Exploring evolution of feather function in early birds and dinosaurs

Feathers are key innovations that underpin the evolutionary success of birds, and biologists have achieved a solid understanding of modern feather types and their functions. Nonetheless, the unexpected recent discoveries of several specialized feather morphologies in extinct birds and related dinosaurs, challenges our views of the overall evolution of feathers. Such discoveries raise large evolutionary questions in a wider group than simply birds (i.e., dinosaurs as well as pterosaurs). These are related, for instance, to the initial function of feathers, subsequent feather diversification and functions, and potential links between such evolution and external factors. We have differentiated and inventoried fossil feather types based on their general morphological structure, and coded these as traits that relate to feather functions. We analyse the dataset through computational phylogenetic comparative methods, including ancestral state reconstructions, to identify the points of origin for each feature and estimate patterns and rates of evolution. Monofilamentous integumentary structures appear synapomorphic to Avemetatarsalia. A loss of monofilamentous integumentary structures occurred within Pennaraptora. While the presence of pennaceous feathers is synapomorphic for Pennaraptora, the presence of pennaceous feathers on the hindlimbs is a synapomorphy of Paraves. There is greater complexity, however, in feather evolution, with uncertainty over convergence and uniqueness of some feather types not seen in modern birds. The analysis allows some connection from feather morphology evolution to the sequence of regulatory gene switches in modern feather ontogenetic development, but the fossils suggest a richness of evolution not directly seen in studies of feather evo-devo.