An armour of acid-neutralising bone helped four-legged vertebrates to survive when they first crawled out of the ocean, say researchers.
Palaeontologist Dr Christine Janis, from Brown University, and colleagues, report their hypothesis today in the Proceedings of the Royal Society B.
"Nobody knew what this bone was for in these early guys," says Janis. "Now information from the physiology of modern animals helps explain it. I find that very exciting."
Around 360 million years ago, early tetrapods that looked like "clunky crocodiles" started coming onto land.
These creatures had an extensive bony armour including over the roof of their skull and top of their shoulder girdle.
This 'dermal bone' - so-called because it was formed within skin, rather than in cartilage - were covered in ridges and furrows that carried a lot of blood vessels.
They were flexible, not weight-bearing and while some had thought they might be used for protection, their function has been a mystery.
Janis and colleagues now argue this armour played a crucial role in enabling early tetrapods to survive on land.
Clue from modern reptiles
The researchers got their clue from modern crocodiles and turtles, which also have an armour of dermal bone.
Such reptiles are able to stay submerged under water for several hours at a time. In doing so, carbon dioxide builds up in their blood and this can lead to acidosis, acidification of the blood.
Physiological studies have shown that alkaline ions from calcium and magnesium in the dermal bone enter the reptile's blood.
"In living animals this kind of dermal bone can act, when they are under water and can't breathe, to neutralise the acid," says Janis.
She says it makes sense to use the mineral content of dermal bones as a buffer, since unlike other bones they are non-structural.
And the high degree of vascularisation of dermal bones means there is easy exchange between them and body fluids.
Getting rid of CO2 is a challenge on land, especially for the early tetrapods, which unlike humans and reptiles, did not have expandable ribs.
Palaeontological evidence suggests they would have to pump their lungs in a similar way to current-day amphibians - by raising and lowering the floor of their mouth.
"This is fine for getting enough oxygen into the lungs but it's not very efficient for getting rid of CO2," says Janis.
So, for early tetrapods, acidosis would have been a problem when they were on land, rather than in the water, as it is for modern reptiles.
Frogs and other modern amphibians are small enough to get rid of the CO2 through their skin, because they have a large surface area to volume ratio. But early tetrapods would have been too big to do this.
"They wouldn't have been able to get rid of CO2 the way that modern amphibians do. They wouldn't have been able to get rid of CO2 the way that reptiles do. What were they doing?" says Janis.
Janis and colleagues argue that the evidence in modern reptiles helps solve the puzzle: the tetrapods used their dermal bone to buffer acidosis once they crawled onto land.
"Suddenly it now makes sense why these first tetrapods would have had this kind of specialised dermal bone," says Janis.
The researchers hope that they might one day find a chemical or structural signature in the acid-neutralising dermal bone of modern animals that can be also identified in early tetrapod fossils.
But meantime, says Janis, the hypothesis is supported by evidence from the fossil record that shows tetrapods with more dermal bone were more terrestrial than those that had less dermal bone.
Also, tetrapods with expandable ribs had less dermal bone than those with a fixed rib cage.
And finally, she says, tetrapods that were small, more like the size of amphibians that could eliminate CO2 through their skin, had less dermal bone than their larger relatives.