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.
These ancient ancestors must have relied on accessory breathing apparatus. 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.
Read more about our commitment to Open Access. Glenn Tattersall talks about his experiences using a thermal imaging camera in South Africa, the Galapagos Islands, Scotland and Brazil. European nightjars continually change height, sometimes reaching 5km, as they migrate to find the most favourable flight conditions and conserve energy. When we breathe in, the muscle below the rib cage called the diaphragm is pulled down, and air gets sucked into the rib cage, filling the lungs.
Blood cells circulating through tiny blood vessels near the lungs pick up oxygen and carry it around the body to the sites of respiration.
Air is then forced out of the lungs as the diaphragm bows upwards. So do all animals breathe in the same way?
Birds are different from humans in many ways. As you probably know, they fly, and their bodies are well adapted for flight. Their lungs are very efficient: they take in much more oxygen per breath than other animals do. Because they get this extra oxygen they have lots of energy to direct to the flight muscles in their wings - they can flap away for hours!
Frogs and toads have lungs, but when they are in water they can also breathe through their skin. Vertebrates exhibit a multitude of such mechanisms, but the authors were able to exclude each of them except for a diaphragm. And indeed, the closest living relatives of caseids are the mammals. The researchers conclude that already the least common ancestor of caseids and mammals had a diaphragm more than million years ago -- that is about 50 million years earlier than previously assumed.
An efficient respiratory system is intertwined with the evolution of warmbloodedness, which in turn molded our entire behavior. This assumed early origin of the diaphragm now demands a reevaluation of these developments. A fossil diaphragm probably will never be found, simply due to its bad chances to become preserved. Science, according to Prof. Perry, therefore depends on functional approximations to trace the origin of this important evolutionary novelty.
Lambertz: "We still don't know that much about these animals. It has been a long way towards mammals, but the origin of the diaphragm marked a turning point on it. Materials provided by University of Bonn. Note: Content may be edited for style and length. Science News. Sponge-like bones as in osteoporosis Dr. Story Source: Materials provided by University of Bonn. Shelton, Frederik Spindler, Steven F.
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