What is the only characteristic currently known to be unique to all dinosaurs and why is it important

feathers

Explanation:

The characteristic that has been unique for the dinosaurs and not the other groups of animals were the feathers. From the evidence that are obtained it seems that the dinosaurs have developed feathers very early in their evolution, and the feathers remained a common feature among all of the dinosaur branches. Some dinosaurs seems to have lost the feathers because of their lifestyle, but they are less in number. The feathers are a very important feature among the dinosaurs because it means that they are warm blooded animals, endothermic. Being able to control the body temperature independently and not rely on nature for it was definitely a huge advantage for their evolution, as it meant that they can live in pretty much all environments that were present on the planet.