Tuesday, 7 April 2015

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



Saturday, 24 January 2015

The Lost Tyrannosaurid of Kazakhstan


Tyrannosaurids are best known from the Late Cretaceous deposits of North America and Asia and, it is fair to say, that we have a very reasonable fossil dataset from both continents. And it is also worth reiterating that our understanding of tyrannosaurid paleobiology and phylogeny is one of the fastest moving disciplines in palaeontology. This should surprise nobody as they are amongst the most interesting and impressive of all dinosaurs and, as such, will always inspire a fervent following – myself included.

In recent years we have seen the arrival of new tyrannosaurids such as Teratophoneus, Bistahieversor, Lythronax, Nanuqsaurus and Zhuchengtyrannus - and there are more to come. For instance we already know that there is a second species of Daspletosaurus in the pipeline for publication (Carr & Varricchio 2014) which I will, obviously, blog about in due course and there are one or two other surprises coming in the future as well. Even the relatively unknown Alioramus has undergone an extensive redescription recently due to new fossils being found (eg Brusatte et al 2009, Brusatte et al 2012, and Lü et al 2014) and we have a much greater understanding of the so called “narrow snouts” than we ever did.

So we appear to know a lot about them but, in real terms, we still understand very little and interpretation of the fossils is often subject to debate. Tyrannosaurids of all taxa are actually very similar animals and, to be fair to all the palaeontologists concerned, it is easy to see why there are so many discussions when so many of the anatomical details are often slight. One man’s new taxon is another man’s different ontogentic stage of a current taxon. Morphological change throughout ontogeny is, without a doubt, the biggest single determining factor in dinosaur phylogeny.
     
The origins of Tyrannosauridae and its distribution is also a matter of contention - some people prefer an Asian origin whilst others, by way of example, now favour a radiation that evolved in southern Laramidia. I favour neither nor disregard either and I believe that it is fair to say that there were obviously multiple migration events with taxa of different  groups from both Asia and North America moving back and forth across Beringia which, unsurprisingly, may also give strange or false phylogenetic signals on occasion.  

The earliest true tyrannosaurid is undoubtedly yet still to be discovered and we do not have the best dinosaurian fossil record beyond the Campanian and it would be terrific if there were fossils to be found in the Santonian and probably the Coniacian too – but where would be anyone’s guess.
So with all of this in mind it seems that, despite the relative wealth of fossil material that we have of tyrannosaurids, we are in need of a hell of a lot more to increase the dataset. So it can cause consternation amongst palaeontologists’ when what may be important specimens to science are sold off at auction. Thomas Carr recently highlighted a number of Tyrannosaurus specimens that are held in private hands and, as such, are off limits to professional researchers.

This is unfortunate. There are many who would say that since there are multiple specimens of Tyrannosaurus already in institutions then what is the point of having more? Without increasing the aforementioned dataset then we will continue to have large gaps in our knowledge and never be able to fully understand the paleobiology, morphology and ontogeny of these majestic creatures.

And just to amplify how important it is to have a large sample we need look no further than by using another Hell Creek giant, Triceratops, as an example. There are hundreds of skulls of this animal – hundreds! In fact nobody is really sure just how many there are but when compared with good skulls of Tyrannosaurus (perhaps 20?) then it is amazing that there is still debate regarding the amount of Triceratops species there are, whether Torosaurus is the adult morph of Triceratops and, indeed, the amazing morphological change that is displayed throughout ontogeny.  Indeed, this is just as applicable to other ceratopsians as well.

Hadrosaurs are another extremely well known group and, although there are many interpretations of their fossils, they do not appear to induce the often emotional discourse that follows tyrannosaurids and, to a lesser degree, ceratopsians. And we have not even got to the world of dromaeosaurids and troodontids for which there is a dearth of GOOD fossils but no shortage of opinion and speculation.

Therefore tyrannosaurid specimens are important – all of them and, as we can see, this relates to all clades of dinosaurs as well. So when I learnt that a rather stunning tyrannosaurid maxilla and premaxilla from the Republic of Kazakhstan had come to the surface I became rather excited. There is a diverse fauna of Cretaceous dinosaurs that are recognised from the ex-soviet republic including Hadrosauridae, Ankylosauridae, Sauropoda and Tyrannosauridae amongst others.

