What were T. rex’s tiny little arms for anyway?
American palaeontologist Henry Osborn first described T. rex but initially expressed doubts that the diminutive arms he found belonged to this enormous animal. After investigating further and finding it to be true, he considered their purpose and advanced the first theory in 1906: that they were used as ‘graspers’ or stabilizers during copulation.
But this was only the start of a debate that still rages. In 1970 British palaeontologist Barney Newman suggested that the small arms braced the beast's body as it stood up.
Another, contrasting theory suggested that the small limbs are vestigials (degenerated organs that have lost their use).
To add to the confusion it seems that the muscular of the T. rex was very well defined and though the arms were disproportional to the animal's body, they were still very strong. So some people consider that the most useful function of the limbs was to be used as meat hooks while the animal fed. This theory is supported by the beautifully sharp claws on T. rex’s two fingers.
In contrast, Greg Erickson has recently commented on a biomechanical analyses of a new specimen. The aim of this study was to try and understand the physical capacities of T. rex's diminutive structures. Erickson has concluded that the elbow could not be extended beyond 90° and though the arms were very strong (they could lift about 180 kg or 400lbs) they had a very limited side-to-side and up-and-down motion.
It is also known that T. rex's arms were often broken (and mended) during its life, suggesting that the arms were poorly suited for their function that these animals could go without using their arms for extended periods of time.
I read an interesting theory a while back that suggested the arms were most useful in juveniles, as a counterbalance to the young dinsaurs fast quickly. The suggestion was made that the arms were proportionally longer in childhood compared to adulthood. However, as far as I know this theory did not gain popularity.
The truth is that no one knows the answer. Scientific inquiry into this issue has been going on for a century and it doesn’t seem likely that the mystery will be solved soon. If you have any other ideas, please let me know!
12 comments:
Nice post; I've actually been wondering lately what T. rex would have been like if evolution was allowed to continue for a while longer, whether the arms would become further reduced or lost altogether. I guess we'll never know, although I would imagine they had some use if all tryannosaurs seem to have them (I would imagine there would be able time for them to be lost if they were totally useless, if this was even possible).
I was curious though; when was the first actual T. rex forearm found? I had heard that we didn't have the actual forelimb from Tyrannosaurus until relatively recently (within the last 25 years) and prior to that its arms were based upon those of Albertasaurus and its close relatives.
Just to follow up, I found out that all we had of the T. rex arm was the humerus until 1989 when the Wankel rex was found; Sue and other specimens since then have reconfirmed this. Previously, the arms were based off those of Gorgosaurus, and even prior to that T. rex sported a three-clawed hand for a while.
I can't seem to find the paper I'm thinking about here, but there was a study on the allometry of limb proportions in Tyrannosaurus. So juveniles have relatively longer limbs than adults. Many animals go through allometric growth (like our own species; kids have relatively larger head compared to their bodies) so I wouldn't be surprised if the forelimbs of T. rex were longer and more functionally significant in juveniles.
Laelaps - thanks for the info, I hadn't realized that the first T. rex forelimb was found so recently
Mambo-Bob - I remember reading about allometric growth in T. rex forelimbs some time ago. The theory is interesting but in this particular paper, the measurements did not suggest significant differences between adults and juveniles.
Anyway, on the whole I have a hard time buying into the vestigal theory because the forelimbs are proportionally small in many of the large theropods and nature tends to be very 'efficient'.
"Anyway, on the whole I have a hard time buying into the vestigal theory because the forelimbs are proportionally small in many of the large theropods and nature tends to be very 'efficient'."
I agree; they must have had some advantage or use. Obviously more research needs to be done, otherwise we get into the "Yes, it is" "No, it isn't" shouting matches that seem to surround issues like this.
there are people with every frickin kind of interest out there.
thank you so much for letting me in on these facts about dinosaurs.
im so scared, and hate that i havent chose a path for me yet.
People are in general only talking about the arms of T-rex, but its relatives had like Albertosaurus had also only very small arms. And even those already small arms looks still comparably usefull in contrasts to the tiny, but still four-fingered stumps of Carnotaurus. Perhaps they had still some sort of funtion in tyrannosaurids, but in Carnotaurus they were surely vestigal.
Interestingly, the re-evolution of clawed arms in phorusrhacids like Titanis walleri has several parallels to T-rex. The arms were in contrast to the complete animal very small, especially compared to the huge head, but they were very strong, although they had only two claws and also a very limited range. They had surely not evolved, if they had not had any function.
Has anyone concidered that the scientits just might have put it together wrong? Most animals are proportinal to their size, maybe someone put the wrong pieces together, is it soo wrong to believe that? Hello?
Evolution? Why not creation? I've heard it being told that there's an infinite chance that life evolved, but if that were true, why is everything so perfectly made? Ex- the trees, how they work with a water circulation system, going way over 300 feet, without a sound. Alot of things in nature we could not recreate ourselves, and if we did, we could not match up to. The human body, for example. We could not recreate the human eye, to such precision. 1 in a million chance everything worked out so well? And if we were evolved, Woman would have 4 arms instead just two just to look after her children. And a million years have passed already, and I'm not seeing any stubs under my arms yet... From Anonymous,
03 November 2008 02:35
Is that a serious question? Why don't women have four arms yet? I guess the answer must be that it's your fault. You should try harder.
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