Tuesday, 21 May 2013

Fossils are Important ( All of Them )


 
One of the most important things for me in palaeontology as a whole is never to take fossils for granted. It does not matter that some of them are astonishingly abundant and are so well known that they have little scientific importance. We must never lose track that these wonderful artefacts have survived millions of years and give us a vivid picture of life many millions of years ago. It is truly a miracle of nature that anything got fossilised at all when you consider the conditions that are required for fossilisation. Just look at the delicate preservation in the clay of the crustacean Mecochirus pearcei.
Taking such factors into consideration then it comes as no surprise that there are those fossils which are so unusual, so rare and so revealing that their fossilisation appears nigh on impossible. And there are those fossils which, although may actually be quite abundant in some places, bring us closer to the living breathing animal – the very essence of past life.
The most obvious candidates for examples of the latter are trackways of dinosaurs and their contemporaries that wandered over the coastal flood plains and river banks leaving their footprints behind them. These trace fossils can reveal a wealth of information and today’s progressive digital technology widens our knowledge and has opened up a catalogue of data that was unheard of only 25 years ago.
I do not profess to have a great knowledge of trackways and there are many palaeoichnologists out there far better qualified to discuss tracks than I am but I am, never the less, fascinated by them for here is the proof that the bones and skeletons of the animals we find were living, breathing and often fast moving components of a long vanished ecosystem. From trackways we can accumulate detail that helps us determine gait, stance, stride pattern, speed, weight and, most interesting of all, possible behaviour.
Fossilised impressions and remnants of skin also give us tangible evidence of what some dinosaurs may have looked like although there is always an element of caution due to preservational issues. We are all familiar with the mummified hadrosaurs of which Leonardo, a specimen of Brachylophosaurus, is the most well documented of previous times and these fossils have given us considerable insight into the appearance of some dinosaurs. Leonardo, of course, is an exceptional specimen since nearly all of its body is covered in skin revealing various textures and different sized scales and tubercles.
 
A recent specimen of Triceratops is also sending shockwaves throughout the palaeocommunity due to its exceptional skin preservation and we look forward to the publication of the associated paper.  As noted in my previous post we even have skin impressions from Tyrannosaurus and a whole host of other dinosaurs. And, of course, we have the well documented feathered dinosaurs from China that has given us a whole new dimension when it comes to visualising dinosaurs. Even colour is slowly being decrypted from some fossil feathers and this is an area that I expect to see a lot more exciting developments in the future.
It is hard to believe that any soft parts of an animal would fossilise and yet, in exceptional circumstances, this too has happened. The most famous example of this is the little theropod Scipionyx from the Lower Cretaceous of Italy. This wonderful little specimen has the muscles, intestines – even the liver are all preserved and give us many clues to aid in our understanding of dinosaur physiology (Dal Sasso & Maganuco 2011). And we must not forget too, the remarkable work by palaeontologist’s such as Mary Scweitzer et al (2007) who have actually managed to recover blood vessels and other soft structures from the bones of a 68 million year old Tyrannosaurus.
Away from dinosaurs there are other superb fossils that must be noted.  There are occasions when specimens are found that, although flattened, are so well preserved in the fine grained sedimentary rocks that the outline of the whole body is preserved.  The wonderful ichthyosaur specimens from Holzmaden enabled scientists to reconstruct these marine reptiles accurately for the first time due to these outlines in the shale.  Initially reconstructed without a dorsal fin or the crescent shaped tail we are all now very familiar with the dolphin-esque shape of the ichthyosaur.
Similarly, the lithographic limestones of Solnhofen have opened a window into the past of the Upper Jurassic landscape of Bavaria. Rightly famous for the fossils of Archaeopteryx, there are also wonderfully preserved examples of the animals that shared Archaeopteryx’s environment and, of particular note, are the exquisitely preserved specimens of fish, marine reptiles and even pterosaurs with clearly preserved outlines of the wings.
Amber fossils are amongst the most beautiful examples of ancient life that are preserved to an almost unparalled degree. We are all familiar with the notion of insects captured in amber, most famously eulogised in Jurassic Park, and these are the most common animals preserved but, occasionally, even vertebrates are trapped and these are the rarest amber fossils of all and have included examples of frogs and lizards.
 
