Avialae

Avialae is a clade of theropod dinosaurs comprising the last common ancestor of Archaeopteryx and modern birds and all of its descendants. In many cladistic schemes, Avialae is the broader group that predates the crown group Aves, so birds are nested within Avialae. The exact content of Avialae varies among authors, but it generally includes both extinct avialans and living birds.

Among its members are Archaeopteryx, ichthyornitheans, hesperornithines, enantiornitheans, and ornithuromorphs. The clade also contains all living birds (Aves), which form the crown group within Avialae.

Anatomically, avialans show features associated with flight, including feathers and wing bones. Some early avialans were capable of powered flight, while others may have relied on climbing or gliding. Teeth were common in many early avialans, but later members and birds generally lacked teeth.

Fossil record and time: Avialae originated in the Middle to Late Jurassic, roughly 160 to 150 million years ago. Archaeopteryx from Europe is among the earliest members. During the Cretaceous, avialans diversified into major lineages such as enantiornithines and ornithuromorphs; following the mass extinction at the end of the Cretaceous, only Aves survived.

Nomenclature and phylogeny: The use of Avialae is sometimes debated, with some sources treating Avialae as including Aves and others using Avialae as the stem lineage to Aves. Regardless, the clade provides a framework for understanding the evolution of birds and their extinct relatives.