Mammalian ancestors became warm-blooded some 20 million years later than previously estimated, according to European scientists who analyzed inner ear fossils to solve "one of the greatest mysteries of palaeontology”.
https://ca.sports.yahoo.com/news/scientists-reveal-origin-mammal-evolution-164538839.html
Mammaliomorphs are the ancestors of mammals. They would have been the first to be hot-blooded.
PHOTO: UNIVERSITY OF LISBON/LUZIA SOARES