One major change, in the group of reptiles that gave rise to the dinosaurs, was in the animals' posture. This changed from the usual "sprawling" mode, where the limbs jut sideways, to an erect posture, with the limbs held directly under the body. This had major implications for locomotion, as it allowed much more energy-efficient movement.
The dinosaurs , or "terrible lizards", fall into two major groups on the basis of their hip structure: the saurischians (or "lizard-hipped" dinosaurs) and the ornithischians (misleadingly known as the "bird-hipped" dinosaurs). Ornithischians include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus). Saurischians are further subdivided into theropods (such as Coelophysis and Tyrannosaurus rex) and sauropods (e.g. Apatosaurus). Most scientists agree that birds evolved from theropod dinosaurs.
Although the dinosaurs and their immediate ancestors dominated the world's terrestrial ecosystems during the Triassic, mammals continued to evolve during this time.
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