Most vertebrates engage in combat either over territory, mates, or food. It's not surprising that dinosaurs did, too, especially since we can observe combat in the closest living relatives of extinct dinosaurs, crocodiles and birds.
One 65-million-year-old skeleton of Tyrannosaurus rex discovered in South Dakota has telltale battle scars. One eye socket in the skull is nearly closed by abnormal healing of the surrounding bone. On another part of the skull, the broken tip of a tooth from another tyrannosaur in embedded. Around this tooth are the characteristic marks of bone growth that occurs during healing, indicating that the animal survived the attack.