November 18, 2013 | 52
Some weeks ago now, myself and a team of colleagues (Gareth Dyke, Roeland de Kat, Colin Palmer, Jacques Van der Kindere and Bharathram Ganapathisubramani) – all of whom are based at the University of Southampton – published the results (in Nature Communications) of our study on the aerodynamic performance of Microraptor, a small, long-winged dromaeosaurid theropod from the Lower Cretaceous of China (Dyke et al. 2013). Several previous analyses have looked at the aerodynamic performance of Microraptor: what makes our study different, and what did we conclude?
Microraptor – first reported from the famous Yixian Formation of China’s Liaoning Province in 2000 – is well known for being a four-winged dinosaur: it has long feathers on its forelimbs, like a bird, but equally long feathers on its hindlimbs. The rest of the animal is thickly covered in plumage as well, and a large, fan-like array of feathers also surrounds the end of its tail.
Microraptor was originally described from a single fragmentary fossil that revealed few of its peculiarities. However, it has since been described from numerous specimens and its osteology, dentition and plumage can now be considered reasonably well known (Hwang et al. 2002, Xu et al. 2003, Hone et al. 2010, Li et al. 2012). Three species have been named (M. zhaoianus, M. gui and M. hanqingi) but it’s been argued that all are synonymous. A fourth species, Cryptovolans pauli, is also generally regarded as synonymous with M. zhaoianus.
Building a Microraptor: controversy and consensus
What, exactly, did we do in our new study? Colin made an accurate, poseable scale model of a Microraptor, kitted out with genuine feathers (from ducks and pigeons) attached in anatomically correct positions and orientations. Those who have been following the development of ideas on the life appearance of non-avian maniraptorans will be interested to note that our model’s head, neck and body has the streamlined look more typical of modern birds than of conventional reconstructions of Mesozoic dinosaurs: an inescapable consequence of our considering the full extent of all the feathering present in these animals.
Previous reconstructions have sometimes gotten things wrong as goes feather attachment: the hindlimb feathers of Microraptor have asymmetrical vanes (as do the forelimb feathers), showing that they were arranged in a specific way relative to airflow (that is, the narrower side of the vane must have been arranged along the wing’s anterior edge). However, one previous reconstruction has the hindlimb feathers arranged perpendicular to airflow (Xu et al. 2003) and thus is very likely incorrect.
Based on feather and hindlimb anatomy, it seems that Microraptor held its legs such that the long feathers projected laterally or posterolaterally, their leading edges near-perpendicular to the sagittal plane. This raises lots of questions as to how mobile the feathers were: were they fixed in this pose, or were there muscles and ligaments in place that allowed them to be moved and folded away when not in use? We don’t know (yes yes, we’re fully aware that similar feathers are present in some domestic chicken and pigeon breeds). In fact, our understanding of these feathers is in its infancy: we don’t even have a nomenclature for them yet, plus there are all sorts of questions about the genetics involved in the evolution of these structures. Stay tuned.
If the hindlimb feathers of Microraptor were held projecting laterally (remember that the longest feathers are on the metatarsus), this means that we agree approximately with the biplane-type configuration proposed by Chatterjee & Templin (2007) (though it doesn’t follow that the hindlimbs were held pressed together as Chatterjee and Templin supposed). Incidentally, it should be noted that some members of our team tried really hard to check, and test, Chatterjee and Templin’s proposed configuration. As we explain in the paper (Dyke et al. 2013), this is hard because it hasn’t been possible to replicate their methods; furthermore, Chatterjee and Templin didn’t demonstrate how their biplane configuration might generate lift.
One debate that surrounds the aerodynamic performance of Microraptor concerns hindlimb posture. In the very first study to discuss Microraptor’s possible flight abilities, it was depicted as being capable of a full-on sprawl, its hindlimbs projecting laterally in parallel with its arms (Xu et al. 2003). This sprawling pose was also promoted in another study (Alexander et al. 2010). Given that the form of the theropod femur and hip socket generally prevents the hindlimb from being abducted this far from the sagittal plane (there are proximally placed trochanters on the femur, and supra-acetabular shelves and antritrochanters on the ilium that prevent this sort of posture), this is surely incorrect (Brougham & Brusatte 2010).
However, there have been suggestions that microraptorine dromaeosaurids had a more spherical femoral head than most other theropods, and hence that they were indeed more capable of hindlimb abduction than is typical for this group. While it’s difficult to provide precise measurements on joint orientation, we manipulated our poseable model such that its femora were abducted for the amount that we deemed reasonable based on the fossils (especially those of Hesperonychus, a Canadian relative of Microraptor known from three-dimensional remains (Longrich & Currie 2009)). We didn’t favour the idea that Microraptor could sprawl.
