Within the diverse theropoda, one branch, the coelurosaurs, would evolve into our most familiar dinosaurs.
“Coelurosaurs started out as the small critters running around the feet of the sauropods, fearing for their lives,” Persons says. “They had one thing going for them: They were very fast. With long legs, a complex ankle and a stiff spine, they were built for speed. Because they were successful, they got bigger.”
Those bigger coelurosaurs included the tyrannosaurs (starring, yes, the king of them all, T. rex). Another line of coelurosaurs adapted to arboreal living. “They had broad feathers on their limbs and tail, and glided from tree to tree, like flying squirrels,” says Persons. One branch of these gliding dino-squirrels led to scythe-clawed raptors, such as the coyote-sized Velociraptor; the other branch led, eventually, to roadrunners (and all other birds).
Timespan: 165 mya-present