From feathers to flying dinosaurs

The next group to take flight were dinosaurs, a group of archosaurs of whom the only known survivors are birds.

One unique aspect of dinosaur flight is that wing's airfoil is primarily comprised of modified insulatory structures- the feathers. Feathers started out as hollow keratin fibres. Over time, these grew more and more complex, becoming bundles of fibres, then developing unbranched barbs, then barbs and barbules, and finally becoming fully-developed flight feathers. Fully-developed flight feathers are found on birds and on their closest relatives, such as dromaeosaurs. Many of the more distantly related theropods had fibres or bunches of fibres- "protofeathers" as it were. Additionally, filamentous structures were found on some ornithischian dinosaurs. This suggests the precursors of feathers may go all the way back to early basal dinosaurs in the Triassic (Clarke, 2013).

Figure 1: The evolution of feathers. 1- Simple fibres. 2- Bundles of fibres. 3- Unbranched barbs. 4- Barbs and barbules. 5- Fully-developed flight feathers.  Artist: Emily Willoughby.

As with the pterosaurs, a sophisticated system of air sacs evolved in certain dinosaurs, specifically in theropods (except the earliest ones) and in sauropods. The air sac systems of theropods, sauropods and pterosaurs all appear to have evolved independently of one another (Paul, 2010).

Also as with the pterosaurs, gliding does not have appear to have factored into the evolution of flight in the dinosaurs. While it has been proposed dinosaurs like Archaeopteryx and Microraptor were gliders, studies of their skeletal structure show them to have been true flyers. Incidentally, the therizinosaurs may have come from gliding ancestors but rather than evolving flight they, on the contrary, became more ground based (Paul, 2010).

Figure 2: A male Therizinosaurus shows off to an unimpressed female. Therizinosaurs may have come from gliding ancestors but rather than evolving into flying creatures these gliding ancestors evolved into large ground based forms.  Artist: Mark Witton.

How dinosaurian flight first evolved is not completely clear. It is quite possible the precursors of flying dinosaurs were, like the precursors of the pterosaurs, arboreal leapers. Alternatively, flying dinosaurs may have evolved from ground dwelling running forms. These hypothetical ancestors would have run and jumped into the air and flapped their feathery arms, which over time became a more and more sophisticated airfoil structure (Shipman, 1998).

Figure 3: The Ascent of Bird.  Artist: Matthew Martyniuk.

Before moving onto birds, we shall next see some of the non-avian dinosaurs that also evolved flight.

References
Clarke, J. (2013). Feathers Before Flight. Science. 340 (6133), pp. 690-692.
Paul, G. S. (2010). Dinosaurs: A Field Guide. A & C Black Publishers Ltd: London.
Shipman, P. (1998). Taking Wing. Touchstone: Rockefeller Center.

Image sources
Figure 1: Accessed April 7, 2015, from http://fc02.deviantart.net/fs70/i/2012/190/2/1/feather_evolution_by_ewilloughby-d56msug.png
Figure 2: Accessed April 7, 2015, from  https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEijkQFc6zmdlTerZuehDU8dvxlXXzZqH613zK9zC0JVC4RN_2wwYD9Kh4vTapIPyaFCT_QuIVllkU9NU7qCdxLqSHGEGTChXZ-gMzs3i2liqLC7gj66dHxMFNcGKZce7evkuvMr8ZPChK4/s1600/She's+not+interested+2015+Witton+low+res.jpg
Figure 3: Accessed April 7, 2015, from http://fc00.deviantart.net/fs71/i/2013/093/a/7/the_ascent_of_bird_by_mattmart-d5jygt0.png