An investigation of a 415 million year-old fish skull strongly suggests that the last common ancestor of all jawed vertebrates, including humans, was not very shark-like. It adds further weight to the growing idea that sharks are not ‘primitive’.
Has anyone ever defined what it means for a life form to be “primitive”? How easy is it for any life form to just randomly originate?
Chondrichthyans [cartilaginous fishes, like sharks] have often been viewed as primitive, and treated as proxies for what the ‘ancestral’ jawed vertebrate would have looked like. A key component of this view is the lack of a bony skeleton in cartilaginous fishes.
‘The results from our analysis help to turn this view on its head: the earliest jawed vertebrates would have looked somewhat more like bony fishes, at least externally, with large dermal plates covering their skulls,’ said Sam Giles of Oxford University’s Department of Earth Sciences, first author of the report. ‘In fact, they would have had a mix of what are now viewed as cartilaginous- and bony fish-like features, supporting the idea that both groups became independently specialised later in their separate evolutionary histories.’
So the researchers think that sharks were originally bony, and that the cartilaginous stuff was an adaptation:
The team then used X-ray CT (computed tomography) to ‘virtually’ cut through the fossil. Different materials attenuate X-rays to different amounts — just as in a hospital X-ray, bones show up brighter than muscles and skin. This same principle can be applied to fossils, as fossilised bone and rock attenuate X-rays to different degrees. This technique was used to build a 3D virtual model of the fossil, enabling its internal and external features to be examined in great detail. Traces left by networks of blood vessels and nerves, often less than 1/100th of a centimetre in diameter, could then be compared to structure in a variety of jawed vertebrate groups, including sharks and bony fishes.
‘Losing your bony skeleton sounds like a pretty extreme adaptation,’ said Dr Friedman, ‘but with remarkable discoveries from China, Janusiscus strongly suggests that that the ancient ancestors of modern sharks and their kin started out just as ‘bony’ as our own ancestors.’
So, readers, which multicellular life forms can genuinely claim to be “primitive”? What must a life form do to be “primitive” and still be life?
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