Are birds just dinosaurs? The surprising history of the Avialae clade

Nature · Published 3 October 2026 Swipe more like this →

In short: The line between dinosaur and bird is thinner than you think. Avialae, the group containing modern birds and their closest extinct relatives, reveals that flight was not a sudden invention but a slow, messy evolution. From feathered Jurassic fossils to the bizarre creatures of the Cretaceous, our understanding is constantly shifting.

Avialae, or bird-winged dinosaurs, represents the clade that includes modern birds and the theropods most closely related to them. While the famous Archaeopteryx from the Late Jurassic of Germany was long considered the definitive 'bird,' its status has become increasingly complicated. Discovered in the 1860s and 1870s, it was the first fossil skeleton found with feathers, cementing its place in history. However, as paleontologists have unearthed a wealth of new specimens in China, the 'birdiness' of Archaeopteryx has been debated. Some studies now place it within the Anchiornithidae, a group of small theropods that includes Anchiornis, which is known from thousands of specimens and exhibits remarkable soft-tissue preservation.

The evolution of flight within this group was not a straightforward progression. Despite artistic reconstructions, evidence from shoulder joint orientation suggests that basal avialians were likely poor at powered flight compared to dromaeosaurids like Microraptor. Instead, these early forms often possessed features that would later be lost, such as long feathers on their hind limbs—sometimes called 'hind wings'—which may have aided in aerial maneuvering. During the Cretaceous, avialians diversified significantly. While early forms like Jeholornis retained long, bony tails, more advanced groups evolved the pygostyle, a fused tail bone that characterizes the Pygostylia clade.

Taxonomy remains a point of intense contention. Researchers frequently disagree on where to draw the boundaries of 'Aves.' Some follow a traditional usage where Aves includes Archaeopteryx, while others restrict the term to Neornithes, the crown group containing only the last common ancestor of all living birds and its descendants. This confusion is compounded by the fact that some Late Cretaceous creatures, like the double-sickle-clawed Balaur from Transylvania, appear to be 'extinct living fossils'—anatomically primitive forms that survived long after more advanced avian lineages had already begun to flourish. Ultimately, Avialae serves as a reminder that the transition from ground-dwelling theropod to feathered flyer was a complex, multi-step process.

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