Thursday, 22 September 2011

Tyrannosaurid Forelimbs - Why, What or Something Else?

The small arms of Tyrannosaurus rex have provoked discussion and reaction since the animal was first discovered in 1902 (not including the 1900 specimen BMNH R7994 “Dynamosaurus imperiosus”). It is a fact, however, that very few examples of the forelimb were recovered until MOR 555 was found south of Fort Peck Lake in 1988 where a nearly complete left arm and manus were excavated. And because of the increase in specimens found since the 1990’s, there has been more material made available which has enabled further study.

The original theory, proposed by Henry Fairfield Osborn in 1906, was that they were an adaption to assist in the act of copulation although he only proposed this after accepting that the relatively small humerus found with the specimen actually belonged to the animal. Other theories have suggested that the arms were used to help brace the animal and push it up as it rose to stand up (Newman 1970) or that, indeed, these limbs were actually of no consequence whatsoever and were degenerate and were actually in the process of being lost.

For me, one of the real issues with trying to demonstrate theories about the forelimbs of Tyrannosaurus is that we don’t think laterally enough. First of all, reduced forelimb size is almost universal throughout Theropoda. Since dinosaurs first evolved in the Triassic, theropods were bipedal, largely carnivorous and the forelimbs were less than two thirds the length of the hindlimb (Holtz & Osmolska 2004). There were general reductions in the length of some digits and metacarpals and the manus generally ended with sharp recurved claws.

Bipedality obviously demanded a reduction in the size of the forelimb – for me that is a given. However, tyrannosaurids have taken this reduction to an extreme and here is another point to bear in mind. If we look at the clade Tyrannosauridae as a whole, then we can see that all tyrannosaurids have greatly reduced forelimbs so we are not looking to determine the use of these forelimbs only in Tyrannosaurus – and that’s important.

With odd exceptions then (Deinocheirus being the most obvious), theropods all have short forelimbs to various degrees. Early coelophysoids had fairly useful forelimbs as did both basal and derived tetanurans whilst ornithomimosaurs had extremely beneficial elongate forelimbs. At the other extreme are the abelisaurids and Carnotaurus displays the smallest forelimbs possible for an animal approaching 25 feet in length – and seemingly useless (but see Ruiz et al 2011). They make tyrannosaur forelimbs appear positively huge. Also of significance is that no theropod could reach its mouth with its manus.

Did theropod forelimbs help with balancing, turning and agility? Possibly. Smaller and lighter forelimbs would certainly help the animal turn and it has also been suggested that theropods held their arms backward and against their bodies in situations where speed and agility was required (Carrier et al 2001). One thing seems certain and that is that reduced sized arms would certainly not help any theropod if it tripped whilst walking or running and this was one of the principle objections against large theropods being able to run fast. And yet there are multiple examples of theropods with healed fractures on bones such as the ribs that are indicative of this very scenario.

So with this overall view of theropod arms, it can be seen that tyrannosaur arms do follow a general pattern but they are different in other ways. Significantly, there appeared to be no further reduction in limb size once it had been established in Tyrannosauridae since the ratio between forelimb and hindlimb was fairly constant from the Campanian to the end of the Maastrichtian.

The forelimbs of tyrannosaurids were strong, agile and capable of coping with powerful stress forces but they had a limited range of motion. The two claws faced in opposing directions and were ideally designed to act like barbed fishing hooks and would not have easily been dislodged from the flesh of a prey animal. The biceps were extremely large and were the driving force behind the forelimbs ability to bear weight – in the case of Tyrannosaurus, that is estimated to exceed 400 pounds.

As if to provide evidence that the forelimbs were subjected to substantial forces, the bones in tyrannosaur arms are often found with pathologies – that is they have been fractured or broken and have healed up accordingly. This demonstrates that tyrannosaurs could cope without the use of an arm or two for a period of time – certainly long enough for the bones to heal. Some think that this is actual evidence that the arms were not up to the job and that they were poorly adapted but then why would they be subjected to such pressures?

Perhaps the forelimbs were much more useful to juveniles and allometric studies of limb proportions in tyrannosaurs suggests that the forelimbs were relatively longer in juveniles and thus of more functional consequence, especially if their dietary requirements were different when juvenile. Recent study of the well publicised Tarbosaurus juvenile (Tsuihiji et al 2011) seems to complement this observation by uncovering other allometric implications for different feeding strategies in the skull.

So where does this all this lead? What conclusions can be made about the use of these forelimbs in tyrannosaurids? Well actually very little. For me, the belief that they were vestigial organs and gradually being lost does not quite ring true since, if that was the case, then why were they still so powerful? Nature tends not to bestow natural power for nothing and certainly not more power than the animal needs.

