Glossary

An **avian dinosaur** is a member of the clade Dinosauria that belongs to the lineage encompassing modern birds (Aves) and their closest fossil relatives within Avialae. Under phylogenetic taxonomy, birds are not merely descendants of dinosaurs—they are dinosaurs, nested within the theropod suborder as part of Maniraptora, a clade that also includes dromaeosaurids and troodontids. Birds evolved from small feathered theropods during the Late Jurassic, approximately 165–150 million years ago, acquiring a suite of features incrementally over tens of millions of years: feathers, hollow pneumatized bones, a fused clavicle forming the furcula (wishbone), a semi-lunate carpal enabling the wing-folding mechanism, and eventually toothless beaks in more derived lineages. These traits were not acquired simultaneously but were assembled piecemeal, with many features serving non-flight functions before being co-opted for powered flight. The term "avian dinosaur" carries particular significance in the context of the Cretaceous–Paleogene (K-Pg) mass extinction approximately 66 million years ago, when all non-avian dinosaurs perished while certain beaked bird lineages survived. Today, the approximately 10,000–11,000 living bird species represent the sole surviving branch of the dinosaur family tree, making avian dinosaurs the most speciose group of land vertebrates on Earth.

A **feathered dinosaur** is any non-avian dinosaur or early bird for which fossil evidence of feathers or feather-like integumentary structures has been confirmed. The majority of known feathered dinosaurs belong to the Theropoda, particularly within the clade Coelurosauria, although feather-like filamentous structures have also been identified in some ornithischian dinosaurs and pterosaurs. Preserved integumentary structures range across the full evolutionary spectrum of feather morphology, from simple monofilaments to branched downy filaments, symmetrical pennaceous feathers, and asymmetrical flight feathers. Feathers appear to have originated as simple filamentous epidermal structures that served primarily a thermoregulatory (insulation) function, subsequently undergoing adaptive radiation into roles including visual signalling (display and camouflage), egg brooding, and ultimately gliding and powered flight. Since the landmark 1996 discovery of *Sinosauropteryx prima* in the Yixian Formation of Liaoning Province, China, dozens of feathered non-avian dinosaur taxa have been described. These discoveries have decisively corroborated the hypothesis that birds evolved from small theropod dinosaurs, fundamentally transforming the study of dinosaur biology and avian origins.