To the wind tunnel!
The model was placed in one of the several University of Southampton wind tunnels and subjected to various simulations. In view of the controversy about hindlimb posture, we tested the model’s performance with sprawling limbs, with the limbs projecting straight downwards, and with the hindlimbs entirely removed. Aerodynamic performance was best when the limbs were in the straight-down posture (Dyke et al. 2013) – a satisfying result given that this is the configuration we regard as most likely. The tail operated as a lift-generating structure, meaning that Microraptor can accurately be described as a five-winged flier, not just a four-winged one.
Notably, Microraptor was never an efficient flier: it suffered from extensive drag in all simulations and was aerodynamically unstable, performing best when moving quickly. It was well able to glide no matter what the feather or wing configuration: in fact, we concluded that all Microraptor needed in order to glide effectively was a flat wing surface – feather asymmetry, anatomy and configuration didn’t make that much difference (Dyke et al. 2013), a discovery which supports the view that the evolution of theropod wing and feather anatomy did not occur within an aerodynamic context.
The integrated Microraptor
What might this mean for the ecology, biology and behaviour of Microraptor? Like several of the long-feathered Mesozoic paravians, Microraptor is something of an enigma. After all, it seems to combine a ‘typical’ non-bird theropod skeletal bauplan – traditionally associated with terrestriality and cursoriality – with massive, fully vaned feathers that seemingly have hallmarks of aerodynamic function. The fact that Microraptor was a half-decent glider, but not really a great one (it incurs all that drag, don’t forget), suggests that it was an occasional glider that could indulge in this behaviour, but didn’t rely on it as a specialisation: in other words, it wasn’t an aerial specialist, or a ‘flying squirrel dinosaur’ that spent most of its time leaping from tree to tree, relying on arboreal resources.
As we say (Dyke et al. 2013), I think that this is what we would predict based on what we already know about the morphology of Microraptor. It also matches what we know about its diet and lifestyle. There are indications from the claw curvature work that I published with Alexander Birn-Jeffery and colleagues late last year (Birn-Jeffery et al. 2012), for example, that Microraptor could climb, and the idea that Microraptor occasionally ate fish (Xing et al. 2013) is certainly contradictory to the view that it was a specialised, dedicated aerialist.
I think that our conclusions on the aerodynamic performance of this peculiar dinosaur are in agreement with our general ideas on its ecomorphology – and I haven’t discussed all the ramifications and conclusions of our study: those especially interested will want to read the paper. Applying sophisticated (and complex) aerodynamic modelling and the use of wind-tunnels to the study of Microraptor was great fun, but our work is – of course – definitely not the last word on this issue. Furthermore, Microraptor is but one of many: the Mesozoic fossil record has become populated by a growing number of near-bird paravians with particular wing and hindlimb configurations and feather arrangements, and we still know but oh so little.
For previous Tet Zoo articles on non-bird paravians and other maniraptorans, see…
Refs – -
Alexander, D. E., Gong, E., Martin, L. D., Burnham, D. A. & Falk, A. R. 2010. Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid Microraptor gui. Proceedings of the National Academy of Science 107, 2972-2976.
Birn-Jeffery, A. V., Miller, C. E., Naish, D., Rayfield, E. J., Hone, D. W. E. 2012. Pedal claw curvature in birds, lizards and Mesozoic dinosaurs – complicated categories and compensating for mass-specific and phylogenetic control. PLoS ONE 7(12): e50555. doi:10.1371/journal.pone.0050555
Dyke, G., de Kat, R., Palmer, C., van der Kindere, J., Naish, D. & Ganapathisubramani, B. 2013. Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight. Nature Communications 4, Article number: 2489 doi:10.1038/ncomms3489
Hone, D. W. E., Tischlinger, H., Xu, X. & Zhang, F. 2010. The extent of the preserved feathers on the four-winged dinosaur Microraptor gui under ultraviolet light. PLoS ONE 5(2): e9223. doi: 10.1371/journal.pone.0009223
Li, Q., Gao, K.-Q., Meng, Q., Clarke, J. A., Shawkey, M. D., D’Alba, L., Pei, R., Ellison, M., Norell, M. A. & Vinther, J. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335, 1215-1219.
Xing, L., Persons IV, W. S., Bell, P. R., Xu, X., Zhang, J., Miyashita, T., Wang, F. & Currie, P. J. 2013. Piscivory in the feathered dinosaur Microraptor. Evolution 67, 2441–2445.