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 mesozoic era, while smaller birds survived?

1. The fossil evidence that will relate the dinosaurs and birds could be a transitional form of organism, or body structures that share the same characteristics.

Explanation:

It is generally accepted that the birds are descendants of the dinosaurs, or rather a separate branch of animals that has evolved from the dinosaurs and has remained as their last representatives. The scientists only recently though managed to figure this out, as initially there was no proof of something to connect the two.

The proof came in the form of a fossil of an organism that had characteristics of both the dinosaurs and the birds. Clearly a transitional form, this organism stunned the scientist, as it had wings and features, with the body shape being something in between a reptilian and a bird, but still had a tail and teeth. With the finding of more and more fossils from different periods, it became clear that the birds actually evolved from small, bipedal dinosaurs which tried to avoid competition by occupying the canopies and later flying.

2. The large dinosaurs were too specialized, while the small birds were much more opportunistic when it came to food sources.

Explanation:

As we all know, the dinosaurs are extinct, with only one branch of their family managing to survive until the present, the birds. The dinosaurs were the dominant life form on the planet during their existence, which in turn made them evolve in manner that made them highly specialized, and that turned out to doom them when the impact happened.

When the impact happened, the environment got heavily disturbed and the habitats around the globe changed in matter of hours or days. This left all of the animals that were specialized in particular food source to starve to death, as there is no way they could have evolved in few days to save themselves. On the other hand, the animals that were small and highly opportunistic when it came to food sources, like the small birds and mammals, managed to survive. They were able to feed on variety of different things, smaller animals, insects, roots, plants, so they survived.