Some Theropods, such as this one, were smaller (about the height of a German Shepherd) carnivores that predated today's birds. Their smaller size allowed them to explore alternate natural resources, habitats, and even prey. As the Theropods decreased in physical size, they became more and more able to chase insects, climb trees, leap and glide, and eventually develop powered flight.

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From the gallery of "Dinosaurs Revealed": https://pbase.com/britestar/dinosaurs_revealed_journey_across_america_