Ancient bony fish forces rethink of how sharks evolved

Date:
September 7, 2020
Source:
Imperial College London
Summary:
Sharks' non-bony skeletons were thought to be the template before
bony internal skeletons evolved, but a new fossil discovery
suggests otherwise.



FULL STORY ========================================================================== Sharks' non-bony skeletons were thought to be the template before bony
internal skeletons evolved, but a new fossil discovery suggests otherwise.


==========================================================================
The discovery of a 410-million-year-old fish fossil with a bony skull
suggests the lighter skeletons of sharks may have evolved from bony
ancestors, rather than the other way around.

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.

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.

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.

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.

And if further evidence supports an early evolution of endochondral bone,
it could point to a more interesting history for the evolution of sharks.

Dr Brazeau said: "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."

========================================================================== Story Source: Materials provided by Imperial_College_London. Original
written by Hayley Dunning. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Brazeau, M.D., Giles, S., Dearden, R.P. et al. Endochondral bone
in an
Early Devonian `placoderm' from Mongolia. Nat Ecol Evol, 2020 DOI:
10.1038/s41559-020-01290-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200907112329.htm

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