The Structure of a Pterosaur
Life restoration of Anhanguera piscator.
This is a multi-layered diagram of the main structural elements of a
pterosaur, based on Anhanguera (including A. piscator and
A. santanae). Much of it is highly speculative, as obviously, not
much of the soft tissue is preserved in the fossil record. There are many
clues, however, such as fossil impresssions of skin, some muscle1, muscle scars on the bones, and
pneumatic foramina (holes and depressions in the bones) for air sacs.
The Skin and Integument
Pterosaur wings were a complex membrane made of skin, muscle and collagen fibres. The wing-membrane configuration is still controversial - most pterosaur palaeontologists uphold that the membrane attached to the ankle (as it clearly does in some pterosaurs), but there are aerodynmic reasons for thinking that the membrane may have attached to the hip or thigh in large dynamic soarers. The configuration shown here was suggested by Mike Habib as a possible ankle attachment that would have aerodynamic advantages. Pterosaurs were covered with fur. Fur impressions from small pterosaurs show that it was fairly extensive, unlike many illustrations portray. Colouration is obviously guesswork, as no colour has been fossilised. General seabird flavour is the order of the day.
The Musculature
As flying animals, pterosaurs had very powerful forelimb muscles, which allowed even the biggest of them to flap their wings. Some of the larger muscles can be restored with a reasonable degree of confidence, other muscles can be inferred from living relatives. The main wing depressor muscles (for the downstroke) ran from the keeled sternum, and to a flange on the coracoid to a massive deltopectoral crest on the humerus. Pterosaurs didn't use a pulley mechanism like birds for the upstroke2, but instead had a more conventional arrangemnt of muscles tha ran from the back and scapular (shoulder blade) to the top of the humerus.
The wing finger was controlled with long tendons from muscles on the forearm. Somewhat counter-intuitively, the finger flexed (i.e. moved towards the palm) to move it into flight position, and hyper-extended to fold.
The Lungs and Air Sacs 3
Like birds, pterosaur's lungs were attached to an extensive series of air sacs. The air sacs had several functions, firstly to act as "bellows" for the lungs. This allows the air to flow through the lungs in only one direction, which is a more effecient system to our bi-direction lungs. Secondly, they were used to invade the bones - essentially fill them with air - which meant the bones could be bigger, without being any heavier. Thirdly, the air sacs may have have played a structural role in holding wing shape and posture, as they to in some birds.
The Skeleton
The skeleton of pterosaurs was highly modified for flight, with the fourth digit massively enlarged to form the outer wing spar. Their torso's were rigid and braced against the forces induced by flapping.
Often, pterosaur skeletons are descibed as "light" - but this is not correct. Although many of their bones were filled with air, they were larger absolutely, so the skeletal mass remains in line with other animals. The way to picture the situation is to imagine inflating a bone like a baloon: you end up with just as much "baloon" as before, just with a thinner wall.
Notes:
1Helmut Tischlinger's beautiful UV-light photos of a new anurongathid clearly show impressions of forelimb and hindlimb muscles. The photo can be seen in David Unwin's The Pterosaurs: From Deep Time.
2Birds have a specialised system for pulling the wing up during flight. The supracoacoideus originates on the sternal plate, and loops over the top of the shoulder to attach to the top of the humerus. It has been suggested that pterosaurs had a similar arrangement, but that is no longer considered to be the case. Bats also lack a supracoacoideus pulley, so it is not necessary for powered flight.
3 The configuration of the air sacs is taken entirely from birds - I have little doubt that pterosaurs differed somewhat, but that will have to wait on addditional research. They would also have been more extensive than shown here - which is somewhat simplified.
References:
Bennett, S. C., 2003. Morphological evolution of the pectoral girdle of pterosaurs myology and function, in Buffetaut, E., and Mazin, J-M. (eds) 2003, Evolution and Palaeobiology of Pterosaurs, geological society of London, 2003 217 pp. 191-215 Kellner, A. W. A., and Tomida, Y., 2000, Description of a new species of Anhangueridae (Pterodactyloidea) with comments on the pterosaur fauna from the Santana Formation (Aptian-Albian), northeastern Brazil: National Science Museum Monograph, n. 17. Hudson, E. H., Lanzillotti P. J., 1955. Gross anatomy of the wing muscles in the family Corvidae, The American Midland Naturalist, 53:1 pp. 1-44 Ritchison, G.. Bird Respiritory System (website) Meers, M. B., 2003. Crocodylian forelimb musculature and its relevance to the Archosauria, The Anatomica Record, Part A Unwin, David M., 2006. The Pterosaurs: From Deep Time. New York: Pi Press, 246. ISBN ISBN 0-13-146308-X. Wellnhofer, P., 1991. Weitere Pterosaurierfunde aus der Santana-Formation (Apt) der Chapada do Araripe, Brasilien, Palaeontographica, A, 215. Witmer W.M., Chatterjee, S., Franzosa, J. and Rowe, T. 2003. Neuroanatomy of flying reptiles and implications for flight, posture and behaviour. Nature 425, 950-953