Herrerasaurus ( Herrera's lizard") was one of the oldest and most primitive Theropods, or meat-eating dinosaurs, though in its day it was relatively hyper-advanced. And its body displayed many of the same features of the later theropods. It walked on its hind legs, and its arms ended in powerful clawed hands for grasping prey. Its teeth - like those of most theropods - were shaped like blades and had knife-like serrations running up the front and down the back. Its lower jaw had a special hinge about halfway along its length. This joint would have helped Herrerasaurus to better hold on to struggling victims. Many later theropods also had this hinge.
Although Herrerasaurus shared the basic body design of future rulers of the Earth (like Allosaurus, Giganotosaurus, and Tyrannosaurus Rex), it lived at a time when dinosaurs were not the most powerful predators. Herrerasaurus would have had to run away from the much larger Saurosuchus, a giant land-dwelling crocodile relative, and the even larger Fasolasuchus tenax, which was the largest meat-eater in Argentina during the beginning of the Age of Dinosaurs.