The growing knowledge of tetrapod flight

So why did flight evolve? While flying does have its disadvantages, such as high energy use, it has many advantages as well. Flying animals can cover far greater distances than flightless ones and can also more easily pass over geographical barriers such as valleys, highlands and bodies of water. Flying animals can also reach environments that flightless animals might find hard to reach such as islands and treetops, and this could allow flyers to find safer areas to rest or reproduce. Furthermore, getting away from predators is easier when one can just take to the air. All of these factors likely helped promote the evolution of flight in pterosaurs, flying dinosaurs, and bats alike (Witton, 2013).

Figure 1: A couple of anachronistic friends. A Darwinopterus- a pterosaur from the Jurassic, and a European robin- a dinosaur from the Quaternary.  Artist: Mark Witton.

New discoveries are constantly expanding our knowledge of tetrapod flight. Just a few weeks ago, an incredible new winged dinosaur was discovered. Yi was a scansoriopterygid that lived in China around 160 million years ago. Each of its hands had an extremely long third finger as well as a strange long styliform that grew from the wrist. These may have supported a wing membrane, as evidenced by fossilised membranous tissue found in the wrist area. While Yi may not have been capable of true flight, it is still an interesting example of how maniraptoran dinosaurs tried out more than just one wing design (Padian, 2015). Some classify scansoriopterygids as basal avialans (Martyniuk, 2012). Others classify them as more primitive non-avialan maniraptorans (Paul, 2010).

Figure 2: Yi, a membrane winged scansoriopterygid. While it may not have been capable of true flight, perhaps it could flutter.  Artist: Emily Willoughby.

Research has also helped destroy old prejudices about pterosaurs and bats. Once thought to be poor flyers, pterosaurs are now known to have actually been the most effective flying animals ever, with amazingly efficient wings and launching capabilities (Witton, 2013). Bats too, often overshadowed by birds, have been shown to be quite successful and diverse, indeed some of the most diverse of all mammals (Altringham, 2011).

Figure 3: Representatives of each flying tetrapod group flying together with an airplane- a machine invented by some upright apes who wanted to fly too.  Artist: Matteo Bachin.

What does the future hold? Will any other tetrapod group ever evolve flight many millions of years from now? Who knows. Only time will tell.

References
Altringham, J. D. (2011). Bats: From Evolution to Conservation. Oxford University Press: Oxford.
Martyniuk, M. P. (2012). A Field Guide to Mesozoic Birds and Other Winged Dinosaurs. Pan Aves: Vernon, New Jersey.
Padian, K. (2015). Dinosaur up in the air. Nature. 521 (7550), pp. 40-41.
Paul, G. S. (2010). Dinosaurs: A Field Guide. A & C Black Publishers Ltd: London.
Witton, M. P. (2013). Pterosaurs. Princeton University Press: Princeton, New Jersey.

Image sources
Figure 1: Accessed May 26, 2015, from: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVXAWWt6i-CAOhAYeIKbrpa3Ei9ROTbEWGg1LxKfhN5j8YXUWFiKYanExv6iB8rc1BVAq9Mw2wsICHsbVLe-Mt2mydTLnm4dp7sfgZwkc9RD8UP-WmkL4lHKirrF47pJq4gC7CEtGJgpw/s1600/Darwinopterus+and+Robin+Witton.png
Figure 2: Accessed May 26, 2015, from: http://upload.wikimedia.org/wikipedia/commons/5/56/Yi_qi_restoration.jpg
Figure 3: Accessed May 26, 2015, from: http://pterosauria.weebly.com/uploads/2/2/0/7/2207195/189247.jpg?595

Bat diversity

While bats never reached as great a level of diversity as pterosaurs and birds, they are nevertheless quite diverse. Today bats are the second largest group of mammals, with more than 1100 known living species. The largest group, rodents, have more than twice as many species, but they are less biologically diverse than the bats (Altringham, 2011).

