Reptiles and birds are divided into two classes, reptilia and aves. what fossil evidence would be the best support for this division?

The answer is Archaeopteryx.

Archaeopteryx is a bird-like dinosaur. It lived in an archipelago of islands that later formed Europe 150 millions of years ago. It was a raven-size. It had a feather and wing and was able to fly. Despite that, it was more similar to the dinosaurs than the modern birds, with its sharp teeth, claws, and skeleton. Nevertheless, it represents transitional fossil between reptilia and birds.

The zone your looking for is the intertidal zone.