Pretty pycnofibres, wondrous wings and amazing air sacs

The first important feature needed to be a flying tetrapod is a warm-blooded metabolism in order to produce enough energy to sustain powered flight. Pterosaurs had this. For insulation they had a thick pelt of hair-like structures called pycnofibres. A pterosaur's pelt could be quite dense. For example, the Jurassic pterosaur Rhamphorhynchus had about 20 pycnofibres per square millimetre and each one was 2-3 millimetres long (Benton, 1998).

Figure 1: On this Rhamphorhynchus the hair-like pycnofibres are clearly visible. Artist: Luis Rey.

The most important feature for flight though are of course wings and the pterosaur wing was one of the most amazing structures to have ever evolved. The fourth finger had become extremely elongated, while the fifth finger had vanished. From the elongated fourth finger was attached a large membrane that served as an airfoil surface. This membrane was comprised of layers of skin, muscle tissue and air pockets. Supporting the wing were special fibres called the aktinofibrils. As well as reinforcing the flight membrane, the aktinofibrils also prevented wobbling in flight and allowed pterosaurs to change their wing shape for better maneuverability (Wellnhofer, 1991).

Pterosaurs launched into the air in a quadrupedal fashion where the hind limbs were used to provide forward momentum while the forelimbs (wings) were used to provide a vertical heft that launched the pterosaur into the air (Habib, 2008). At least some pterosaurs were even able to launch from water (Habib & Cunningham, 2010).

Figure 2: Cretaceous pterosaur Ornithocheirus launching from water. Artist: Mark Witton.

 Pterosaurs also had another amazing feature. As well as lungs they also possessed an advanced system of air sacs that extended into their skeleton leaving openings in the bones. The only other animals known to possess such a system are saurischian dinosaurs (inlcuding birds). As with the dinosaurs, the air sacs of the pterosaurs allowed air to flow in a single direction and get utilised as efficiently as possible. Pterosaur air sacs, however, also ended up evolving a feature not possessed by dinosaur air sacs. The larger pterosaurs could inflate subcutaneous air sacs in their wings to alter their mechanical properties, such as relative stiffness (Claessens et al, 2009).

Figure 3: This cutaway of Cretaceous pterosaur Anhanguera shows lungs (red), neck air sacs (green) and wing air sacs (blue). Artist: Mark Witton.

Next up will be pterosaur diversity before moving onto the next group of flyers- birds and other dinosaurs.

References
Benton, M. (1998). The Reign of the Reptiles. Eagle Editions: Hertfordshire.
Claessens, L. P. A. M., O'Connor, P. M., & Unwin, D. M. (2009). Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PloS one, 4 (2).
Habib, M. B. (2008). Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana. B28, pp. 159-166.
Habib, M. B. & Cunningham, J. (2010). Capacity for water launch in Anhanguera and Quetzalcoatlus. Acta Geosci. Sin 31, pp. 24-25.
Wellnhofer, P. (1991). The Illustrated Encyclopedia of Pterosaurs. Crescent Books: New York.

Image sources
Figure 1: Accessed March 24, 2015, from http://pterosaur.net/species/Rey%20Rhamphorhynchus.jpg
Figure 2: Accessed March 24, 2015, from http://www.markwitton.com/communities/6/004/009/119/796/images/4618976279.jpg
Figure 3: Accessed March 24, 2015, from http://i.livescience.com/images/i/000/002/908/i02/090217-pterosaur-ballooning-02.jpg?1296073588