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Abstract

The evolutionary origin of feathers has long been obscured because no morphological antecedents were known to the earliest, structurally modern feathers of Archaeopteryx. It has been proposed that the filamentous integumental appendages on several theropod dinosaurs are primitive feathers; but the homology between these filamentous structures and feathers has been disputed, and two taxa with true feathers (Caudipteryx and Protarchaeopteryx) have been proposed to be flightless birds. Confirmation of the theropod origin of feathers requires documentation of unambiguously feather-like structures in a clearly non-avian theropod. Here we describe our observations of the filamentous integumental appendages of the basal dromaeosaurid dinosaur Sinornithosaurus millenii, which indicate that they are compound structures composed of multiple filaments. Furthermore, these appendages exhibit two types of branching structure that are unique to avian feathers: filaments joined in a basal tuft, and filaments joined at their bases in series along a central filament. Combined with the independent phylogenetic evidence supporting the theropod ancestry of birds, these observations strongly corroborate the hypothesis that the integumental appendages of Sinornithosaurus are homologous with avian feathers. The plesiomorphic feathers of Sinornithosaurus also conform to the predictions of an independent, developmental model of the evolutionary origin of feathers.

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The evolution of dinosaurs.

P. C. SERENO (1999)

The ascendancy of dinosaurs on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous. The dinosaurian radiation, launched by 1-meter-long bipeds, was slower in tempo and more restricted in adaptive scope than that of therian mammals. A notable exception was the evolution of birds from small-bodied predatory dinosaurs, which involved a dramatic decrease in body size. Recurring phylogenetic trends among dinosaurs include, to the contrary, increase in body size. There is no evidence for co-evolution between predators and prey or between herbivores and flowering plants. As the major land masses drifted apart, dinosaurian biogeography was molded more by regional extinction and intercontinental dispersal than by the breakup sequence of Pangaea.

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An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China

Pei-ji Chen, Zhi-Ming Dong, Shuo-nan Zhen (1998)

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Development and evolutionary origin of feathers.

Richard Prum (1999)

Avian feathers are a complex evolutionary novelty characterized by structural diversity and hierarchical development. Here, I propose a functionally neutral model of the origin and evolutionary diversification of bird feathers based on the hierarchical details of feather development. I propose that feathers originated with the evolution of the first feather follicle-a cylindrical epidermal invagination around the base of a dermal papilla. A transition series of follicle and feather morphologies is hypothesized to have evolved through a series of stages of increasing complexity in follicle structure and follicular developmental mechanisms. Follicular evolution proceeded with the origin of the undifferentiated collar (stage I), barb ridges (stage II), helical displacement of barb ridges, barbule plates, and the new barb locus (stage III), differentiation of pennulae of distal and proximal barbules (stage IV), and diversification of barbule structure and the new barb locus position (stage V). The model predicts that the first feather was an undifferentiated cylinder (stage I), which was followed by a tuft of unbranched barbs (stage II). Subsequently, with the origin of the rachis and barbules, the bipinnate feather evolved (stage III), followed then by the pennaceous feather with a closed vane (stage IV) and other structural diversity (stages Va-f). The model is used to evaluate the developmental plausibility of proposed functional theories of the origin of feathers. Early feathers (stages I, II) could have functioned in communication, defense, thermal insulation, or water repellency. Feathers could not have had an aerodynamic function until after bipinnate, closed pennaceous feathers (stage IV) had evolved. The morphology of the integumental structures of the coelurisaurian theropod dinosaurs Sinosauropteryx and Beipiaosaurus are congruent with the model's predictions of the form of early feathers (stage I or II). Additional research is required to examine whether these fossil integumental structures developed from follicles and are homologous with avian feathers. J. Exp. Zool. (Mol. Dev. Evol.) 285:291-306, 1999. Copyright 1999 Wiley-Liss, Inc.