If one includes gliding as a form of flight, then the earliest flying tetrapods would have been the weigeltisaurs of the Permian (Benton, 1998). Since then, gliding has independantly evolved in numerous different groups of tetrapod. However, if one is referring to true powered flight then the first flying tetrapods were the pterosaurs, who first appeared in the Triassic. Later, powered flight evolved in certain maniraptoran dinosaurs, including the birds. Later still it evolved in a group of mammals, the bats.
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| Coelurasauravus, a gliding creature from the Permian. |
In all of these groups, the forelimbs evolved into a new type of structure- wings. A wing is an aerofoil structure, with a curved top and a flat bottom. This causes air flowing across the wing to "stretch" over the top, resulting in lower air pressure above the wing than below it and resulting in the higher air pressure below pushing the wing upwards. Each of the groups of flying tetrapod had forelimb-derived wings utilising this basic principle. However, each group had a very different type of wing design. The pterosaurs had "finger wings" in which an elongated fourth finger supported a membranous wing reinforced by stiffening fibres. Birds and other flying dinosaurs had "arm wings" comprised primarily of feathers. Bats had "hand wings" in which a leathery elastic membrane stretched between adjacent fingers. (Raymer, 1988 & Shipman, 1998). These different designs of structures used for the same purpose show how evolution can produce differing solutions to a single problem.
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| Bird, bat and pterosaur wings all derive from the forelimbs but each in a different way. |
Future blog posts will go into further detail on the origins and evolution of the pterosaurs, the birds and other flying dinosaurs, and the bats, and on the wide diversity that evolved within all these groups of flyers.
References
Benton, M. (1998). The Reign of the Reptiles. Eagle Editions:Hertfordshire.
Raymer, J. M. W. (1988). The evolution of vertebrate flight. Biological Journal of the Linnaean Society, 34 (3), pp. 269-287.
Shipman, P. (1998). Taking Wing. Touchstone: Rockefeller Center.
Image sources
http://f.tqn.com/y/dinosaurs/1/S/j/a/-/-/ABcoelurasauravus.jpg
http://ncse.com/files/images/Wing_morphology.img_assist_custom.jpg
This is a very fascinating topic. Do you think that gliding is a necessary pre-requisite for powered flight, or has powered flight evolved independently of the ability to glide? I’m really intrigued about what will come in future blogs.
ReplyDeleteI think powerful flight evolved independently of the ability to glide. Indeed, as I shall show in future posts, none of the flying tetrapods seem to have come from gliding ancestors and the very idea that gliding leads to flight is actually erroneous.
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