A 410 million-year-old fossil of an armoured fish skull is expected to turn accepted ideas of sharks’ evolutionary history on its head. Unlike modern-day sharks, the Minjinia turgenensis fish had a skeleton made of bone.
Shark skeletons, with the exception of their teeth, are made entirely out of a softer material called cartilage.
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It was a very unexpected discovery
Dr Martin Brazeau
Dr Martin Brazeau of Imperial College London, the study’s lead author, said: “It was a very unexpected discovery.
“Conventional wisdom says a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us.”
This ancestor to modern-day sharks is likely to have been even larger than today’s apex predator, the great white.
Some descendants of the placoderm (jawed fish) species may have grown to 30ft (10m) or more in length.
However, Minjinia turgenensis is believed to have been significantly smaller, at only a mere one foot (30cm) in length, although it is thought to have made an enormous impact.
The researchers believe this suggests sharks once had bone and then evolved to lose it.
As well as having a bony skeleton, the shark had bony plates over its head and shoulders acting as shields.
Dr Brazeau described this as “extensive armour” as it scoured the ancient seas for prey.
The find boosts the once-controversial theory endochondral bone – which constitutes human skeletons – may have played a significant role in shark evolution.
This likely helped early sharks survive and evolve for more than 400 million years.
Dr Brazeau said: “If sharks had bony skeletons and lost it, it could be an evolutionary adaptation.
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“Sharks don’t have swim bladders, which evolved later in bony fish, but a lighter skeleton would have helped them be more mobile in the water and swim at different depths.
“This may be what helped sharks to be one of the first global fish species, spreading out into oceans around the world 400 million years ago.
“This kind of flips it on its head, because we never expected really for there to be a bony internal skeleton this far down in the evolutionary history of jawed vertebrates.
“This is the type of thing [that suggests] maybe we need to re-think a lot about how all of these different groups evolved.”
Dr Daniel Field, a University of Cambridge vertebrate palaeontologist not involved in the work, is amazed by the research.
He said: “Evolutionary biologists were long guided by the assumption that the simplest explanation – the one that minimised the number of inferred evolutionary changes – was most likely to be correct.
“With more information from the fossil record, we are frequently discovering that evolutionary change has proceeded in more complex directions than we had previously assumed.
“The new work by Brazeau and colleagues suggests the evolution of the cartilaginous skeleton of sharks and their relatives surprisingly arose from a bony ancestor – adding an extra evolutionary step and illustrating that earlier hypotheses were overly simplistic.”
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