Stomach contents are yet another source of data. Although not commonplace they do occur on occasion – Baryonyx, most famously, had the remains of both fish and an iguanodontid preserved. The aforementioned Leonardo’s last meal was also exquisitely preserved and consisted of ferns, magnolias and conifers and further examination of pollen grains in the gut reveal at least forty different species of plant. Specimens of Sinocalliopteryx preserved both dromaeosaurid and bird remains (Xing et al 2012) whilst various marine reptile remains reveal diets that comprise, not only of their reptilian contemporaries, but also ammonites, belemnites and, in one case, the osteoderms from a thyreophoran dinosaur although this cannot be categorically confirmed due to possible taphonomic distortion.
Naturally enough, what goes in must come out and coprolites are another of those extremely common fossils that are greatly underrated. Many are simple phosphatic nodules that may be identified by their peculiar structural patterns and are often outwardly rather plain looking. But some show clear inclusions and these are very much more interesting. They reveal a great deal about an animal’s diet and although it is nigh on impossible to identify which animal may have produced these mineralised faeces we can still take some educated guesses. The most famous coprolite in recent years is the specimen that almost certainly came from a T.rex and was found to contain a mass of pulverised bone (Chin et al 1998).
There are other examples of evidence which also indicate possible behaviour. For identifying predator/prey relationships it is obviously fossils that display signs of predation that you need. These include puncture marks and drag marks that sometimes even display the marks of the jagged edges of serrated teeth. Although these bones are not that common, I suspect that there are many more in collections than are actually realised and that the marks have either been missed, misinterpreted or disregarded as preservational phenomena.
Of course, tooth and claw marks on bone do not actually confirm that the feeding animal had actually killed the prey item – only that it fed on the remains. Much more interesting are bones that show signs of predation but have healed up because then they obviously survived being attacked which is indeed evidence of attempted predation. These are much rarer specimens but both hadrosaur and ceratopsian dinosaur bones are known displaying such pathologies (eg Carpenter 1998, Happ 2008).
So it can be seen that we have this great wealth of information that enables us to interpret how animals lived and died in the past, what they may have looked like, how they moved and what the environment was like that they lived in. And yet there is still so much that we don’t know and we are still scratching the surface of certain aspects of palaeontology and desperately need more fossils and so much more data.
I, and so many others, are constantly referring to sampling and preservational issues with good reason – there are still so many gaps in the fossil record and without more specimens in numbers to fill those gaps then there will always have to be elements of conjecture and speculation – amply supported by the available science of course.
This is why fossils are so important and is why we must never ever take them for granted – they are, after all, a finite source. I never have forgotten this and I am still inspired every time I see a fossil that has seen the light of day for the first time in perhaps millions of years. So next time you are casually throwing aside the odd brachiopod as you look for more “significant” remains just remember how lucky we all are that they are even there in the first place.
References
Carpenter, K. 1998. Evidence of predatory behavior by theropod dinosaurs. Gaia 15: 135 - 144.
Chin, K., Tokaryk, T.T., Erickson, G.M. and Calk, L.C. 1998. A king-sized theropod coprolite. Nature 393: 680 - 682. 

Dal Sasso, C. & Maganuco, S. 2011. Scipionyx samniticus (Theropoda: Compsognathidae) from the Lower Cretaceous of Italy: Osteology, ontogenetic assignment, phylogeny, soft tissue anatomy, taphonomy, and plaeobiology. Memorie della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano. XXXVII (1) 1 - 281pp.
Happ, J. (2008). "An analysis of predator-prey behavior in a head-to-head encounter between Tyrannosaurus rex and Triceratops". In Larson, P.; and Carpenter, K. (editors). Tyrannosaurus rex, the Tyrant King (Life of the Past). Bloomington: Indiana University Press. pp. 355–368.
Schweitzer, M.H., Suo, Z., Avci, R, Asara, J.M., Allen, M.A., Teran Arce, F. & Horner, J.R. 2007. Analyses of soft tissue from Tyrannosaurus rex suggest the presence of protein. Science 316: 277 - 280.
Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012.

