According to new research from the University of Bristol, the ancestors of both mammals and birds became warm-blooded at the same time, about 250 million years ago, at a time of mass extinction in late Perm.
The ultimate Permian extinction, also known as the Permian-Triassic extinction and the Great Death, is the worst mass extinction on Earth, peaking about 252.3 million years ago.
The disaster wiped out almost 96% of all marine species and 70% of terrestrial vertebrate species on the planet over thousands of years.
Calculations of seawater temperature show that at the peak of extinction, the Earth experienced hot global warming, in which the equatorial ocean temperature exceeded 40 degrees Celsius (104 degrees Fahrenheit).
Among the possible causes of this event, and one of the longest-running hypotheses, is that mass coal burning has led to catastrophic global warming, which in turn has been devastating to life.
Two main groups of tetrapods survived – synapsids and archosaurs, including the ancestors of mammals and birds, respectively.
Paleontologists have found signs of warm-bloodedness (endothermia) in survivors of the Triassic age, including data on the diaphragm and possible whiskers in synapses.
More recently, similar evidence has emerged for the early origin of feathers in the ancestors of dinosaurs and birds.
Both in synapsids and in Triassic archosaurs, the bone structure shows the characteristics of warm-bloodedness.
Evidence that mammalian ancestors had hair from the early Triassic has long been suspected, but the assumption that archosaurs had feathers 250 million years ago is new.
But a strong hint of this sudden origin of warm-bloodedness in both synapsids and archosaurs at the time of the Permian-Triassic mass extinction was found in 2009.
In their research, Mike Benton, a professor at the University of Bristol, and Tai Kubo, a graduate student, analyzed the fossilized footprints and found that all medium and large tetrapods moved from a stretched to an upright posture right on the border of the Permian and Triassic.
Paleontologists have examined a sample of hundreds of fossil trails, and they were surprised to see that the change in posture occurred instantly, rather than stretching for tens of millions of years, as suggested. This also happened in all groups, not just the ancestors of mammals or birds.
“Modern amphibians and reptiles are stretched, keeping the limbs partially to the side,” said Professor Benton.
“Birds and mammals have upright postures, the limbs are directly under their bodies. This allows them to run faster, and especially further.
“There are great benefits to being upright and warm-blooded, but the price is that endotherms have to eat much more than cold-blooded animals, just to strengthen their internal temperature control.”
Evidence of a change in posture and the early origin of hair and feathers, which occurred simultaneously, suggests that this is the beginning of a kind of “arms race.”
“The Triassic was a wonderful time in the history of life on Earth. Today, you see birds and mammals on land, while amphibians and reptiles are often quite hidden, ”said Professor Benton.
“This revolution in ecosystems was caused by the independent origin of endothermy in birds and mammals, but until recently we did not understand that these two events could be coordinated.”
“This is because only a small number of species have survived the Permian-Triassic mass extinction – the survivors depended on intense competition in a tough world.”
“Because some of the survivors were already endothermic in a primitive way, everyone else had to become endothermic to survive in the new fast-paced world.”
The study was published in the journal Research of Gondwana.
Michael J. Benton etc. The origin of endothermy in synapsids and archosaurs and the Triassic arms race. Research of Gondwana, published online September 3, 2020; doi: 10.1016 / j.gr.2020.08.003