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. 2019 Dec 24;15(12):20190622.
doi: 10.1098/rsbl.2019.0622. Epub 2019 Dec 4.

Flight, symmetry and barb angle evolution in the feathers of birds and other dinosaurs

Xia Wang  1 , Ho Kwan Tang  2 , Julia A Clarke  3
Affiliations

Flight, symmetry and barb angle evolution in the feathers of birds and other dinosaurs

Xia Wang et al. Biol Lett. .

Abstract

There has been much discussion over whether basal birds (e.g. Archaeopteryx and Confuciusornis) exhibited active flight. A recent study of barb angles has suggested they likely could not but instead may have exhibited a gliding phase. Pennaceous primary flight feathers were proposed to show significant shifts in barb angle values of relevance to the inference of flight in these extinct taxa. However, evolutionary trends in the evolution of these barb angle traits in extant volant taxa were not analysed in a phylogenetic frame. Neither the ancestral crown avian condition nor the condition in outgroup dinosaurs with symmetrical feathers were assessed. Here, we expand the fossil sample and reanalyse these data in a phylogenetic frame. We show that extant taxa, including strong flyers (e.g. some songbirds), show convergence on trailing barb angles and barb angle asymmetry observed in Mesozoic taxa that were proposed not to be active fliers. Trailing barb angles in these Mesozoic taxa are similar to symmetrical feathers in outgroup dinosaurs, indicating that selective regimes acted to modify primarily the leading-edge barb angles. These trends inform dynamics in feather shape evolution and challenge the notion that barb angle and barb angle ratios in extant birds directly inform the reconstruction of function in extinct stem taxa.

Keywords: barb angle; bird; dinosaurs; evolution; feathers.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Ancestral state reconstruction for leading (a) and trailing (b) vane barb angles. Taxa are coloured with different flight styles. Red, continuous flapping ‘CF’; blue, flapping and soaring ‘FS’; purple, flapping and gliding ‘FG’; green, passerine type flight ‘PT’; brown, flightless ‘FL’; black, Mesozoic fossil taxa. * non-volant taxa. Leading and trailing barb angle values for Aves and Avialae and Caudipteryx are labelled.
Figure 2.
Figure 2.
Ancestral state reconstruction for barb angle asymmetry. Taxa are coloured with different flight styles. Red, continuous flapping ‘CF’; blue, flapping and soaring ‘FS’; purple, flapping and gliding ‘FG’; green, passerine type flight ‘PT’; brown, flightless ‘FL’; black, Mesozoic fossil data. * non-volant taxa. The most parsimonious ancestral asymmetry values are labelled at the split of Caudipteryx, Archaeopteryx, Confuciusornithidae, the base of Aves and Passerines. The lowest values in the flightless Aves (Gallirallus rovianae) and flighted Aves (Junco hyemalis) are labelled. Insets show feathers of Archaeopteryx and crown bird (Hirundo rustica) with leading and training angle measured.
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