Bats are divided into two groups- the microbats and the megabats. The microbats are the more original bats, while megabats are a younger group with the oldest known megabat, Archaeopteropus, living around 35 million years ago. The names "microbat" and "megabat" are somewhat misleading as there do exist big microbats and small megabats. The real differences in the groups are features such as the more elongated, dog-like, faces of the megabats and the fact that most megabats have lost their echolocation abilities (Altringham, 2011).

Figure 1: A blossom bat pollinates a flower. Despite its small size, this is actually a megabat, showing that size is not the defining feature of the two bat groups.  Photographer: Mike Trenerry.

While most bats have retained the insectivore lifestyle of their ancestors, there are many who have exploited other niches. The megabats and some microbats have become frugivores and nectarivores. There are also bats who have evolved a fishing lifestyle, such as Latin America's bulldog bats, who use echolocation to detect the ripples caused by fish near the surface of rivers and ponds. There are also a number of predatory bats, such as the false vampire bats and the frog eating bats (Levy, 1999).

Figure 2: A bulldog bat catches a tasty fish.  Photographer: J. Scott Altenbach.

There are even a few bats who evolved to feed on blood. Namely Latin America's vampire bats. As well as the standard echolocation ability, vampire bats also possess two heat sensing pits in their face, to help seek out warm blood vessels near their prey's skin surface. Their razor-sharp triangular teeth shave away overlying tissue and nick the blood vessel and special anticoagulants in their saliva keeps the blood flowing (Levy, 1999).

Figure 3: A vampire bat shows off the tools of the trade- razor-sharp triangular teeth for nicking blood vessels.  Photographers: Michael Fogden & Particia Corbis.

While no bat ever became flightless, New Zealand's lesser short-tailed bat spends much of its time down on the ground, foraging for insects, berries and nectar on the forest floor. Unlike other bats its wings are not attached to the hindlimbs, so it can move about on the ground more easily than other bats can (Fitter, 2009).

Figure 4: A lesser short-tail bat foraging around in a New Zealand forest.  Photographer: Rod Morris.

Next shall be this blog's conclusion.

References
Altringham, J. D. (2011). Bats: From Evolution to Conservation. Oxford University Press: Oxford.
Fitter, J. (2009). New Zealand Wildlife. Bradt Travel Guides Ltd: England.
Levy, C. K. (1999). Evolutionary Wars: A Three-Billion-Year Arms Race. W. H. Freeman and Company: New York.

Image sources
Figure 1: Accessed May 19, 2015, from: http://www.wettropics.gov.au/site/resized/80-08082012022826-0-119-990-389-990x389-cropped-mtlittleblossombat2.jpg
Figure 2: Accessed May 19, 2015, from: http://www.vnews.com/csp/mediapool/sites/dt.common.streams.StreamServer.cls?STREAMOID=H_kR6yEGutZtjodlwCi2aM$daE2N3K4ZzOUsqbU5sYv$HB5zCCfOaoycRvYQB8Y5WCsjLu883Ygn4B49Lvm9bPe2QeMKQdVeZmXF$9l$4uCZ8QDXhaHEp3rvzXRJFdy0KqPHLoMevcTLo3h8xh70Y6N_U_CryOsw6FTOdKL_jpQ-&CONTENTTYPE=image/jpeg
Figure 3: Accessed May 19, 2015, from: http://images.nationalgeographic.com/wpf/media-live/photos/000/005/cache/common-vampire-bat_505_600x450.jpg
Figure 4: Accessed May 19, 2015, from: http://blogs.scientificamerican.com/media/inline/blog/Image/CrawlingBat.jpg

The origin of the bats

The most recent group of tetrapods to evolve flight were a type of mammal. The bats.

The earliest known bat fossils date to around 52 million years ago. However, these early bats were very much like modern microbats in morphology, and their only really primitive features were unfused ribs, some finger features, and the retention of certain teeth absent in modern bats (Benton, 2005). Furthermore, the very early known bats were already very widespread, being found in Eurasia, North America and Australia (Long et al, 2002). This suggests bat origins go back further. A 75 million year old fossil of a noctuid moth egg in Massachussets (noctuids are a group of moths adapted to hear bat echolocation) suggests bats may have been around as long as 75 million years ago (Altringham, 2011).

