“The Castorocauda lutrasimilis, the largest mammal of its time ever found, is characterized by fur, beaverlike scales on its wide, flat tail and soft tissue webbing between its hind toes. It lived and died about 164 million years ago. …Its discovery, reported in today’s issue of the journal Science, showed for the first time that mammals diversified much earlier than was thought, even in an age dominated by dinosaurs.” – wpost
“Paleontologist Jason A. Lillegraven, a Docodontans specialist retired from the University of Wyoming, noted that the world’s first mammals appear to have arisen about 200 million years ago, with their history paralleling that of the dinosaurs right up to the moment of the dinosaurs’ extinction 65 million years ago.”
By comparison, the oldest known hominid footprints found near Olduvai Gorge in Tanzania are over 3.7 million years old. The earliest known footprints of our species Homo species are about 385,000 years old at most. So, we might now crudely say that 160 million years of evolution could get mammal complexity and size from that of a beaver to a human.