Other suggestions include nest or bed scraping (unlikely and surely they would use the foot?) and egg rotation (just unlikely). Getting back to the more popular and realistic theories, the powerfully constructed forelimb has recently been determined to be a not insignificant aid in predation and would have helped the tyrannosaur keep hold of its prey (Lipkin & Carpenter 2008). They may have also helped the animal manoeuvre the carcass as the animal fed.

Strangely, and going back to the very first theory put forward by Osborn, procreation has also been suggested in as much that the forelimbs helped the male cling onto the female during copulation, and perhaps there is something in this. This idea has some merit since tyrannosaurs obviously displayed at least some intraspecific interaction as demonstrated by the well documented face biting injuries (Tanke & Currie 1998).

For me, it also seems a possibility that perhaps the forelimbs may have actually been used for courtship purposes. Because it is a behavioural implication, it is completely untestable, but perhaps they were subtle signalling devices demonstrating a male’s intention to mate and the females signal that she was receptive. If courtship was a body rubbing issue, perhaps the forelimbs were also used to “stroke” and groom each other in all the right places.

This appears to be a reasonable suggestion since you would imagine that animals as powerful and as dangerous as tyrannosaurs would need some form of mating ritual so that they avoided hurting each other unnecessarily during the mating season. In the end, this continual speculation about the purpose of tyrannosaur forelimbs will go on but you always hope that the next fossil found may provide the answer to this fascinating and enduring question.


Carrier, D.R., Rebecca M. Walter and David V. Lee (2001). Influence of rotational inertia on turning performance of theropod dinosaurs: clues from humans with increased rotational inertia. Journal of Experimental Biology (Company of Biologists) 204 (22): 3917–3926. PMID 11807109.

Holtz, T.R., Jr. & H. Osmólska. 2004. Saurischia. Pp. 21-46, in D.B. Weishampel, P. Dodson and H. Osmólska (eds.), The Dinosauria. Second Edition. University of California Press.

Lipkin, C., and Carpenter, Kenneth (2008). Looking again at the forelimb of Tyrannosaurus rex. In Carpenter, Kenneth; and Larson, Peter E. (editors). Tyrannosaurus rex, the Tyrant King (Life of the Past). Bloomington: Indiana University Press. pp. 167–190. ISBN 0-253-35087-5

Newman, BH (1970). Stance and gait in the flesh-eating Tyrannosaurus. Biological Journal of the Linnean Society 2: 119–123.

Osborn, H.F., Brown, Barnum (1906). Tyrannosaurus, Upper Cretaceous carnivorous dinosaur. Bulletin of the AMNH (New York City: American Museum of Natural History) 22 (16): 281–296.

Ruiz, J., Angélica Torices, Humberto Serrano and Valle López (2011) The hand structure of Carnotaurus sastrei (Theropoda, Abelisauridae): implications for hand diversity and evolution in abelisaurids. Palaeontology 54 (5) Article first published online: 19 Sep 2011 DOI: 10.1111/j.1475-4983.2011.01091.

Tanke, D.H., and Currie, Philip J. (1998). Head-biting behavior in theropod dinosaurs: paleopathological evidence. Gaia (15): 167–184. ISSN 0871-5424.

Tsuihiji, T., M. Watabe, K. Tsogtbaatar, T. Tsubamoto, R. Barsbold, S. Suzuki, A. H. Lee, R. C. Ridgely, Y. Kawahara, and L. M. Witmer 2011. Cranial osteology of a juvenile specimen of Tarbosaurus bataar from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. Journal of Vertebrate Paleontology. 31(3).


Anonymous said...

Many theropods do possess shortened forelimbs, but this is, IMO, due to anteriorization - the mathematics of the dinosaurian body and its balance of mass and size.

For example, the frontal body of the theropod has three main organs, the forelimbs, neck and the head. By theory, if the head is small, the neck will probably be elongated and there will be very long forelimbs. Ornithomimus or a sauropod could fit this.
Another example: if a theropod has a large head, by theory evolution will produce a short neck and small arms to level it with the rest of the body. This also results in a longer tail, in the case of Tyrannosaurus, to balance its heavy skull.

The exact puropse of this anteriorization process in Tyrannosaurus rex is beyond me, but I think if we want to know more about the evolution of tyrannosaur forelimbs, we should look more at the anatomy and evolution of the head in T. rex. Tyrannosaurus evolved a large head, obviously, to eat more in a plentiful of food.

Mike said...

Tooth picks. It eats by tearing great chunks of meat and bone and swallowing like a crock. Some of that gets hung between the teeth. Tooth picks.

Andrea Cau said...

Compared to basal tyrannosauroids, tyrannosaurids' arms are clearly reduced in the size of muscular attachment areas and hand/phalangeal proportions. It's possible that tyrannosaurids were in the beginning of the reduction trend that abelisaurids almost had completed. It is interesting that both abelisaurids and tyrannosaurids show more rounded and globode humeral heads compared to other large theropods: although largely speculative, this could mean that these reduced arms restricted most of their mobility at the shoulder joint.

