Researchers have suggested that a partial skull of an armored fish floating in the ocean 100 years ago could turn the history of shark development on its head.
Bone fish, such as salmon and tuna, as well as almost all terrestrial vertebrates, from birds to humans, have skeletons made up of bones. However, shark skeletons are made from a soft material called cartilage – even in adults.
The researchers have long explained the difference that the last common ancestor of all jaw vertebrae was the inner skeleton of cartilage, the skeletal skeleton emerging after the shark had already developed. Development was considered so important that as a result living vertebrates are divided into “bone vertebrae” and “cartilaginous vertebrates”.
In other evidence for the theory, the remains of fish known as placoderms – creatures with bone armor plates, which also form part of the jaw – show that they had internal skeletons made of cartilage.
But a surprising new discovery has disproved the theory: researchers have discovered a case of partial skull-roof and brain of bone placoderm.
About 410 mm old and reported in the Journal of Natural Ecology and Evolution, the fossil was discovered in western Mongolia in 2012, and belongs to a placoderm that has been dubbed. Minginia tergenesis And its length is about 20-40 cm.
“This relic is probably the most amazing thing I’ve ever done in my career. “I never expected to meet this,” said Dr. Mart, of Imperial College London, the first author of the research. Said Martin Brezau.
“We know a lot about this [placoderm] The anatomy and we have hundreds of different species of these things – and none of them have ever shown this type of bone. ”
He said the new findings cast doubt on the idea that sharks cut down the tree of development of the spinal cord of the jaw before the inner skeleton of the bone develops.
“This kind of flips it over his head, because we never expected an internal skeleton of the bottom bone in the evolutionary history of Javad vertebrates.” “This is kind of the thing [that suggests] Maybe we need to think a lot about how all these different groups evolved. “
While the team says one possibility is that the Bonnie skeleton could have evolved twice – once giving birth to a newly discovered placoderm species and once the ancestor of all living skeletal vertebrates – more likely the ancestor of sharks and skeletal vertebrae was actually a skeletal skeleton. But at some point in their evolutionary history, sharks lost their ability to make bones.
Brezau said the new findings add weight to the idea that the last common ancestor of all modern jaw vertebrates does not resemble “some kind of supernatural shark”, as is often shown in textbooks. Instead, he said, such ancestors probably resembled a placoderm or a primitive bony fish.
Vertebrate paleontologist at Cambridge University, Dr. Daniel Field, who was not involved in the work, welcomed the findings. “Evolved biologists have long been guided by the assumption that simple explanations – one that reduces the number of predicted evolutionary changes – have true potential. With more information from the fossil record, we often find that evolutionary change has progressed in more complex directions than we expected..
“The new work by Brezau and colleagues suggests that the cartilaginous skeleton of sharks and their relatives evolved surprisingly through a single bone ancestor – adding an additional evolutionary step and showing that previous hypotheses were much simpler.”
