What type of fossil evidence may exist that would lead scientists to believe that dinosaurs were ancestors of birds? why might large dinosaurs have died out at the end of the mesozoic era, while smaller birds survived?

Discovery of ancient fossils of dinosaurs has revealed that some had feathers akin to today’s birds. A renowned example of this feathered dinosaur is the Velociraptor and T. rex. Fossils have shown that these dinosaurs had short forelimbs and well developed powerful hindlimbs. In some discovered fossils, there were complex feathers attached to the forelimbs akin to the wings of birds.

It is believed that the dinosaurs were eliminated by a massive meteor strike. The resulting dust cloud from the impact obscured the sun and stifled plant life on the planet (due to lack of light for photosynthesis). The bigger dinosaurs that required great amount for foraging to maintain their size, starved faster than the little dinosaurs that required less food. Natural selection, therefore, favored same dinosaurs that are believed to have evolved into today’s birds.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.

the areas around the equator