It's now believed that many dinosaurs had which feature?
And the answer: feathers.

In recent years, one of the most surprising developments in paleontology has been the discovery that, like modern-day birds, many dinosaurs had feathers. Scientists believe the feathers evolved from reptile-like scales, and that they were initially used to keep the dinosaurs warm.
Though an image of a furry T-rex or Brachiosaurus is a bit jarring, recent studies have revealed that the idea might be closer to reality than previously thought. A group of two-legged carnivorous dinosaurs called theropods seems to have evaded the great dino extinction event 66 million years ago by developing feathers, bigger and more adaptable brains, and smaller, more airborne forms.

However, this idea isn't quite new. Scientists have been commenting on the distinct birdlike appearance of dinosaur fossils and footprints for centuries. Modern technological advances have resurrected the similarities between birds and theropod dinosaurs nearly a century later.
Aside from feathers, researchers have found dinosaurs that display a host of other bird-like traits. For example, recent scans of the insides of dinosaur skulls show that the parts of the brain that control sight, flight, and high-level memory functions were every bit as expanded in theropods as they are in living birds. And researchers have found fossils of small predators protecting their nests of eggs, as well as large dinos with the same hollow bone structures that make birds light enough to fly.
However, the search for answers is far from over. Check out the video below for an in depth explanation of what we know about these massive (feathered) creatures.
