. This development was considered so important that as a result the living vertebrae were divided into “bone vertebrae” and “cartilaginous vertebrae”.

Among other proofs of this theory, the remains of early fish known as placoderms – plated animals with bone armor that also formed a part of the jaw – show that their internal skeletons were made from cartridges.

However, a startling new discovery has flooded the theory: Researchers have found a partial skull-roof of the placoderm made up of bones and the brain.

The 410-meter-old fossil was published in the Journal of Natural Ecology and Evolution in 2012. It was discovered in West Mongolia in 2012 and belongs to a placoderm that has been dubbed. Minjinia targenensis And the length should be about 20-40 cm.

“This fossil is probably the most amazing thing I have ever done in my career. I never expected to find it, ”said Dr. Martin Brazzau of Imperial College London, the first author of the study.

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The Plugoderm fossil was discovered in 2012 in Mongolia.



Placoderm fossils discovered in Mongolia in 2012 Phot Photograph: ERC

“We know a lot about this [placoderm] Anatomy and we have hundreds of different species of them – and we have never seen such bones in them. ”

He said the new discovery casts doubt on the idea that the evolutionary plant of the spinal cord had spread in a frightening response before the inner skeleton had developed into bone.

“This kind of thing goes over the head, because we never expected that there would be such a low internal skeleton in the evolutionary history of the Zayed spine,” Brazio said. “It’s kind of the thing [that suggests] We need to rethink a lot about how all of these different groups evolved. “

Although the team said there was a possibility that skeletal bones could have evolved twice – once the newly discovered Plcoderm species and once the ancestor of all living vertebrates – it is more likely that the ancestor of sharks and vertebrates was actually an unstable skeleton. Was, but at one stage in their evolutionary history the ability to make sharks was lost on sharks.

Brazzau said new research has reinforced the notion that the last common ancestor of all modern jaw spines has not been like the “somehow strange shark”, as is often portrayed in textbooks. Instead, he said, this kind of ancestor probably resembles a placoderm or a primitive bone fish.

Dr. Daniel Field, a versatile paleontologist at the University of Cambridge who was not involved, welcomed the findings. “Evolutionary biologists have long been guided by the idea that simple explanations – which have reduced the number of supposed evolutionary changes – were probably correct. With more information from the fossil record, we often discover that evolutionary change has progressed to a more complex level than we previously thought, “he said..

“The new work by Brazau and colleagues suggests that the evolution of the cartilaginous skeletons of sharks and their relatives was surprisingly from the ancestor of bones – adding an additional evolutionary step and illustrating that previous hypotheses were too simple.”

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