Mark Wildman said...

Thanks for the comments everyone.

Some interesting points there but although anteriorization may be a contributing factor, I'm not convinced. The heads on tyrannosaurs are indeed massive relative to the forelimbs, but why reduce them? Why not get more massive overall or evolve a bigger tail to take the increased weight of the skull? Of course, you may very well be correct and I confess it is a hard nut to crack.

An old favourite this one but, as already stated, theropods could not reach their mouths with their forelimbs, let alone pick their teeth!

Thanks for the detail Andrea and, yes, the Ruiz paper is indeed indicative of this.

Anonymous said...

Some interesting points.
I'm going to preface this with 'I'm only an artist who draws dinosaurs' before launching into my comment....

I think there's something to be said for the idea that the size of the arms helps maintain the balance between the front and the back of the animal. The big heavy head as the main 'prey interaction device' in adult Tyrannosaurs getting priority for weight distribution.
The arms may have had more use for prey interaction in young tyrannosaurs, where they are proportionately bigger and prey items were possibly more diverse.
So they are retained in adults but only as a artifact of their juvenile requirement....
Could be talking out of my bum.

Mark Wildman said...

It would be interesting to know if there has been any research or, indeed, if there is any ongoing research into how tyrannosaurs maintained their balance and whether or not the size of the forelimbs would have made any significant difference.

Perhaps it would be possible to "morph" allosaur-sized arms onto a tyrannosaur and run a comparison to see what the differences and implications might be. Any takers?

Anonymous said...

I say, look at the T-Rex's cousin,the chicken,as far as body proportions,from the neck down.We all laugh at Rockie as he chased one,but I saw how an animal of 40 ft.move,as it ran,and recovered when it fell.

Anonymous said...

Good summary of the different theories of the tyrannosaur forelimb function. I also believe the forelimbs had some function.

But what do you mean with "the Ruiz paper is indeed indicative of this". Indicative of what? Limited mobility of the upper arms in abelisaurids?

Mark Wildman said...

Thanks for the kind words. Andrea was suggesting what Ruiz et al were also implying that the arms of Carnotaurus represent a dichotomy in as much that they display great strength in the lower arm region and yet have greatly resticted movement between the humerus and scapulacorocoid.

I will dig out the paper and run through it again to be sure.

Anonymous said...

As I said, I to think tyrannosaurs had use of their arms (despite limited motion). Regarding the idea of tiny arms compensating a large head, it is interesting that “big-armed” allosaurs like Acrocanthosaurus, also had lightweight, pneumatic skulls with thin bones,[1] compared to tyrannosaurs massive skulls and jaws. It makes sense for tyrannosaurs to have small forelimbs, especially as they had shorter tails than allosaurs. As Currie pointed out in his excellent paper on tyrannosaur ontogeny (I actually found it for free, really difficult…),[2] most tyrannosaur genera had similar body and skull proportions, and forelimbs of a shared “standard” size (~1 meter). The exception is Tarbosaurus, with forelimbs somewhat smaller than the “standard”.[2] Yet, Tarbosaurus had a relatively light skull.[3] One might wonder then why Tarbosaurus could not evolve larger forelimbs, if it had a lighter skull?

Some seems to imply that Tarbosaurus forelimbs (with humerus usually 25 % the length of the femur) might represent a more “derived” state in regard to small forelimb[4] (also see comment by Andrea Cau, 23/9 2011). But I do not think so: there is some individual variation in tyrannosaurs proportional arm size, even within the same genus, and it might even overlap with other genera. As Currie pointed out, the shortest tyrannosaur humerus is actually found in TMP 86.64.1, an Albertosaurus.[2] I think Tarbosaurus descended from an ancestral population which had forelimbs in the lower limit of the tyrannosaur “standard”. Regarding why Tarbosaurus with its light skull did not get larger forelimbs; if we study dinosaurs in a naturalistic context (unlike new age-paleontologists who believe in evolution as a thinking, planning being), it seems easier for organisms to brake or preserve a feature/organ than to improve/re-evolve it…

Regarding limited movability of the tyrannosaur and abelisaur arm: it does not necessarily mean they were useless. Limited arm motion is also the case in Acrocanthosaurus,[1] yet their arms were certainly not useless! But it is interesting to note that juvenile tyrannosaurs like “Jane” had more flexibility in the shoulder than adult Tyrannosaurus.[4]

1. Monsters resurrected, ep. 4 (Discovery Channel, 2009).
2. Currie P.J. 2003, "Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia", Canadian Journal of Earth Sciences 40(4):p. 651-665, .
3. , read 12-9-2012.
4. Currie P.J. 2003. “Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada”, Acta Palaeontologica Polonica 48(2): p. 191-226.
5. , read 12-9-2012.

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