The origin and early evolution of feathers: implications, uncertainties and future prospects

As a defining feature of the clade, feathers are key to understanding bird biology. Discoveries of spectacular dinosaur and pterosaur fossils preserving feathers and feather-like integumentary appendages demonstrate trends of increasing complexity in gross morphology and microstructure through avemetatarsalian evolution, and the acquisition of complex flight feathers before the origin of birds. Moreover, this material shows some early feathers differed from modern feathers morphologically, ultrastructurally, biochemically and developmentally, revealing integumentary evolutionary pathways absent in modern taxa. These advances have changed conventional understanding of dinosaurs and impacted conceptions of both birds and feathers. However, it remains unknown if 'true' feathers originated at the base of Avemetatarsalia or within Theropoda. The former scenario implies multiple feather losses, the evolutionary and developmental mechanisms of which require investigation; the latter suggests pterosaurs and ornithischians independently evolved filamentous integumentary appendages, which might have shared genetic regulatory networks with theropod feathers. Answering these questions requires additional data on avemetatarsalian integument, particularly for sauropodomorphs, early diverging theropods and dinosaur outgroups, and more information on those taxa with known integumentary features. An integrative approach combining morphological, developmental, biochemical and taphonomic data, including extinct and extant taxa, is essential for a clearer understanding of feather origin and evolution.