What is Extreme?
What's extreme about mammals? How about a four-ton tongue? A tooth bigger than you? A bumblebee-sized bat? Usually we call something "extreme" when it departs significantly from the normal, average, or ancestral condition. But what's normal?
The bizarre-looking Uintatherium was one of the first giant mammals to evolve after the extinction of the large dinosaurs, but it had many normal mammalian features. These included a single lower jawbone and teeth with cusps and ridges for chewing its plant diet. However, Uintatherium's huge size, small brain for its body size, dagger-like teeth, and bony horns are all extreme for mammals.
The opossum, on the other hand, is a fairly typical mammal. It's about average size for a mammal, has four kinds of teeth (incisors, canines, premolars and molars), walks on four legs, and is comfortable both on the ground and in trees--all normal for mammals. Opossums do have some extreme features--like their prehensile tails and grasping thumbs. They are marsupials, carrying their immaturely-born babies in a pouch--which is extreme compared to most mammals but normal for marsupials.
How about you? Are you extreme?
Well, yes and no. On the normal side, we are warm-blooded, have hair, nurse our young and have three middle-ear bones. Like ancestral mammals, we have sharp front teeth and grinding back teeth.
On the extreme side, our brains are remarkably big for our body size. Our thumbs can close against our fingers with both strength and delicacy. Our tail is just a remnant of a few hidden bones, and our body hair is very sparse.
How about walking around on two legs? What might seem to humans like the most normal thing in the world is actually one of our most unusual features. The only other mammals that travel primarily on two legs all hop like kangaroos. Walking human-style is rarer in mammals than laying eggs!
So the terms "extreme" and "normal" are relative--it depends on what you are comparing.