The origin of endotherm in synapsids, including the ancestors of mammals. The diagram shows the development of the main groups through the Triassic, and the scale from blue to red is a measure of the degree of warm-bloodedness, which was reconstructed using various indicators of bone structure and anatomy. Photo credit: Mike Benton, University of Bristol. Animal pictures are from Nobu Tamura, Wikimedia
Mammals and birds are warm blooded today, and this is often seen as the reason for their great success.
Bristol University The palaeontologist Professor Mike Benton identifies himself in the journal Gondwana research that the ancestors of mammals and birds became warm-blooded about 250 million years ago at the same time as life was recovering from the greatest mass extinction of all time.
Der Permian-Trias Mass extinction killed up to 95 percent of life, and the few survivors were faced with a turbulent world repeatedly plagued by global warming and the ocean acidification crisis. Two main groups of Tetrapoden survived the synapsids and archosaurs, including the ancestors of mammals and birds, respectively.
Paleontologists had found evidence of warmblood or technical endothermia in these Triassic survivors, including evidence of a diaphragm and possible whiskers in the synapses.
More recently, similar evidence of the early formation of feathers in ancestors of dinosaurs and birds has emerged. Both in synapses and archosaurs of the Triassic period, the bone structure shows characteristics of warm blood.
Evidence that mammalian ancestors had hair since the beginning of the Triassic has long been suspected, but the suggestion that archosaurs had feathers from 250 million years ago is new.
Attitude shift at the end of the Permian 252 million years ago. Before the crisis, most reptiles were sprawling; Then they walked upright. This could have been the first sign of a new pace of life in the Triassic. Photo credit: Animal drawings by Jim Robins, University of Bristol
Strong evidence of this sudden origin of warm blood in both synapses and archosaurs at the exact time of extinction of the Permian-Triassic mass was found in 2009. Tai Kubo, then a student of paleobiology in Bristol and Professor Benton found that all medium and large tetrapods switched from spreading to upright posture right on the Permian-Triassic border.
Their study was based on fossilized footprints. They looked at a sample of hundreds of fossil traces, and Kubo and Benton were surprised to see that the shift in posture happened instantly rather than strung for tens of millions of years as suggested. It also happened in all groups, not just mammalian or avian ancestors.
Professor Benton said: “Modern amphibians and reptiles are spreaders that hold their limbs partially to the side.
“Birds and mammals have an upright posture with the limbs lying directly under their bodies. This enables them to run faster and, in particular, further. Upright posture and warmblood have great advantages, but the cost is that endotherms need to eat much more than cold-blooded animals just to help their internal temperature control. ”
Evidence of posture change and the early appearance of hair and feathers, all occurring at the same time, suggested that this was the beginning of some sort of “arms race”. In ecology, arms races occur when predators and prey have to compete with one another and when adaptations can escalate. The lion evolves to run faster, but the wildebeest also evolves to run faster or twist and turn to escape.
Something like this happened in the Triassic 250 to 200 million years ago. Warm-blooded animals can now live in cold areas all over the world and remain active at night. They also show intense parental care, feed their babies, and teach them complex and clever behaviors. These adaptations gave birds and mammals the advantage over amphibians and reptiles, and enabled them to dominate more parts of the world in the current cool world.
Professor Benton added, “The Triassic was a remarkable time in the history of life on earth. You can see birds and mammals everywhere on land today, while amphibians and reptiles are often quite hidden.
“This revolution in ecosystems was triggered by the independent origins of endothermics in birds and mammals, but until recently we did not realize that these two events might have been coordinated.
“It happened because only a tiny number of species survived the mass extinction of the Permian Triassic – whoever survived depended on intense competition in a tough world. Since some of the survivors were already primitively endothermic, everyone else had to become endothermic in order to survive in the new fast-paced world. ”
Reference: “The origin of endothermia in synapses and archosaurs as well as in arms races in the Triassic” by Michael J. Benton, September 3, 2020, Gondwana research.
DOI: 10.1016 / j.gr.2020.08.003
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