A fossil track of the future?

 

Saturday, 11 May 2013

Hypothesis or Proof?


 
Premiering back in 1980, Carl Sagan’s Cosmos led us on an enthralling tour of our universe and took us on a crash course of science. We watched as he strolled around the heavens and our planet as he guided us through the very essence of what it meant to be human in the natural order of things. Sagan was a wonderful guide who was clearly ahead of his time, highlighting such issues as the possibilities of nuclear Armageddon and global warming and there are many quotes by Sagan that are still cited today.

Chief amongst them and most often used by palaeontologists of today, is this one:

“The absence of evidence is not the evidence of absence.”

Most people would tend to agree with this, as would I, and yet should we? On the face of it this is a very strange concept since we are suggesting that we should accept certain issues as a given despite a lack of physical proof. In other words perhaps what we should actually be saying is:

“The absence of evidence is the evidence of absence.”

Let me explain what brought this on. The well documented, and almost universal, criticism of the makers of the already much hyped Jurassic Park 4 for not having feathered dromaeosaurids is scientifically well founded since we have extensive fossils now of these dinosaurs clearly displaying such coverage. It is bizarre that the producers have seen fit not to feather their raptors and their argument that it is best to maintain a form of continuum from the previous films is a weak excuse. Perhaps it is a blind to throw us off the scent and they will indeed unleash feathered dromaeosaurs but I fear not.

Then last week the trailer for the new Walking with Dinosaurs 3D movie went on line globally and generally met with a pretty good reaction. Scott Hartman, over at Skeletal Drawing, naturally enough featured a post about it since he was involved as one of the anatomy design team for the production. One of the comments on his blog mentioned the fact that there were no shaggy gorgosaurs in the trailer and Scott suggested that the movie world is not quite ready for that yet and I tend to agree with that statement.

In the comments section I pointed out the following:

And yet, technically, the absence of shaggy gorgosaurs is correct since Tyrannosauridae is still fuzz free for now albeit phylogenetically likely. We cannot really moan about the film makers in this instance because they are depicting tyrannosaurids accurately - in as much as the fossil record permits.”

To which Scott replied:

”That doesn't necessarily follow - absence of evidence is not evidence of absence here. The phylogenetic data is really the only data we have, so it's what we should be relying on.”

Perfectly reasonable comment and yet my initial reaction was to be a little bemused. We can justly criticise film makers for not including feathered dromaeosaurs because there is ample physical proof and yet can we justify criticism of film makers for not including feathered tyrannosaurids when there is no actual physical evidence currently available?

The phrase itself demands closer scrutiny. In effect, and in the simplest of terms, the following shows a classic example:

Many people take glucosamine and chondroitin tablets to ward off the effects of osteoarthritis (OA) and yet there is no solid data that there is any form of relationship between the two ingredients and the prevention of OA. Therefore absence of evidence is clear but, in reality, those who manufacture or believe in the properties of these tablets would simply point out that just because there is no proof currently available it does not mean that the tablets do not work – therefore absence of evidence is not evidence of absence.

Now I confess that this has very little relevance in palaeontological terms but it does show the dichotomy and how easy it is to manipulate the lack of data into possibility – even probability. However, in the case of tyrannosaurids, we have a completely different picture. I suggested that since there is no physical proof of feathers or fuzz in tyrannosaurids then we should not necessarily take it as a given simply because, and as Scott pointed out, the phylogenetic data is highly indicative that they were.

So in this case, does the absence of physical evidence mean that there is no evidence for feathered tyrannosaurids at all? Well of course not – in fact virtually the whole coelurosaurian clade displays integument of one form of another and Tyrannosauridae is currently the last group devoid of physical evidence. Very important here – we are referring to tyrannosaur-ids and not tyrannosaur-oids of which there are indeed animals displaying integument of which the most famous in recent times is Yutyrannus (if, in fact, it is a tyrannosauroid).

