University of Bristol palaeontologist Professor Mike Benton has said that the ancestors of both mammals and birds became warm-blooded at the same time, some 250 million years ago.
Around this time, life was recovering from the greatest mass extinction of all time - the Great Dying.
The Great Dying mass extinction killed around 90 per cent of life on Earth and the very few survivors faced a turbulent world, repeatedly hit by global warming and ocean acidification crises.
But two main groups of tetrapods survived - the synapsids and archosaurs, including ancestors of mammals and birds respectively.
Palaeontologists had identified indications of warm-bloodedness, or technically 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 the archosaurs had feathers from 250 million years ago is new.
Professor Benton said: 'Modern amphibians and reptiles are sprawlers, holding their limbs partly sideways.
'Birds and mammals have erect postures, with the limbs immediately below their bodies. This allows them to run faster, and especially further.