The ancient reptile Thrinaxodon, an ancestor of mammals, had skull features suggesting that it had sensory whiskers  If Thrinaxodon had whiskers, it clearly also had hair on other parts of its body, which would have served as insulation that regulated body temperature
Ttherefore, Thrinaxodon was probably warm-blooded, for such insulation would be of little use to a cold-blooded animal
It is offered as an explanation of the phenomenon described by the argument's main conclusion, but it is not itself used to provide support for that conclusion.