Figure 1: Icaronycteris, a bat that lived in Wyoming around 52 million years ago.  Artist: unknown.

Earlier in this blog we saw how gliding played no role in the evolution of flight in pterosaurs and dinosaurs. Again, gliding played no role in the evolution of bat flight either. In a study conducted by Kevin Padian and Kenneth Dial, it was found that bat anatomy, physiology and behaviour pointed to an origin as climbing mammals who would hang upside down and fall down towards insect prey, using proto-wings to control their descent in a manner quite different to that of any glider. In time this early fluttering developed into true flight (Kaplan, 2011).

Figure 2: Baby bats cannot fly but if they fall they can flutter to slow their descent, likely a vestige of the fluttering ancestry of bats.  Photographer: Peet van Schalkwyk.

Bat wings are essentially webbed hands. These hand wings are somewhat better airfoils than the feather wings of birds and other flying dinosaurs, though not quite as good as the multi-layer wings of pterosaurs. Bats lack air sacs but compensate by having enormous lungs. Perhaps the bats' most amazing feature though is the ability of echolocation (though this has been lost in most megabats). By emitting ultrasonic pings that bounce off solid objects bats can work out the exact distance and location of said objects regardless of whether they can actually see them. Bat echolocation is remarkable in its precision. Indeed, it is around 1 trillion times more effective than our best sonar detector technologies (Levy, 1999).

Figure 3: A simple explanation of how bat echolocation works. The bat emits ultrasonic pings that bounce off solid objects. From this the bat can determine things like distance, location and, in the case of the moth shown here, which direction something is moving in.

Next post shall be exploring the diversity of these remarkable creatures.

References
Altringham, J. D. (2011). Bats: From Evolution to Conservation. Oxford University Press: Oxford.
Benton, M. (2005). The Rise of the Mammals. Quantum Publishing Ltd: London.
Kaplan, M. (2011). Ancient bats got in a flap over food. Nature.
Levy, C. K. (1999). Evolutionary Wars: A Three-Billion-Year Arms Race. W. H. Freeman and Company: New York.
Long, J., Archer, M., Flannery, T. & Hand, S. (2002). Prehistoric Mammals of Australia and New Guinea. University of New South Wales Press Ltd: Australia.

Image sources
Figure 1: Accessed May 12, 2015, from:  http://age-of-mammals.ucoz.ru/_si/2/24863285.jpg
Figure 2: Accessed May 12, 2015, from: http://farm3.static.flickr.com/2629/3965523629_c86b1ce4e2.jpg
Figure 3: Accessed May 12, 2015, from: http://paulmirocha.com/wp/wp-content/uploads/2011/11/echolocation1.jpg

Flightless birds

As seen earlier in this blog, flight was gained and lost on a number of occasions amongst certain non-avian dinosaurs. The avian dinosaurs are no different, as birds have become flightless many times.

Already by the late Cretaceous, there were various groups of flightless bird, including the marine dwelling hesperorniths and the ground dwelling patagopterygiforms (Buffetaut & Angst, 2013). The ratites also appeared at this time, and throughout their history, ratites have become flightless not just once but numerous times (Harshman et al, 2008).

Figure 1: Garganuavis, a huge patagopterygiform bird from late Cretaceous France. Though it looks a bit like a ratite the wing claws give away its identity as a member of a more primitive group.  Artist: Joschua Knuppe.

Following the Cretaceous-Palaeogene Extinction 65.5 million years ago, there evolved some enormous flightless birds. The gastornids were enormous relatives of ducks who lived in Eurasia and North America around 56-41 million years ago. Once thought to be predators, studies of their bone isotopes and jaw musculature show them to have been large herbivores (Angst et al, 2014). The phorusrhacids, on the other hand, were definitely predatory. Relatives of cranes, they lived in the Americas, Europe, Africa and Antarctica and existed from around 60 million-15,000 years ago (Dyke & Kaiser, 2011).

