Uintatherium

Saber-toothed mammals evolved many times from non-saber-toothed ancestors. The core of this saber adaptation is an enormously enlarged upper fang shaped like a sword or dagger – flat on the sides and sharp-edged in the front and back. Like many saber-toothed animals, Uintatherium evolved a special chin-guard that protects the fangs when its mouth is closed. To allow the animal to stab and slice with its sabers, the jaw joint lets the mouth open exceptionally wide. And, to increase the deadliness of the bite, the neck muscles are arranged to pull the head down, driving the fang further into the target.