A newly described fossil is as old as the “first bird,” Archaeopteryx, and represents a birdlike dinosaur that might have specialized in running or wading instead of flying

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edit: further https://www.scientificamerican.com/article/how-birds-evolved...

A remarkable fossil record of the dinosaurs that led to birds reveals how evolution produces entirely new kinds of organisms

This is not 'the truth!!!', it's just science's best shot at explaining. It will be wrong in parts, it could be wrong overall, but it's the best we've got.