As discussed indirectly, more and more evidence is leaning toward the bipedal, carnivorous dinosaurs as the origin of the aven group. In fact, birds may have been a close relative to the Velociraptor. A few skeptics still regard crocodilomorphs, basal archosaurs, or even a variety of non-archosaurian diapsids, as the ancestors. These theories are not as well supported with facts, as they are with the need to hold on to old ideas.
It has been shown in the recent
fossils found in the Liaoning Province of northeastern China that the Late
Jurassic birds are larger than any of the known Early Cretaceous flying
birds. This indicates that those birds may have undergone a size reduction
at about the same time that they developed improved powers of flight. This
may also be related to the development of a keeled sternum and the ability
to land on and take off from level surfaces. Much of the characteristics
of birds were not truly developed and effective in their function of flying
until mid to late Cretaceous. This is seen in the newly-found group Iberomesornis
In fact this is the same time when we get the explosion/diversification
of the aven group.
