Phylogenetic Context for the Origin of Feathers1

Abstract

A number of hypotheses have been suggested for the origin of birds and feathers. Although distributions of functional complexes have frequently been used to test phylogenetic hypotheses, analysis of the origin of feathers remains hampered by the incomplete fossil record of these unmineralized structures. It is also complicated by approaches that confuse the origins of birds, feathers, and flight without first demonstrating that these relate to the same historical event. Functional speculation regarding the origin of feathers usually focuses on three possible alternatives: (1) flight; (2) thermal insulation; or (3) display. Recent fossil finds of Late Cretaceous feathered dinosaurs in China have demonstrated that feathers appear to have originated in taxa that retained a significant number of primitive nonavian features. Current evidence strongly suggests that birds are theropod dinosaurs, and that the most primitive known feathers are found on non-flying animals. This further suggests that feathers did not evolve as flight structures. Thermoregulatory, display, and biomechanical support functions remain possible explanations for the origin of feathers. As the earliest function of feathers was probably not for aerial locomotion, it may be speculated that the transitional animals represented by the Chinese fossils possessed skin with the tensile properties of reptiles and combined it with the apomorphic characteristics of feathers.