Some remains can be locally abundant – hadrosaurs in particular whilst others are known from rather scrappy and indeterminate remains.  Averianov et al (2012) reported on a right dentary from an unspecified tyrannosaurine that was recovered from Late Cretaceous rocks in Kara-Cheku which is located in Almaty Province. This was collected back in 1950 and labelled as belonging to Tyrannosaurus sp. – actually delightfully labelled as “Tirannosaurus”.

Without decrying the specimen too much, it is not the most aesthetically pleasing example but it is, never the less, an important fossil and the authors were able to confirm its assignation to a tyrannosaurine tyrannosaurid. The fact that this is one of the better tyrannosaur fossils from Kazakhstan again reinforces how poor the record is from this country and we hope that more specimens will be discovered in due course.



So where on earth did this new specimen come from? The documentation, such as it is, is fairly vague. It was collected in, and, I quote “…the second half of the previous century...” from the Zhetysu region of Eastern Kazakhstan. If correct then this is certainly of interest since the eastern region has been considered fairly non-productive for Cretaceous vertebrates (Averianov et al 2012) so one wonders about the collecting locality of this specimen. The specimen itself is striking and represents a right maxilla with its accompanying premaxillary attached and is approximately 360mm in length albeit with about <>5% restoration . As you can see, there are at least 12 maxillary teeth in situ and it appears that the four premaxillary teeth are also present and correct. All the teeth are genuine and in position as found.



The maxilla displays a gently convex anterior edge but the posterior appears not to be as acutely convex as is the case in Zhuchengtyrannus (Hone et al 2011). The 12 maxillary alveoli is the typical count for both Tyrannosaurus and Tarbosaurus. The rugose surface is also common in derived tyrannosaurids although this sculpting is more heavily defined in mature specimens – which this does not appear to be.  The subnarial foramen is readily apparent but detail is hard to define. But then this is the real point about this specimen. In comparison with known tyrannosaurid material from Kazakhstan this specimen is actually quite spectacular and really important and we should all look forward to its description.



But there is a disturbing caveat here. The only reason I am aware of this specimen is that I was tipped off that it was going to auction and all this “detail” I have is only because I referred to the auction website and inspected the images. I do not intend to go into detail here but the specimen was sold into private hands for a not inconsiderate amount of money and these pictures I am sharing with you today may very well be the only public record of this specimen now that it has probably disappeared forever.

Now it may be that this specimen is not from Kazakhstan at all and when you have a specimen with so little recorded history then it is a little bit of a punt whether the specimen appears to be what it says it is. Regardless of this, and in the correct repository, careful research should be able to address the taxanomic issues and, perhaps, where the fossil originated from. If it is from Kazakhstan then it is important, important, IMPORTANT!

It does not matter what your opinion is on the buying and selling of fossils – for there are many but one thing remains absolutely certain. It is awful that important specimens, like this tyrannosaurid maxilla, are lost forever in the eternal blackness that is private ownership never to see the light of day and I wish that we could convince these people to donate these specimens to the correct repositories so that they are available for research and so that we can, indeed, fill in these missing gaps in our evolutionary history.

References

Brusatte SL, Carr TD, Erickson GM, Bever GS, Norell MA (2009). A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proceedings of the National Academy of Sciences of the United States of America106(41), 17261–6. doi:10.1073/pnas.0906911106

Brusatte SL, Carr TD, Norell MA (2012). The osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia.  Bulletin of the American Museum of Natural History Number 366, 197 pp., 82 figures, 11 tables Issued February 29, 2012.

Carr, T & Varricchio, D. 2014. A new species of Daspletosaurus from the Upper Two Medicine Formation (Late Campanian, Cretaceous) of Montana and evidence for anagenesis in tyrannosaurine evolution. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp103-104. 

Hone D, Wang K, Sullivan C, Zhao X, Chen S, Li D, et al (2011). A new, large tyrannosaurine theropod from the Upper Cretaceous of China. Cretaceous Research 32 (4): 495–503.  

Lü J, Yi L, Brusatte S, Yang L, Li H, Chen L (2014). A new clade of Asian Late Cretaceous long-snouted tyrannosaurids. Nat. Commun. 5:3788 doi: 10.1038/ncomms4788 (2014).

Averianov, A. O., Sues, H. D., & Tleuberdina, P. A. (2012). The forgotten dinosaurs of Zhetysu (eastern Kazakhstan; Late Cretaceous). Proceedings of the Zoological Institute RAS Vol.316, No.2, 2012, pp.139-147.