So we have lots of phylogenetic data to support the probability of feathered tyrannosaurids but this still needs to be quantified and rigorously tested. When this is done then the dataset will provide a statistical probability of confidence in the hypothesis. If there is confidence in the dataset then you must turn the process on its head and try to prove that there is NOT a probability of feathered tyrannosaurids. This is an extremely important part of the process and lends substantial credence to the theory.

As a result, the phylogenetic signal is highly indicative that tyrannosaurids were indeed covered in some form of fuzz or feathered plumage. And yet for all that detail, all that probability – indeed all of this “evidence” – there is still no proof. And this is where we have to be a little careful since the available data is so compelling that we sometimes forget that physical evidence is required for confirmation.
BHI 6230 (Wyrex) - No feathers here...
 
The very essence of all science is based on this fundamental process and now the onus is on finding fossils of tyrannosaurids that may preserve impressions of fuzz/feathers. There are many specimens already in collections that need re-examining as well as those specimens that may still reside in their jackets and, of course, there are still many wonderful fossils out there awaiting discovery. And, if it exists, then we should be able to find the evidence since our detection techniques are much more advanced these days – especially with UV light.

But, for the sake of argument, what if we never find a feathered tyrannosaurid? What then? I mean we have the phylogenetic data but still no proof. So we will rerun our data, perhaps add more updated information and check and double check – but still no fossil. And then we will question ourselves again since we keep performing the science and keep coming out with the same results. And then if we still find no fossils then what? Perhaps we have to consider the possibility we may be wrong.

But this is unlikely to happen and we will continue to search for feathered tyrannosaurids because they must exist – the phylogenetic data insists it to be true. And this is the point – we must not then fall into the trap of “just because there are no feathered tyrannosaurids, this does not mean they did not exist”.

“Absence of evidence is not evidence of absence” must not become a given in science – not without the proof to back it up. Circumstantial evidence, no matter how strong, can lead to the wrong conclusion and we must all be aware of this. What this lack of feathered tyrannosaurid fossils does indicate, however, is that they are, if they exist, at the very least exceptionally rare but this, again, is not actually proof!

There are so many variables as well – sampling and preservational biases are probably the biggest issues here. It is also worth pointing out that the phylogenetic dataset also has vast gaps in it although there is still a wealth of relevant data available. As usual more fossils are required – especially when you consider how few tyrannosauroids are actually known throughout the Cretaceous.

So “absence of evidence is evidence of absence” is kind of true but, at the same time, is as equally fallible as the other. I accept the near certainty that tyrannosaurids had some form of integument but it is not enough to merely want to believe. In the end we have the data, we have the hypothesis and all we need now is the proof.

Wednesday, 1 May 2013

Tyrannosaur Forelimbs ......... Again


 
Let me set the scene. It had been a long day and as I walked through the front door of my place I thought I would head straight for the sofa and have a five minute chill before getting some dinner ready. As I lay on the sofa and relaxed I looked upward and there was something that caught my eye that made me realise something about tyrannosaurid forelimbs.
Allow me to explain. Above the sofa there are two wall mounted shelves which have a few fossils on display. These include a mosasaur dentary, a woolly rhinoceros cervical vertebra, a piece of wonderfully preserved tree bark from the Oxford Clay and one of those polished belemnites from Morocco. But central to the display is a cast of a right forelimb from Albertosaurus libratus and, as I looked up, I saw the wickedly hooked manus unguals peering over the edge of the shelf and it occurred to me that tyrannosaur forelimbs were actually very functional. In fact they were not vestigial at all.
Let me explain. I have discussed theropod forelimbs before – particularly those of tyrannosaurs – and one of the more common explanations for the reduction in the size of  tyrannosaur forelimbs was that they actually served no use anymore and that they were, indeed, becoming vestigial. There is a school of thought that had the dinosaurs escaped the end of Cretaceous extinction then tyrannosaur forelimbs would have disappeared altogether.
There is a wonderful depiction by palaeoartist Greg Paul that features in the popular Scientific American Book of Dinosaurs (2000) in which he depicts how dinosaurs may have evolved post Cretaceous and, central to this typical Paul-ian scene, are derived tyrannosaurs with only little stubs remaining where there were once forelimbs. Interesting but I suspect probably incorrect (I would share this image but suspect this may cause a problem).
 