Figure 2: Some foolish borhyaenids attempt to steal some prey- a small ungulate- from a phorusrhacid. This will probably not end well for them.  Artist: Renata Cunha.

Today flightlessness exists in 18 surviving bird families (Harshman et al, 2008). In some cases, flightless birds took to a more aquatic lifestyle. This was the case with the penguins, who first appeared around 60 million years ago (Ksepka & Clarke, 2010). Penguin wings have become flippers allowing them to "fly" underwater. It is possible future penguins may evolve into fully aquatic animals.

Figure 3: A couple of Gentoo penguins about to go for a swim. Penguins have become superbly adapted to an aquatic lifestyle, with the wings having become flippers.  Photographer: Robert Heil.

Another thing that can lead to flightlessness in birds is living in island habitats. An excellent example of this is New Zealand, which is inhabited by various flightless birds including the kiwi- a ratite, three species of flightless swamp hen, and the kakapo- the world's only flightless parrot (Fitter, 2009).

Figure 4: A kakapo, the world's only species of flightless parrot.  Photographer: Thomas Gibson.

So why did so many birds become flightless? The answer is, surprisingly enough, that birds are just not exceptionally good flyers. Feather wings can be rendered flightless by something as simple as shortening the feathers and, furthermore, as feathers are made of dead tissue the wings are mostly unable to take in sensory information. Additionally, birds can only launch with their hindlimbs and must flap two to three times just to get airborne (Shipman, 1998). No wonder then that many became flightless.

The next post shall introduce the final group of flyers, the bats.

References
Angst, D., Lecuyer, C., Amiot, R., Buffetaut, E., Fourel, F., Martineau, F., Legendre, S., Arbourachid, A. & Herrel, A. (2014). Isotopic and anatomical evidence of an herbivorous diet in the Early Tertiary giant bird Gastornis. Implications for the structure of Palaeocene terrestrial ecosystems. Naturwissenschaften. 101 (4), pp. 313-322.
Buffetaut, E. & Angst, D. (2013). New evidence of a giant bird from the Late Cretaceous of France. Geological Magazine. 150 (1), pp. 173-176.
Dyke, G. & Kaiser, G. (eds.). (2011). Living Dinosaurs: The Evolutionary History of Modern Birds. John Wiley & Sons Ltd: West Sussex.
Fitter, J. (2009). New Zealand Wildlife. Bradt Travel Guides Ltd: England.
Harshman, J., Braun, M. J., Braun, E. L., Huddleston, C. J., Bowie, R. C. K., Chojnowski, J. L., Hackett, S. J., Han, K., Kimball, R. T., Marks, B. D., Miglia, K. J., Moore, W. S., Reddy, S., Sheldon, F. H., Steadman, D. W., Steppan, S. J., Witt, C. C. & Yuri, T. (2008). Phylogenomic Evidence for Multiple Losses of Flight in Ratite Birds. Proceedings of the National Academy of Sciences of the United States of America. 105 (36), pp. 13 462-13 467.
Ksepka, D. T. & Clarke, J. A. (2010). The Basal Penguin (Aves: Sphenisciformes) Perudyptes devriesi and a Phylogenetic Evaluation of the Penguin Fossil Record. Bulletin of the American Museum of Natural History. 337 (337), pp. 1-77.
Shipman, P. (1998). Taking Wing. Touchstone: Rockefeller Center.

Image sources
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Figure 2: Accessed May 5, 2015, from http://41.media.tumblr.com/tumblr_mdbbqeRTk21rqeszyo1_1280.png
Figure 3: Accessed May 5, 2015, from http://images.nationalgeographic.com/wpf/media-live/photos/000/247/cache/gentoo-penguins-jumping-in-water_24700_600x450.jpg
Figure 4: Accessed May 5, 2015, from http://kleberly.com/data_images/wallpapers/9/282092-kakapo.jpg