Forelimbs, Wings & Other Things

The following blog post was really scheduled for a couple of months ago but I just never got round to publishing it. Having said all that, the science is still cutting edge and just as fascinating as it ever was so I hope you find it of interest.

Forelimb reduction in theropod dinosaurs has long since fascinated. It is a constant source of interest that some of the largest carnivorous dinosaurs reduced the size of their forelimbs to such a degree that it would appear that they were almost becoming vestigial or, at the very least, were only capable of speculative rudimentary function. Chief amongst palaeontologists looking at this enduring mystery is Sara Burch of Ohio University.

To tackle these enduring questions, Burch approached the issue in quite a unique way – firstly by constructing a model of the ancestral forelimb muscle arrangement highlighting any associated plesiomorphies. Then, by looking at the phylogenetic signals generated by examining the evolutionary processes in various theropod lineages, Burch has come up with some useful data.

Both allometric and evolutionary trends suggest that there is no evidence for a general reduction in forelimb size throughout Theropoda. A clade by clade study also revealed interesting myological trends – particularly in Abelisauridae and Tyrannosauridae. Abelisaurids display a quite unique morphology and yet, despite their forelimbs appearing to be useless, the study reveals them to be still functional. It is unlikely that they were capable of much, however, and it may be that the forelimbs were used for sexual stimulation – speculative, of course, but not the first time this has been suggested for abelisaurids or, indeed, tyrannosaurids.

Tyrannosaurids themselves have robust and muscular arms and one suggestion to try and describe a particular use for these forelimbs was that it enabled the animal to push itself up from the ground when required. This particular hypothesis was modelled accordingly with the outcome that there is no justifiable support for the theory. But what about possibly using their arms for grasping or holding prey? Muscle correlation concludes that this was indeed possible supporting the evolutionary trend that reduction in forelimb size is not necessarily about vestigiality but rather to satisfy an evolutionary demand. As with abelisaurids, the theory that tyrannosaurids may have used their forelimbs intraspecifically remains untestable.

Pterosaurs currently remain the biggest flying animals of all time and yet was there a size limit that dictated how big a pterosaur could be and still manage to take off, fly and land? Colin Palmer, of the University of Bristol, and Mike Habib of the University of Southern California, have been addressing these very issues.

 Azhdarchids were the giants of the pterosaur world but estimates of their weight and mass vary considerably. For example, Chatterjee & Templin (2004) suggested a body weight in the region of 70Kg whilst Mark Witton (2008) and Witton and Habib (2014), more realistically for an animal approaching the size of a giraffe, estimated 260Kg. The general consensus is that the latter is probably correct.    

As for wingspan it appears that the most common sizes vary from 5 metres to 9 metres with an upper limit, perhaps, of twelve metres – true giants. Computer models were generated for pterosaurs with 6, 9 and 12 metre wingspans which were as structurally and aerodynamically as accurate as the fossil record allows bearing in mind most of the data is derived primarily from ornithocheirids.

For an azhdarchid to remain in the air depends, essentially, on the available power that is generated by the muscles. The model indicates that these pterosaurs were capable of generating sufficient power to maintain station once airborne and, interestingly, that the ability to stay up does not limit the size of the pterosaur. In other words, pterosaur maximum size is limited by the amount of tensile stress generated but is not limited by size alone. Therefore, it is quite feasible for a pterosaur with a 12 metre wingspan to remain airborne.

What about landing? A large extant bird requires a stopping distance of around 4 metres per second and the models predict that large pterosaurs also fall within this range and the authors point out that the robust hind limbs of azhdarchids make for a pretty sturdy undercarriage so it seems possible that a pterosaur with a 12 metre wingspan could also land safely.

But could a pterosaur of these proportions generate the sufficient power, technique and thrust to launch itself into the air?  It is safe to assume that pterosaurs utilised both forelimb and hind limb musculature to achieve take off and the computer models generated for this research reflect this although, interestingly, 80 to 90% of the required take-off thrust for both birds and pterosaurs is developed from the hind limbs.

Anhanguera forelimb musculature - from Witton & Habib 2010

Taking other factors into account such as oscillation and both launch speed and height indicates that it is problematic for pterosaurs to achieve take off as they get bigger since they could not evolve a sufficient muscular array that would be structurally efficient for take-off. The results suggest that we can predict with confidence that pterosaurs with a 9 metre wingspan could successfully take off but with a lesser degree of confidence that those with wingspans between 9.5 and 11.5 metres could achieve launch.

Thus this research is indicative that 12 metres is currently the absolute wingspan limit for pterosaurs – which we know of. This is really interesting research and it would be fascinating now to see the response if another pterosaur is found with perhaps a 14 or 15 metre wingspan. Unlikely of course but that would really put the fat into the fire because who would not want to know how that would have been achievable and what other factors would need to be considered.

Theropods come in all shapes and sizes and we are familiar with the various sizes and morphological differences in their teeth but what about the actual mandible itself? Is theropod jaw form representative of function or, indeed, is the taxanomic, morphological and functional diversity of the lower jaw a predictor of functional and biomechanical diversity? Emily Rayfield, of the University of Bristol, and her colleagues have been asking these very questions.

A sample size of 103 specimens was analysed using a combination of geometric morphometry and biomechanical metrics. The authors point out that the sample size, although relatively broad, is still nowhere near large enough but included a diverse group sampling which included, amongst others, non-tetanuran theropods, non-maniraptoran tetanurans and maniraptorformes themselves.

  

So do theropods that feed on different things have different shaped jaws? Apparently not since the authors discerned no particular signal from this analysis. What about functional traits in theropod jaws?  The authors recognised 19 traits in 68 of the taxa that are related to jaw robustibility and the enlargement of the coronoid process which contribute to the overall shape and biomechanical variation. As a side note it is worth pointing out the oviraptorosaurs, although included in the analysis, are quite distinct from this overall grouping and were obviously doing something very different.

Theropods that are herbivorous or exhibit omnivorous tendencies display different shaped jaws to traditional carnivorous types which is to be expected. Theropod mandibular evolution throughout the Mesozoic suggests that there is likely to be a link between form and function and this is supported by phylogenetics which does indeed exert a strong signal. However, this is probably exaggerated by the fact that Maniraptora filled many different ecological niches from the Late Jurassic onward which would have demanded more morphological variation. The overall link, therefore, between morphological change  and functional diversity is tenuous at best and suggests that perhaps the shape of the jaw does not always necessarily reflect the evolved and/or derived  state of different theropod jaw mechanics.  

  

References

Burch, S. 2014 Osteological, myological and phylogenetic trends of forelimb reduction in nonavian theropods. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp100. 

Chatterjee, S. and Templin, R. 2004. Posture, locomotion, and paleoecology of pterosaurs. Geological Society of America Special Publication 376: 1-64.

Palmer, C. & Habib, MB. 2014. All time giants of the air: new approaches to calculating the limits to the size of pterosaurs. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp200-201. 

Rayfield, E., Conium, R., Benson, R. & Anderson, P. 2014. Ecomorphological and functional variation in the theropod dinosaur mandible. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp212. 

Witton, M. P. 2008. A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana Reihe B 28: 143-158.

Witton MP, Habib MB (2010) On the Size and Flight Diversity of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on Pterosaur Flightlessness. PLoS ONE 5(11): e13982. doi:10.1371/journal.pone.0013982