From Lambe 1917.
 
The forelimbs of theropod dinosaurs have undergone significant evolutionary change throughout the whole of dinosaurian history. They have displayed various morphologies and evolutionary trends that, although at first they may appear to follow a convergent path ie reduced size due to bipedalism, actually demonstrate significant differences.
For instance, dromaeosaurids and other similar small theropods have well developed, highly functional forelimbs whilst tyrannosaurids, as we are all aware, appear to have highly reduced and less functional forelimbs but, as we have only recently been examining, large theropods such as the therizinosaurs and Deinocheirus, despite them being Late Cretaceous theropods that were contemporaries of tyrannosaurids, evolved spectacular and large specialist forelimbs.
So there is obviously an element of specialisation displayed here. This can evolve in a number of ways that includes loss and/or modification of digits and carpals, the various morphologies in the manus claws and the variable ways that the muscle framework attached.  As far as tyrannosaurids are concerned (and most other large theropods for that matter) the reduction in the size of the forelimbs was largely due, in part, to their large size but this is only part of the story.
The Late Jurassic Ceratosaurus displays a robust, albeit short, forearm adorned with four digits – a primitive trait retained not only by ceratosaurs and abelisaurs but even primitive tyrannosauroids. It is strange that the derived Late Cretaceous abelisaurids, such as Carnotaurus and Majungasaurus also retained the condition since, if these forelimbs were becoming vestigial, one would expect that, since there is around 85 million years of evolution between Ceratosaurus and Majungasaurus, that the forelimbs would have long disappeared – or am I being over simplistic?
We understand, especially from the well sampled Late Cretaceous faunas of North America that the duration of a species, on average, was between 0.9 to 1 million years – in some cases less. Of course, there are exceptions and, by way of example, the hadrosaur Prosaurolophus maximus appears to have survived over a period of 1.6 million years (McGarrity et al 2013) but this, for now, is the exception rather than the rule. It would seem likely that rapid species turnover and evolution would have accelerated the loss of the forelimbs if they were indeed, vestigial.
The caveat here is that, if we use the Dinosaur Park Formation as an example (Mallon et al 2012), theropods actually remain quite stable taxa-wise whilst all around them the mega herbivores are in an almost constant state of evolutionary flux. And yet time and evolution wait for no dinosaur and theropods were no exception. Environmental, ecological and biological pressure would still have driven evolutionary adaption and change.
So was theropod forelimb evolution destined for vestigiality? Hard to quantify but it would appear there is something else going on here. Perhaps one thing that does support vestigiality is the uniform reduction in digit size in these more primitive forms – that is they all appear to be shrinking at the same rate. But this is where tyrannosaurids differ.
Despite having been discussed at length by palaeontologists over the years, the function of the tyrannosaurid forelimb is still no closer to being solved although there are a few good possibilities. However, the one thing that everyone agrees on is that, despite their size, the tyrannosaurid forelimb was actually robust and quiet strong, capable of sustaining high stress loadings and (if it was feasible) able to lift a considerable weight.
 
Not exactly "vestigial".
 
