This is your ulna right here and this is your radius and what happens in people that require Tommy John surgery is they end up having a rupture of this tendon called the ulnar collateral ligament.
The pivot joint between your radius and ulna allow you to rotate your palm forward or anteriorly in a motion called supination, and turning it backward or posteriorly is called pronation.
But they did have two enlarged muscle attachment areas - one on their humerus and one on their ulna - that are similar to what we see in digging mammals today.