What groups of birds are more ancient?

Birds and reptiles have a common ancestor – this is a well-known and scientifically proven fact. Moreover, some scientists call the ancestor of the theropod dinosaur (the theory of the runaway appearance of flight in warm-blooded oviparous, i.e. birds), while others lead the line of metamorphosis from achrosauromorphs (arboreal theory). The remains of Archeopteryx, found in the nineteenth century, though remarkably preserved, contributed to the development of these assumptions. In the modern world, however, even more ancient species of bird-like dinosaurs or so-called primitive birds are known – these are anchiornis, Xiaotingia zhengi and aurornis (they all lived in the Jurassic period. birds.