The ancestors of modern mammals were also emerging as the first dinosaurs did. They were able to generate their own warmth.
The ability to generate heat from within and keep a constant core body temperature has enabled this diverse class of animal to thrive in a variety of environments.
Evolutionary biologists still don't know when warm-bloodedness, or endothermy, first evolved in animals.
The age of the dinosaurs is thought to have started around 233 million years ago, according to a new study.
The evidence was found in the inner ears of ancient mammals.
It was a logical step after they realized that body temperature affects the runniness of the fluid that gushes around the inner ear.
The inner ear's main function is to help detect head motion, which is necessary for balance, vision, and coordinated movements.
Romain David is a study author and paleontologist specializing in the biomechanics of ear canals at the UK Natural History Museum.
We figured out that we could use them to figure out body temperatures.
In the past, different approaches have been used to figure out when endothermy likely evolved. The studies that tried to link metabolism, oxygen usage, and body hair to body temperatures have yielded conflicting results.
They are fairly confident in their new method of analyzing the size and shape of the inner ear to see if animals ran hot or cold.
The inner ear canals of animals with high body temperatures have had to change shape in order to function.
The structure of the inner ear could be used as a guide.
When the researchers analyzed fossils from a group of 56 extinct species from which mammals emerged, they discovered these ancient animals had smaller canals and narrower ducts than similarly-sized, cold-blooded creatures.
Cold-blooded and warm-blooded animals have their inner ear canals. David and Arajo are related.
The researchers found that the changes to the inner ear structures were sudden and correlated with a rise in body temperature.
Simulations show that endothermy evolved much later than paleontologists had thought, and that it was more rapid than they had thought.
At a time when the climate was cooling fast, these ancestors may have sprouted fur at the same time.
Arajo and colleagues write in their paper thatotherapy is a crucial physiological characteristic.
It was not a gradual, slow process over tens of millions of years as previously thought, but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.
The evolution of warm-bloodedness has been good for birds and mammals, but it is not the sole reason they rose to ecological dominance.
A study published in Nature earlier this year used similar methods to conclude that most dinosaurs were not lizards but mammals.
It seems that the warm-bloodedness of birds and mammals didn't help their ancestors survive the mass extinction of the dinosaurs.
When potential answers to a mystery are found, another plot is added.
The study was published in a journal.
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