A theropod is a large, carnivorous dinosaur that typically has short, powerful legs and long, clawed arms. Theropods were the dominant group of terrestrial predators during the Mesozoic Era (252 to 66 million years ago), and include some of the most famous dinosaurs, such as Tyrannosaurus rex, Velociraptor, and Allosaurus.
Theropods first appeared during the Late Triassic Period (approximately 231 million years ago), and quickly diversified into a wide variety of body sizes and lifestyles. Early theropods were small (<2 m in length) and lightly built, with long tails that may have been used for balance or for lashing out at prey. Some early theropods, such as Coelophysis and Herrerasaurus, were bipedal (walked on two legs), while others were quadrupeds (walked on four legs). Many early theropods had teeth adapted for slicing flesh, although some - like Dilophosaurus - had teeth more suited for crushing bones. The majority of theropod species became extinct at the end of the Cretaceous Period (approximately 66 million years ago), along with all other non-avian dinosaurs. However, one group of theropods - birds - survived this mass extinction event and continues to thrive today. Birds are thought to be descendants of small, feathered theropods known as maniraptorans; they share many anatomical features with these ancient animals but also possess unique adaptations that allow them to fly.