October 21, 2016:

byEmily Rayfield

Project description

Quadrupedal launch sequence of a large azhdarchid pterosaur. Image: James Brown, copyright: Colin Palmer.

With a 10m wingspan and 250kg estimated body mass, giant azhdarchid pterosaurs were the largest vertebrates ever to fly. By contrast the largest extant birds have wingspans of 3m and weigh around 20kg, with fossil birds reaching 6-7m wingspan and 70kg mass. Even giant extinct birds are dwarfed by the largest pterosaurs. When birds take off their legs provide the initial work to accelerate their bodies before the wings take over (Earls 2000, Henry et al 2005, Heers & Dial 2014). Once airborne, the legs serve no useful purpose and act as “baggage”. The difficulties that large birds face in becoming airborne (swans, albatrosses, bustards) suggests that take off may be an allometrically constrained factor that limits the maximum size of birds.

Pterosaurs adopted a quadrupedal stance, which meant that their forelimbs were used for both flight and locomotion, including launch, so carried less baggage once airborne. Hypothesised quadrupedal launch behaviour provided a long launch stroke and recruitment of large muscle mass to power take-off (Habib 2008); proportionately larger than that available to birds. While the underlying concept of the quadrupedal launch is now widely accepted, the kinematic and anatomical details are poorly understood, meaning that estimates of available launch power are very imprecise. Increasing our understanding of the take-off process will shed new light on the role it played in pterosaur gigantism and to help refine our estimates of pterosaur maximum size.

The main objective of this proposal is to investigate the effectiveness of the quadrupedal launch and by comparing it with the bipedal launch of birds, test if it was one of the factors that enabled pterosaurs to become much larger than any bird, extant or extinct. The project will establish the differences between the ground launch dynamics of birds and pterosaurs, how these relate to differences in morphology, scale with and enforce upper limits to size, before ultimately establishing the most effective morphology to maximise launch capability. This will be achieved by creating anatomical reconstructions of possible azhdarchid morphologies and using kinematic simulation software to create a simple baseline bipedal launch model, validated against published data for birds and humans and tested for sensitivity to assumptions and modelling detail. This baseline model will then be modified to incorporate quadrupedal launch morphology. The effect of varying muscle morphologies on power output and the effects of varying size and subsequent allometric relationships will be determined. Finally, anatomically correct simulation models will be constructed to model quadrupedal launch capability and potential upper limits to size.

The project would suit a student with an interest in palaeobiology, biomechanics, evolution and engineering. The student will join the large and vibrant Bristol palaeobiology community and receive training in anatomical and computational techniques.
References

K. D. Earls, Kinematics and mechanics of ground take-off in the starling Sturnis vulgaris and the quail Coturnix coturnix. Journal of Experimental Biology 203, 725 - 739 (2000).

M. B. Habib, Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana B28, 159-166 (2008).

A. M. Heers, K. P. Dial, Wings versus legs in the avian bauplan: Development and evolution of alternative locomotor strategies. Evolution 69, 305-320 (2014).

M. P. Witton, Pterosaurs: Natural History, Evolution, Anatomy (Princeton University Press, Princeton, 2013).