There are all kinds of aspects of palaeontology that in some ways we can only guess at how these things might have lived, functioned and behaved as living organisms. However one of the key aspects of science is the ability to make predictions and with careful use of analogy and homology (and of course the fossil record), we can try to work out some of those complexities that otherwise might leave us stumped. My former student Edina Prondvai and I have a new paper coming out in Historical Biology discussing how pterosaurs might have been able to extend that massive fourth finger and keep it stead during flight while minimizing energy expenditure. Edina takes us through it in this guest post:
The dominant characteristic of pterosaurs is the hyper-elongated fourth manual digit, that solely supported the extensive flight membrane. It is by far the longest lever arm along the leading edge of the wing among all actively flying vertebrates. Keeping this in mind it seems to be very odd that no one has paid special attention to the obvious energetic problem faced by pterosaurs: the extension of the enormous wing finger and its stabilization and control during flight. Thus the following questions went through my mind: Could such an extremely enlarged device have been operated exclusively by muscle force? If yes, how big should those muscles have been to be able to operate the wings properly? Is there enough space for them on the wing and shoulder bones at all? If not, how could pterosaurs have gotten around the problem and used as little musculature as possible to minimize the energy output (and the demand of higher neural control and to avoid significant mass increase especially in the distal wing that goes with it) but have still been able to fly? In the light of discoveries of giant pterosaurs such as Quetzalcoatlus or Hatzegopteryx this energetic problem of operating the wings during steady flight with muscles that require constant energy input and neural control to work becomes even more evident.
Despite their obvious evolutionary separation, the only extant flying vertebrates (birds and bats) managed to overcome the same problems in the same way (however, of a much lower magnitude due to the incomparable size-dimensions): biomechanical automatism has been built in their skeletal, muscular and connective tissue system.
The osteological investigation of several 3-D specimens of pterosaurs have made it clear to me that bones alone cannot provide solution for the former problems because there is no indication of a bone-based automatic mechanism in the wing function such as the geometric ‘drawing-parallels’ system in birds. Consequently the solution in pterosaurs must have involved soft tissues to a considerable degree. Since all powered flying animals have to face the same energetic problems here we got to the point where the extant life-style analogues must be used to fill in the gaps of our knowledge about the unpreserved soft parts and their possible significance in pterosaurs.
Based on the morphology, position, role and relative importance of ligaments and tendons in the automatic wing operation of birds and bats we were able to provide two possible models for the wing finger extension in pterosaurs. In the first model we hypothesise the presence of a bird-like ‘propatagial ligament’ or ligamentous system which runs from the shoulder girdle to the wing finger. When the distance between the origin and insertion of these ligaments (hence the tension in them) increases by means of active elbow extension they automatically and passively perform and maintain the extension of the wing finger during flight and prohibit the hyperextension (and dislocation) of the elbow. In the second model, which has been derived from the first, pterosaurs have a more bat-like tendinous extensor muscle system on their forearm that shares the loads of wing finger extension with the bird-like propatagial ligaments. Both models fit with our existing understanding of the muscles and tendons of pterosaurs and where they attach to the various wing bones.
Although merely hypothetical, both models provide a natural solution to avoid unnecessary exertion by: 1. reducing the muscle mass and by extension weight in the distal wing, 2. preventing hyper-extension of the elbow against drag, and 3. automating wing extension and thereby reducing metabolic costs required to operate the pterosaurian locomotor apparatus during flight. Future research is required both to test these models and to look for evidence to support them in the pterosaurian record, by they may help to reveal how an important part of pterosaur flight operated.