Scientists trace the origin of our teeth from the most primitive jawed
fish

Date:
July 9, 2020
Source:
European Synchrotron Radiation Facility
Summary:
Scientists have digitally 'dissected', for the first time, the
most primitive jawed fish fossils with teeth found near Prague
more than 100 years ago. The results show that their teeth have
surprisingly modern features.



FULL STORY ==========================================================================
The origin of our teeth goes back more than 400 million years back in
time, to the period when strange armoured fish first developed jaws and
began to catch live prey. We are the descendants of these fish, as are
all the other 60,000 living species of jawed vertebrates -- sharks, bony
fish, amphibians, reptiles, birds and mammals. An international team of scientists led by Uppsala University (Sweden), in collaboration with the
ESRF, the European Synchrotron (France), the brightest X-ray source, has digitally 'dissected', for the first time, the most primitive jawed fish fossils with teeth found near Prague more than 100 years ago. The results, published today in Science, show that their teeth have surprisingly
modern features.


========================================================================== Teeth in current jawed vertebrates reveal some consistent patterns:
for example, new teeth usually develop on the inner side of the old
ones and then move outwards to replace them (in humans this pattern has
been modified so that new teeth develop below the old ones, deep inside
the jawbone). There are, however, several differences between bony fish
(and their descendants the land animals) and sharks; for example the fact
that sharks have no bones at all, their skeleton is made of cartilage,
and neither the dentine scales nor the true teeth in the mouth attach
to it; they simply sit in the skin. In bony fish and land animals, the
teeth are always attached to jaw bones. In addition, whilst sharks shed
their worn-out teeth entire, simply by detaching them from the skin,
bony fish and land animals shed theirs by dissolving away the tooth bases.

This diversity raises many questions about the origin of teeth. Until
now, researchers have focused on fossils of a group of ancient fish that
lived about 430 to 360 million years ago, called the arthrodires, which
were the only stem jawed vertebrates in which teeth were known. However,
they struggled to understand how they could have evolved into the teeth
of modern vertebrates, as arthrodire teeth are so different in position
and mode of tooth addition in comparison to bony fish and sharks.

Scanning the most primitive jawed fishes A team from Uppsala University, Charles University (Czech Republic), Natural History Museum in London
(UK), National Museum in Prague (Czech Republic) and the ESRF, the
European Synchrotron (France) set out to determine whether this peculiar
type of dentition was really ancestral to ours, or just a specialised
offshoot off the lineage leading towards modern jawed vertebrates.

With this aim, they turned to the acanthothoracids, another early fish
group that are believed to be more primitive than the arthrodires and
closely related to the very first jawed vertebrates. The problem with acanthothoracids is that their fossils are rare and always incomplete. The
very finest of them come from the Prague Basin in the Czech Republic,
from rocks that are just over 400 million years old, and were collected
at the turn of the last century. They have proved difficult to study
by conventional techniques because the bones cannot be freed from the
enclosing rock, and have therefore never been investigated in detail.



==========================================================================
The researchers used the unique properties of the ESRF, the world's
brightest X-ray source and the synchrotron microtomography ID19's
beamline, to visualise the internal structure of the fossils in 3D without damaging them. At the ESRF, an 844 metre-ring of electrons travelling
at the speed of light emits high- powered X-ray beams that can be used
to non-destructively scan matter, including fossils.

"The results were truly remarkable, including well-preserved dentitions
that nobody expected to be there" says Vale'ria Vaskaninova', lead author
of the study and scientist from Uppsala University. Follow-up scans at
higher resolution allowed the researchers to visualize the growth pattern
and even the perfectly preserved cell spaces inside the dentine of these ancient teeth.

Like arthrodires, the acanthothoracid dentitions are attached
to bones. This indicates that bony fish and land animals retain the
ancestral condition in this regard, whereas sharks are specialized in
having teeth that are only attached to the skin -- in contrast to the
common perception that sharks are primitive living vertebrates. Again,
like arthrodires, the teeth of acanthothoracids were not shed.

More different from arthrodires than expected In other ways, however, acanthothoracid dentitions are fundamentally different from those of arthrodires. Like sharks, bony fish and land animals, acanthothoracids
only added new teeth on the inside; the oldest teeth were located right
at the jaw margin. In this respect, the acanthothoracid dentitions look remarkably modern.

"To our surprise, the teeth perfectly matched our expectations of a common ancestral dentition for cartilaginous and bony vertebrates." explains Vaskaninova'.

The tooth-bearing bones also carry small non-biting dentine elements of
the skin on their outer surfaces, a character shared with primitive bony
fish but not with arthrodires. This is an important difference because
it shows that acanthothoracid jaw bones were located right at the edge
of the mouth, whereas arthrodire jaw bones lay further in. Uniquely,
one acanthothoracid (Kosoraspis) shows a gradual shape transition from
these dentine elements to the neighboring true teeth, while another
(Radotina) has true teeth almost identical to its skin dentine elements
in shape. This may be evidence that the true teeth had only recently
evolved from dentine elements on the skin.

"These findings change our whole understanding of the origin of teeth"
says co- author Per Ahlberg, professor at Uppsala University. And he adds: "Even though acanthothoracids are among the most primitive of all jawed vertebrates, their teeth are in some ways far more like modern ones than arthrodire dentitions.

Their jawbones resemble those of bony fish and seem to be directly
ancestral to our own. When you grin at the bathroom mirror in the morning,
the teeth that grin back at you can trace their origins right back to
the first jawed vertebrates."

========================================================================== Story Source: Materials provided by
European_Synchrotron_Radiation_Facility. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Vale'ria Vaskaninova', Donglei Chen, Paul Tafforeau, Zerina
Johanson,
Boris Ekrt, Henning Blom, Per Erik Ahlberg. Marginal dentition
and multiple dermal jawbones as the ancestral condition of jawed
vertebrates.

Science, 2020 DOI: 10.1126/science.aaz9431 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200709141606.htm

--- up 24 weeks, 2 days, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)