Behold the cynodont.
(Sounds like: sign-uh-daunt.)
Cynodonts gave rise to all living mammals about 250 million years ago.
These are pre-mammalian forms. To evolve from reptiles to mammals, there was a transition from supination to pronation—as Jon Zahourek demonstrates so clearly in this video from 1987.
From reptile to mammals. Via cynodonts.
All mammals stand on pronated hands/forearms—allowing a different form of propulsion.
And it’s all thanks to the supinator muscle.
One more thing about that word cynodont.
It’s derived from Ancient Greek:
‘Cyno’ from kúōn: dog.
And ‘odont’ from kúōn: tooth.
Why? Because cynodonts (who were carnivores) were known from their specialized teeth.