The earliest creatures to crawl out of the water onto land may have
concocted antacids out of their own bones, a clever innovation that would’ve
let the animals breathe, researchers now find.
The earliest tetrapods, or four-limbed creatures, made their first
evolutionary forays onto land about 370 million years ago. Breathing air came
with challenges, though. A major one was getting rid of the air’s carbon
dioxide, which, when it builds up, reacts with water in the body and forms an
acid.
Now, growing evidence in modern reptiles suggests that bones that grew
within the skin of early tetrapods may have acted as a natural antacid by
releasing their neutralizing chemicals into the bloodstream. The result would
have bought the creatures time to spend on land before they had to head back to
the water to rid themselves of excess carbon dioxide.
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| The skeleton of Eryops, one of the earliest land-walking tetrapods. | Credit: © Christine M. Janis |
“Now we know that dermal bone can
do this and it’s something we didn’t know before, that gives us a basis that
maybe this is why tetrapods had this feature, which previously we didn’t have a
good explanation for,” study researcher Christine Janis, a paleontologist at
Brown University, told LiveScience. “It’s the discovery of this new feature of
the physiology of these living animals that lets us go back [in time].”
First on land
So let’s rewind the clock: The first tetrapods evolved from fish in the
Devonian period, which spanned from about 416 million years ago to 359 million
years ago. These early tetrapods had broad faces, not unlike frogs, and rather
immobile ribcages. That means they wouldn’t have been able to get rid of extra
carbon dioxide by breathing quickly, as humans and other mammals do with their
longer snouts and flexible ribcages. Nor were the tetrapods small enough to
exchange carbon dioxide and oxygen via their skin, as modern amphibians do.
What tetrapods did have was complex “dermal bone,” or bone that forms
from connective tissue in the skin instead of from cartilage like the long
bones of the arm or leg.The concept of skin bone may seem strange, but it’s
very common: The human skull, for example, is a dermal bone.
Early tetrapod bone showed many pits and furrows, indicating lots of
blood supply, Janis said. Her colleagues, including paper co-author and
biologist Daniel Warren of Saint Louis University, had found another piece of
the puzzle: In modern turtles and alligators, this dermal bone helps the
reptiles tolerate carbon dioxide buildup when they’re under water, unable to
breathe.
Bone breathing
Tetrapods would have the opposite problem, Janis realized: They’d be
able to release carbon dioxide through their skin while in the water, since
their skin was more permeable than an alligator’s tough hide. But out on land,
they’d need another means of release. It seemed very possible that tetrapods
could have used their complex dermal bones as a storage unit for calcium and
other acid-neutralizing minerals, releasing them as needed when body acid levels
got too high, Janis said.
To test the idea, the researchers analyzed the skeletons of tetrapods.
As you might expect, the tetrapods known by the skeletons to spend more time
out of the water had the most complex dermal bones. The evolutionary history of
the animal supports the hypothesis, as well.
“When [the dermal bone] gets lost, it gets lost in the lineage leading
to modern reptiles when they start getting more mobile ribs,” Janis said.
She and her colleagues reported their work Tuesday (April 24) in the
journal Proceedings of the Royal Society B.
End of the early tetrapods
While the evidence is consistent with Janis’ theory, there’s no proof
yet that tetrapods really used their bones in this way. The next step, Janis
said, will be to look for chemical or other clues in modern reptiles who use
their bones as antacid. If any telltale signs are established, researchers can
then hunt for the same signals in ancient tetrapods.
The terrestrial tetrapods studied by Janis and her colleagues went
extinct during the Permian period 299 million to 251 million
years ago. It was a changing world, Janis said, and atmospheric carbon dioxide
was increasing. It’s possible that tetrapods’ bone-dependent breathing wasn’t
as effective in this new atmosphere.
“Who knows?” Janis asked. “I think the point to make is that this was
probably a perfectly good way to live for awhile — millions of years — but in
the end, there were things that had figured out better ways of how to get rid
of carbon dioxide.”
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