So, being simplistic again, if the forelimb was becoming vestigial then why reduce the size of the forelimb only to retain significant strength and power? Of course the dichotomy here is that despite being reasonably functional the reduced size of the forelimb greatly reduces their use! This can drive you to distraction but there are factors worth considering.
First of all, and as we have already mentioned, theropods evolved smaller forelimbs because bipedalism demanded it – that is an undeniable fact. Evolution then had the careful balancing act of ensuring that the forelimb complemented the animals’ lifestyle without compromising balance and agility. This was remarkably successful, not only in terms of dinosaurian evolution, but as one of the greatest terrestrial body plan designs of all time. Theropods were amongst the first and amongst the last dinosaurs ever to exist and this basic blueprint lasted for the duration of their tenure on the planet.
Interestingly, the well-publicised paper looking at the evolution of posture, morphological change and the driving forces behind them in bird-line archosaurs (Allen et al 2013) again highlights the exceptional adaptability of the theropod body plan. One of the key findings in the paper is that as the forelimbs of these bird-line theropods became larger, their centre of gravity edged forward and the gradual evolution of the extant crouching bird position came into effect.
It was originally thought that it was the lightening and shortening of the tail that was the locomotor behind the development of the crouch but this excellent research using sophisticated three dimensional modelling techniques clearly demonstrates that the forelimbs were the major contributing factor.
It came as a surprise to find out just how much influence the forelimbs had on balance and posture especially when compared to the tail but the trend was clear. This new research also has implications for tyrannosaurids in as much as it confirms what we already knew – that is that tyrannosaurs, despite being closely related to the maniraptoran line, retained the large powerful tail to counterbalance the enlarged head and that the reduced forelimbs were clearly not as important to them as it was to those theropods that were heading down the avian road. Obvious I know but nice to have it confirmed by the science.
And yet, just to ensure that there are still more questions to answer, John Hutchinson highlights the fact that since the forelimbs were influential in the development of body mass distribution, then they equally affect how the hind limbs developed and functioned. In other words we have to assess the whole locomotor structure rather than just part. This is obvious in so many ways and yet it is something that we all miss when discussing these kinds of topics. One simple evolutionary trend affects the whole of the animal – everything is affected, everything changes – even if we have no fossil evidence. It is not always easy to remember this but it is always true.
Is it possible then that the tyrannosaurid forelimb affected the balance of the animal? Could they have conceivably had any influence on the agility and stability of a tyrannosaur in motion? It hardly seems possible that these small forelimbs could have any realistic effect and yet I am reminded by the halteres in two winged flying insects. These tiny club-shaped structures are situated behind the wings and enable the insect to maintain a high degree of equilibrium. Insects are not dinosaurs and yet perhaps we should not completely dismiss the possibility of tyrannosaurid forelimbs acting as a balancing aid. 
So if some theropods were clearly evolving their forelimbs for, grasping, flapping, flying and, in the case of Deinocheirus and therizinosaurs, for some as yet undetermined functional use, then tyrannosaurid forelimbs  do appear underwhelming  and yet we have other odd closely related theropods with strange arms – the ornithomimids.
Gallimimus
These appear, on the face of it, to be long and highly adaptable – especially for grasping and yet these forelimbs are surprisingly weak when compared to tyrannosaurs. They demonstrate a surprising lack of manoeuvrability due to an under developed antebrachium (that is the area between the elbow of the wrist) and this limited their ability to pronate or supinate. So, despite their impressive appearance, it appears the arms of ornithomimids were best suited for simple reach-and-grasp use.
The thing that should grab your attention with tyrannosaurid forelimbs is not their relative small size but actually that they are quite robust and that the unguals (claws) are potent barbs although in more derived tyrannosaurines such as Tyrannosaurus they do become blunter. Despite the fact that there only the two digits represented, there is a remnant third metacarpal present and indeed does become more vestigial as tyrannosaurids became more derived - so much so that in taxa such as Tarbosaurus it is nearly completely fused with metacarpal 2.
And this is where we came in. Ceratosauria appears to demonstrate a uniform reduction in digit size throughout their long history although I confess to being mystified as to why it would take so long to lose their forelimbs IF they were becoming vestigial. Tyrannosaurids on the other hand clearly lost their digits in sequence – so much so that we find two digits appearing to gradually disappear throughout the tenure of Tyrannosauroidea although there are massive gaps in the fossil record so we must be cautious. The gradual fusion of metacarpal 3 onto metacarpal 2 in derived tyrannosaurids clearly demonstrates this process as digit 3 disappeared.
Metacarpal 3 (bottom) slowly disappearing in Albertosaurus
But the remaining two digits are clearly of importance to tyrannosaurids and, as always, it is difficult to ascertain what and why they were used. In my previous discussion about tyrannosaur forelimbs I speculated that if tyrannosaurids were feathered to some degree then perhaps they were used in courtship rituals – a kind of “don’t eat me – I’m here to mate” signal. You would think that tyrannosaurids must have had some form of intraspecific communication – even if it was just for the mating act.
Indeed, it was gratifying to hear Thomas Holtz Jr during his recent talk at the Burke Museum in Seattle a short while ago suggest exactly this and the more you study these structures the more intriguing they become. However I have been leaning toward a similar but subtly different use for them.
Pathologies in the forelimbs we have are not uncommon and they demonstrate various fractures, remodelling and scarring right through the entire forelimb right up to, and including, the furcula. There are various reasons for these pathologies and they could be as simple as the animal falling down and breaking an arm. Tyrannosaurids are huge animals and a fall onto a forelimb could have caused considerable damage and pain.
However, it occurred to some of us that perhaps the forelimbs were used to assist in copulation and gave the males some purchase during the act – as well as helping to fend off an over aggressive female. This made some sense since trying to hang onto a large multi-tonned female during copulation would have caused considerable stress to the forelimbs and injury would appear inevitable.
Gorgosaurus sp. (BHI#126846) displaying
 acute pathology to the scapulacoracoid
However, as pointed out by others (eg Rothschild & Molnar 2008), this would seem unlikely since the T. rex Sue (FMNH PR2081) shows considerable pathologies to the humerus and scapulacoracoid and – of course – this specimen is widely accepted to be a female. It is still worth pointing out that sexual dimorphism in tyrannosaurids is unproven although it does appear that there are specimens that display both gracile and robust features so dimorphism seems likely.
But I do not believe that this should disqualify the mating hypothesis since tyrannosaurids led such a hazardous life and multiple pathologies are found throughout their entire skeleton. It is entirely conceivable that the forelimb was used in the act of mating and, at the same time, was equally susceptible to injuries that occurred during hunting, falling and the other day to day hazards that befell any carnivorous dinosaur during life.
Of course (and how many times have I said this?) this is all behavioural inference and likely to be unproven and yet the more you look at these enigmatic structures the more you realise that there is so much more to them than meets the eye. For me, I just cannot accept that what is actually a robust and derived specialist bit of equipment was actually vestigial. But, as always, I remain open minded and look forward to the next hypothesis regarding these extraordinary forelimbs.
And a word of thanks
The viewing figures for this blog recently crashed through the 100,000 mark and I feel really humbled by that. Bloggers, such as myself, who want to make a contribution to our science will never have the aura and pulling power of the uber bloggers such as Dave Hone, Brian Switek and Darren Naish but we all contribute in our own way and long may that continue.
So a very big thank you to all of you who have taken the time to read these pages - it really does make all the difference.
 
