The origin and diversification of birds

Birds are a group of deinonychosaurs who are unique in being the only dinosaurs known to have survived to the present day.

The first bird is traditionally thought to have been Archaeopteryx, which lived in what is now Germany around 150 million years ago. While a somewhat poor flyer it could nevertheless fly. It may also have been able to use its wings to climb and to swim (Paul, 2010). Archaeopteryx's status of "first bird" has however been challenged by China's Aurornis, thought to have lived around 160 million years ago. As well as potentially revising knowledge of bird origins, Aurornis has also added to phylogenetic data suggesting birds are actually most closely related to troodonts rather than to dromaeosaurs (Godefroit et al, 2013).

Figure 1: Archaeopteryx (left) and Aurornis (right) arguing over who gets the title of "first bird".  Artist: Emily Willoughby.

Archaeopteryx and Aurornis still had long tails and lacked such avian features as a beak and a sternal keel (like non-avian flying dinosaurs they used deltoid muscles instead). The first known tailless, beaked, sternal keel possessing bird was Confuciusornis, who lived in China around 142-121 million years ago. Though lacking tails, male Confuciusornis possessed long tail feathers for display, like many birds today (Shipman, 1998).

Figure 2: A male Confuciusornis perches on a tree branch. Though it still had wing claws, Confuciusornis was recognisably avian.  Artist: Kevin Yan.

Confuciusornis belonged to an extinct group of birds called the enantiorniths. However, most known birds, including all modern birds and many extinct groups, belong to another group called the ornithurines, of which the oldest known member is China's Liaoningornis, which was contemporary with Confuciusornis. Interestingly, Liaoningornis and other early ornithurines had toothed jaws. All modern birds belong to a group of toothless ornithurines called the neorniths. The earliest known neornith groups, present by the late Cretaceous, were things like ratites, water fowl, loons, and wading shorebirds (Shipman, 1998). There is also possible fossil evidence of late Cretaceous parrots and possible molecular evidence for many other modern groups having a Cretaceous origin (Stidham, 1998). This is far from conclusive though.

Figure 3: A loon carries its chicks across some water. Loons are a group of birds that have been around since the Cretaceous.  Photographer: Bill Maynard.

Birds have gone on to be the dinosaurs' greatest success story, with more than 10,000 known living species. They have filled numerous niches including fishers, divers, predators, insectivores, frugivores, folivores, nectarivores, filter feeders, and oppurtunists.

Figure 4: Hummingbirds are an excellent example of how modern birds have filled an amazing variety of niches.  Photographer: Finca Lerida.

Next post shall discuss how many birds lost their ability to fly.

References
Godefroit, P., Cau, A., Dong-Yu, H., Escuillie, F., Wenhao, W. & Dyke, G. (2013). A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature. 498 (7454).
Paul, G. S. (2010). Dinosaurs: A Field Guide. A & C Black Publishers Ltd: London.
Shipman, P. (1998). Taking Wing. Touchstone: Rockefeller Center.
Stidham, T. A. (1998). A lower jaw from a Cretaceous parrot. Nature. 396 (6706), pp. 29-30.

Image sources
Figure 1: Accessed April 21, 2015, from http://emilywilloughby.com/gallery-data/images/full/the-earliest-birds.jpg
Figure 2: Accessed April 21, 2015, from http://fc06.deviantart.net/fs9/i/2006/015/3/e/Confuciusornis_sanctus_by_yty2000.jpg
Figure 3: Accessed April 21, 2015, from http://coolwildlife.com/wp-content/uploads/galleries/post-357/Loon%20Pictures%20012.jpg
Figure 4: Accessed April 21, 2015, from http://phenomena.nationalgeographic.com/files/2014/08/Hummingbird-990x618.jpg

Flight in non-avian dinosaurs

It has become a well accepted fact that birds are a type of flying dinosaur. Less well known is that they were not the only dinosaurs to take to the air.

Anchiornis was an early deinonychosaur that lived in what is now China around 160 million years ago. It had some parachuting ability thanks to the large feathers on its arms and legs. These feathers were not proper airfoils despite the great length of the arms, suggesting a reduction in flight ability from a possible flying ancestor (Paul, 2010). This would mean both that flight in dinosaurs goes back earlier than previously thought and that the first flying dinosaurs may have been non-avian.

Figure 1: An Anchiornis chases a small mammal through a late Jurassic forest. Artist: Emily Willoughby.

The dromaeosaurs were a group of deinonychosaurs containing members who possessed genuine powered flight. A good example of flying were the microraptorines, small omnivorous dromaeosaurs living in China and North America around 130-75 million years ago. While microraptorines have often been claimed to have been mere gliders their skeletal features show them to have been not only capable of flight, but in fact better flyers than the "first bird" Archaeopteryx (Paul, 2010). Their shoulders were placed so high they could easily perform full vertical flight strokes with their long forewings. Unlike birds, they did not use a sternal keel to assist in flight but instead used powerful deltoid muscles (Agnolin & Novas, 2013).

Figure 2: A Microraptor flies to a landing perch. Artist: Justine Lee.

Another flying dromaeosaur was the unenlaginine Rahonavis, who lived in Madagascar around 75-70 million years ago. Other known unenlaginines, as well as other larger dromaeosaurs such as Velociraptor and Deinonychus had lost the ability to fly. As adults that is. It is, however, possible the chicks still possessed some flight ability (Paul, 2010).

Figure 3: Some Rahonavis up in some trees. Artist: Vasika Yasnjith Udurawane.

The oviraptorosaurs were a group of short-tailed cousins to the deinonychosaurs who, while flightless, may have descended from flying ancestors (Paul, 2002 & 2010). Additionally, the therizinosaurs were cousins of the deinonychosaurs and oviraptorosaurs who may have come from gliders, who may in turn have come from flyers. As strange as it seems, even such groups as the ornithomimosaurs and the tyrannosaurs may ultimately have come from flying ancestry (Paul, 2002).

Figure 4: A family of Caudipteryx- a type of early oviraptorosaur. Oviraptorosaurs may have been secondarily flightless dinosaurs descended from flying ancestors.  Artist: Emily Willoughby.

Next post shall discuss the most well known flying dinosaurs- the birds.

References

Agnolin, F. L. & Novas, P. E. (2013). Avian Ancestors A Review of the Phylogenetic Relationships of the Theropods Unenlagiidae, Microraptoria, Anchiornis and Scansoriopterygidae. Springer: Netherlands.

Paul, G. S. (2002). Dinosaurs of the Air. The John Hopkins University Press: Baltimore, Maryland.
Paul, G. S. (2010). Dinosaurs: A Field Guide. A & C Black Publishers Ltd: London.

Image Sources
Figure 1: Accessed April 14, 2015, from http://fc02.deviantart.net/fs71/f/2010/082/1/d/Anchiornis___new_version_by_Ferahgo_the_Assassin.jpg
Figure 2: Accessed April 14, 2015, from https://gwawinapterus.files.wordpress.com/2013/04/picture.png
Figure 3: Accessed April 14, 2015, from http://fc04.deviantart.net/fs71/f/2011/357/1/3/rahonavis_ostromi_by_vasix-d4jyevw.jpg
Figure 4: Accessed April 14, 2015, from  https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjo2FX3KUM9e-FogVGhEjxAkmRLmDJnzRC1lXN_-e_XnjQI1Cofk0Xz_3enBapjiDRe7xOjjVJXQuPf2b3OHc5pAqwbcpKsKMIeWQ3siMIHGjXYtrdz_U1grzUrDF9ahFIxlqgZ9IpVcA/s1600/Caudipteryx.jpg

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