Lungs provide air-breathing vertebrates with an efficient and often intricate surface for the exchange of oxygen and carbon dioxide. However, they require a mechanism for the air within them to be repeatedly replenished. Animals achieve this in a variety of different ways, but most suck air into the lungs by generating a negative thoracic pressure. The simplest way to do this is to expand the rib cage with intercostal muscles, and many vertebrates do just that. But in mammals, the primary respiratory driver is the diaphragm: a large crescent-shaped muscle that separates the lungs from the abdomen. When it contracts, it draws in air and inflates the lungs. This effective breathing mechanism, partnered with alveolar lungs, equips mammals with a high capacity for oxygen consumption and may have been key to the evolution of high metabolic rates and warm blood. But when the diaphragm first evolved in the mammalian lineage has remained a mystery.
In search of the earliest evidence of a mammalian diaphragm, Markus Lambertz, from the University of Bonn, Germany, and his colleagues examined fossils of caseids – enormous mammal-like reptiles that occupy the earliest branch of the synapsids, the lineage from which mammals ultimately descended. But even if the caseids had diaphragms, there is little chance it would be written in the stone directly, as muscle is too delicate to fossilize. Thus, the authors reconstructed the respiratory paleobiology of these half-tonne behemoths.
Based on the dimensions and morphology of the caseid trunk, Lambertz and colleagues suggest that these animals had a very limited capacity for directly expanding the rib cage. If they were sluggish and had low oxygen demands, they may have just about managed, but if they had oxygen consumption rates similar to those of active modern reptiles, they would have had to pant over 50 times a minute.
However, the team encountered a game changer when they closely analysed the caseid bones. They were spongy and almost osteoporotic. If, as had previously been assumed, these animals were terrestrial, their fragile bones would have been vulnerable to damage. Further, their short necks would have made the menial tasks of eating and drinking difficult. The team therefore suggest that the caseids were predominantly aquatic beasts.
If these aquatic assumptions are correct, the caseid ventilatory insufficiencies would be tremendously exposed. All known diving animals require large ventilation volumes and are characterised by effective ventilation mechanics. If the caseids relied on their limited rib movements alone, they would be left short of breath. In the authors’ own words, ‘the impossible meets the improbable’. These ancient ancestors must have relied on accessory breathing apparatus. The caseids could have ‘swallowed’ air, pushing it from the mouth to the lungs, like frogs, but their mouths are too small to make a meaningful contribution to ventilation. They also could have possessed specialised muscles like turtles, or something completely unique, but this seems far-fetched and lacks evidence. Instead, Lambertz and colleagues suggest the most likely scenario is that these ancient mammal-like reptiles probably had mammal-like diaphragms to help them catch their breath.