Minjinia turgenensis, evolved before sharks.
On of the great strengths of science, apart from its rigorous fact-checking and peer-review process, is the way scientists constantly reassesses what they thought they knew and change their mind when the facts demand it.
Unlike creationism, there are no sacred truths or dogmas that must never be questioned in science, and the best way to make a name for yourself in science is to overthrow an established consensus, just as Einstein overthrew Newtonian physics and Darwin overthrew the teleological thinking that had dominated biological science.
Creationism, by contrast, have a form of 'Morton's Demon' that filters all information and only allows information that supports and reinforces creationism to enter their world view.
An example of science reassessing the evidence is this paper by an international group of scientists led by Dr. Martin D. Brazeau of the Department of Life Sciences, Imperial College London, Ascot, UK, and the Department of Earth Sciences, Natural History Museum, London, UK. It reassesses the evidence for the origin of sharks, which were assumed to have evolved from the cartilaginous fish that preceded the boney fish, and suggests that they may in fact originate in the bony fish and have since reverted to a cartilaginous skeleton.
The discovery of a 410-million-year-old fish fossil with a bony skull, which is ancestral to both cartilaginous and boney fish, suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.
Sadly, the team's paper in Nature Ecology & Evolution is behind a paywall, but the research is described in an Imperial College new release by Hayley Dunning:
Sharks have skeletons made cartilage, which is around half the density of bone. Cartilaginous skeletons are known to evolve before bony ones, but it was thought that sharks split from other animals on the evolutionary tree before this happened; keeping their cartilaginous skeletons while other fish, and eventually us, went on to evolve bone.The team’s abstract, which is publicly available, give more technical detail:
Now, an international team led by Imperial College London, the Natural History Museum and researchers in Mongolia have discovered a fish fossil with a bony skull that is an ancient cousin of both sharks and animals with bony skeletons. This could suggest the ancestors of sharks first evolved bone and then lost it again, rather than keeping their initial cartilaginous state for more than 400 million years.
The team published their findings today in Nature Ecology & Evolution.
Unexpected discovery
Lead researcher Dr Martin Brazeau, from the Department of Life Sciences at Imperial, said: “It was a very unexpected discovery. Conventional wisdom says that 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.”
Most of the early fossils of fish have been uncovered in Europe, Australia and the USA, but in recent years new finds have been made in China and South America. The team decided to dig in Mongolia, where there are rocks of the right age that have not been searched before.
They uncovered the partial skull, including the brain case, of a 410-million-year-old fish. It is a new species, which they named Minjinia turgenensis, and belongs to a broad group of fish called ‘placoderms’, out of which sharks and all other ‘jawed vertebrates’ – animals with backbones and mobile jaws – evolved.
When we are developing as foetuses, humans and bony vertebrates have skeletons made of cartilage, like sharks, but a key stage in our development is when this is replaced by ‘endochondral’ bone – the hard bone that makes up our skeleton after birth.
Virtual three-dimensional model of the braincase of Minjinia turgenensis generated from CT scan. Inset shows raw scan data showing the spongy endochondral bone inside
Previously, no placoderm had been found with endochondral bone, but the skull fragments of M. turgenensis were “wall-to-wall endochondral”. While the team are cautious not to over-interpret from a single sample, they do have plenty of other material collected from Mongolia to sort through and perhaps find similar early bony fish.
World domination
If further evidence supports an early evolution of endochondral bone, it could point to a more interesting history for the evolution of sharks.
If sharks had bony skeletons and lost it, it could be an evolutionary adaptation. 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.
Dr. Martin D. Brazeau, lead author
Department of Life Sciences
Imperial College London, Ascot, UK.
And the Department of Earth Sciences, Natural History Museum, London, UK.
AbstractWhat creationists will need Morton's Demon to filter out for them, is the fact that, although this work seems to cast doubt on what was thought to be the order of evolution of the two groups of fish, the authors never doubt that they did evolve; the debate is only over the sequence of events, not the underlying mechanisms.
Endochondral bone is the main internal skeletal tissue of nearly all osteichthyans—the group comprising more than 60,000 living species of bony fishes and tetrapods. Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion. Here we report the discovery of extensive endochondral bone in Minjinia turgenensis, a new genus and species of ‘placoderm’-like fish from the Early Devonian (Pragian) of western Mongolia described using X-ray computed microtomography. The fossil consists of a partial skull roof and braincase with anatomical details providing strong evidence of placement in the gnathostome stem group. However, its endochondral space is filled with an extensive network of fine trabeculae resembling the endochondral bone of osteichthyans. Phylogenetic analyses place this new taxon as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.
Brazeau, M.D., Giles, S., Dearden, R.P. et al.
Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia.
Nat Ecol Evol 4, 1477–1484 (2020). https://doi.org/10.1038/s41559-020-01290-2
© 2023 Springer Nature Ltd.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
The other thing they will need their gate-keeper demon to filtered out, is the fact that all this happened on an Earth that wouldn't be created for another 410 million years, according to the creationist mythology.
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