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).