Final answer:
Dinosaurs were a group of reptiles with an upright posture, separate from lizards. They dominated the Mesozoic era and birds are their living descendants.
Step-by-step explanation:
Dinosaurs were a group of reptiles with a distinct upright posture, separate from lizards. They can be described as large, powerful reptiles, and many were very big. Dinosaurs evolved about 230 million years ago and dominated the Mesozoic era, known as the 'Age of Reptiles.' Birds are the only living descendants of one of the major clades of theropod dinosaurs. Some popular dinosaur species include Tyrannosaurus rex, Dilophosaurus, and Velociraptor.