The lumpy bumpy body armor of the world’s earliest four-legged animals might have served a surprising function -- acid relief.
The study, published in the latest Proceedings of the Royal Society B, could help to explain how some sea creatures managed to transition to a more terrestrial lifestyle circa 370 million years ago. It could also solve the mystery as to why these early animals, called tetrapods, had such odd-looking body armor.
“Dermal bones in the skull roof and on the front of the shoulder girdle are a general feature of bony fishes, as is having scales in the skin with a component of dermal bone,” lead author Christine Janis told Discovery News.
“The issue in these early tetrapods is the highly ornate, ‘sculpture’ appearance of these bones that have been seen as a sort of dermal armor,” added Janis, a professor of ecology and evolutionary biology at Brown University.
The inspiration for the research occurred 10 years ago, when co-author Daniel Warren noticed that modern leopard frogs use their skin region bone to buffer acid buildup. Frogs, turtles and caimans do this too, according to the researchers.
Janis explained that all birds, mammals and reptiles use rib breathing to ventilate their lungs, enabling oxygen intake and carbon dioxide release. Amphibians cannot do this, but they are small enough that they can use their skin to release CO2. Without such systems, animals could fill up with acid.
“If CO2 builds up in the blood it basically goes into solution with the water: H2O plus CO2 equals H2CO3, carbonic acid,” she said.
Fish can lose CO2 over their gills, but gills only work in water. Early tetrapods faced numerous challenges when they ventured on land, probably enticed by food. It would be like humans suddenly needing to develop an ocean-based existence without the benefit of technology.
Most of the early tetrapods, like Eryops (aka “drawn out face”), were too big to solve their CO2 buildup through simple skin release. Their lungs were not well suited to rapid breathing because, unlike humans or even reptiles, their ribs were immobile.
As a result, these first four-legged creatures may have drawn upon their nonstructural bone or other mineral deposits to neutralize acidity. Calcium and magnesium ions, when released into the blood, are what buffer the acid.
“This would likely be a temporary solution, sort of like storing up the acid until you could go back to the water and get rid of it via diffusion there,” Janis said. “But the dermal bone would allow the animals to stay out on land longer than they would otherwise be able to do, so you can see it would have a cumulative adaptive advantage.”
The authors support their theory by pointing out that primarily aquatic tetrapods, such as Whatcheeria, had very little dermal bone sculpture, while the more terrestrial relative Pederpes had more. Another prediction is that tetrapods with expandable ribs would also have less dermal lumps and bumps. That too is borne out in species of gator-like anthracosaurs, which are related to reptiles.
The researchers also show that semi-terrestrial prehistoric species, more closely related to modern amphibians, also had less dermal bone sculpture and could have used their skin to eliminate CO2.
Jason Anderson, an associate professor in the University of Calgary’s Faculty of Veterinary Medicine, told Discovery News that the new theory is “possible, but much more research would need to be done to establish that it even occurs in living animals, as they propose.”
“Their idea really requires a smoking gun -- histological signs of dissolution or remodeling in the osteoderm would be a start, but to my knowledge, such data are lacking,” he added.
The authors believe that a direct test of their theory might be made feasible by identifying a chemical or structural component in dermal bone of today’s animals that use it to neutralize acids with stored minerals, and then looking for that same signature in early tetrapod fossils.