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It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.The earliest known avialan fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago.This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae, identified four different definitions for the same biological name "Aves", which is a problem.Gauthier proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves.
Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.
As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred.
Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both.
They are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs according to cladistics.
DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages.