The ancestors of both mammals and birds became warm-blooded at the same time, some 250 million years ago, in the time of the end-Permian mass extinction, according to new research from the University of Bristol.
The catastrophe killed off nearly 96% of all marine species and 70% of terrestrial vertebrate species on the planet over the course of thousands of years.
Calculations of sea water temperature indicate that at the peak of the extinction, the Earth underwent hot global warming, in which equatorial ocean temperatures exceeded 40 degrees Celsius (104 degrees Fahrenheit).
Among the possible causes of this event, and one of the most long-hypothesized, is that massive burning coal led to catastrophic global warming, which in turn was devastating to life.
Two main groups of tetrapods survived, the synapsids and archosaurs, including ancestors of mammals and birds respectively.
Paleontologists had identified indications of warm-bloodedness (endothermy) in these Triassic survivors, including evidence for a diaphragm and possible whiskers in the synapsids.
More recently, similar evidence for early origin of feathers in dinosaur and bird ancestors has come to light.
In both synapsids and archosaurs of the Triassic, the bone structure shows characteristics of warm-bloodedness.
The evidence that mammal ancestors had hair from the beginning of the Triassic has been suspected for a long time, but the suggestion that archosaurs had feathers from 250 million years ago is new.
But a strong hint for this sudden origin of warm-bloodedness in both synapsids and archosaurs at exactly the time of the Permian-Triassic mass extinction was found in 2009.
In their research, University of Bristol’s Professor Mike Benton and Masters student Tai Kubo analyzed fossilized footprints and found that all medium-sized and large tetrapods switched from sprawling to erect posture right at the Permian-Triassic boundary.
The paleontologists looked at a sample of hundreds of fossil trackways, and they were surprised to see the posture shift happened instantly, not strung out over tens of millions of years, as had been suggested. It also happened in all groups, not just the mammal ancestors or bird ancestors.
“Modern amphibians and reptiles are sprawlers, holding their limbs partly sideways,” Professor Benton said.
“Birds and mammals have erect postures, with the limbs immediately below their bodies. This allows them to run faster, and especially further.”
“There are great advantages in erect posture and warm-bloodedness, but the cost is that endotherms have to eat much more than cold-blooded animals just to fuel their inner temperature control.”
The evidence from posture change and from early origin of hair and feathers, all happening at the same time, suggested this was the beginning of a kind of ‘arms race.’
“The Triassic was a remarkable time in the history of life on Earth. You see birds and mammals everywhere on land today, whereas amphibians and reptiles are often quite hidden,” Professor Benton said.
“This revolution in ecosystems was triggered by the independent origins of endothermy in birds and mammals, but until recently we didn’t realize that these two events might have been coordinated.”
“That happened because only a tiny number of species survived the Permian-Triassic mass extinction — who survived depended on intense competition in a tough world.”
“Because a few of the survivors were already endothermic in a primitive way, all the others had to become endothermic to survive in the new fast-paced world.”
The study was published in the journal Gondwana Research.
Michael J. Benton et al. The origin of endothermy in synapsids and archosaurs and arms races in the Triassic. Gondwana Research, published online September 3, 2020; doi: 10.1016/j.gr.2020.08.003