In numerous ways dinosaurs resembled earlier reptiles. Some, such as tyrannosaurs, had protective scales on their bodies. Others sported plumage, although more fuzzy proto-feather than bird feather. Fluffy coloration arose for mating display. This evolutionary impetus would culminate in a functional form that allowed dinosaur descendants to take wing.
The earliest dinosaurs were herrerasaurids, emerging 245 MYA, in the Late Triassic. Eoraptor is exemplary: a 1-meter omnivore, weighing 10 kg.
A million years later, Eodromaeus appeared as a basal theropod: a 1.2-meter carnivore weighing 5 kg. Most theropods were carnivores, though various theropod groups were insectivores, omnivores, or even herbivores. Almost all theropods were bipedal.
Sauropods showed up shortly after theropods. Sauropods were thick-legged herbivores that walked on all fours, with small heads, elongated necks, and long tails.