References

Allen, V., Bates, K.T., Li, Z., & Hutchinson, J.R. (2013) Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs Nature DOI: 10.1038/nature12059
Lambe, L. 1917. The Cretaceous theropodus dinosaur Gorgosaurus, Memoirs of the Geological Survey of Canada. 100, pp. 1–84.
Lipkin, C., & Carpenter, K. (2008) Looking again at the forelimb of Tyrannosaurus rex. In: Tyrannosaurus rex, the tyrant king. Larson, P. & Carpenter, K. (eds.). Indiana University Press. Bloomington & Indianapolis, Larson, P. & Carpenter, K. pp.167-190.
Mallon, Jordan C., Evans, David C., Ryan, Michael J., Anderson, & Jason S. (2012). Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada Palaeogeography, Palaeoclimatology, Palaeoecology Volumes 350-352 pp. 124 – 138.
McGarrity, C.T, Campione, N.E. & Evans, D.C. (2013) Cranial anatomy and variation in Prosaurolophus maximus (Dinosauria: Hadrosauridae).Zoological Journal of the Linnean Society 167(4): 531–568
Paul, G.S. (Editor) 2000. The Scientific American Book of Dinosaurs. St. Martins Press. pp. 384-385. ISBN-10: 0312262264
Rothschild, B. M. & Molnar, R. E. 2008. Tyrannosaurid pathologies as clues to nature and nurture in the Cretaceous. In: Tyrannosaurus rex, the tyrant king. Larson, P. & Carpenter, K. (eds.). Indiana University Press. Bloomington & Indianapolis, Larson, P. & Carpenter, K. pp.287-304.


Obviously not a true comparison but interesting none the less.