Before revision: In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolutionary history, starting with their origin 225 million years ago, and only ending with the demise of the non-avian dinosaurs 66 million years ago .Gerkema MP, Davies WI, Foster RG, Menaker M, Hut RA.
After revision: In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolutionary history, from their origin to after the Cretaceous–Paleogene extinction event, .Gerkema MP, Davies WI, Foster RG, Menaker M, Hut RA.