Tuesday, 17 April 2007

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!

31 comments:

Laelaps said...

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.

Laelaps said...

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.

Mambo-Bob said...

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.

Sarda Sahney said...

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'.

Laelaps said...

"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.

Aaron said...

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.

Sordes said...

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.

Anonymous said...

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?

Anonymous said...

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

Anonymous said...

Nesting might be the answer. Surely T-Rex could not see the nest on the ground very well standing upright. T-Rex would probably not handle the eggs with it's powerful jaws or large feet. I think that tending to the nest while squating on it's belly could possible explain the need to have functional smaller arms. This idea coupled with the need to have a proportionate counterbalance and the use of the large head for eating kinda makes sense. While squating the T-Rex can rotate the eggs and police the nest. Still it is amazing that these arms did not change much in over 150,000,000 years. Did'nt they just discover a small fully grown cousin of the T-Rex in China. Wonder how the evolution stacks up with that specimen.

Anonymous said...

Could the arms be used for picking the teeth?

Warren said...

I have a theory. T-Rex probably used the same strategy as the Komodo dragon, administering a poison bite, probably using a bacteria in it's saliva to poison the blood of it's prey. Then it would follow the sickened animal until it collapsed. Hence it did not need large fore arms to grapple with it's prey.

TheLastBoyScout said...

T-Rex's arms being small or not, it makes no difference, everything has a purpose. The arms were possibly capable of lifting 400lbs? Well I dare say, with that strength and those meat hooks for fingers, the creature could scratch out a divet in the ground for a nest to place its eggs quite nicely, I think.

Anonymous said...

this looks suspiciously similar to a scientific american article on the same subject...

http://www.scientificamerican.com/article.cfm?id=if-t-rex-fell-how-did-it

Kids Cash Connection said...

What about the possibility that they were actually wing appendages similar to the ones found in today's parrot's. There is research in China to suggest that a T-rex they found may have had feathers. I am not suggesting that t-rex flew. I am thinking more along the lines of an ostrich. What to you think? I can find no theories on this.

Anonymous said...

The arms were getting in the way in combat against other carnivores and chronically maimed and subsequently useless. This further drove the strength development of its neck, head, and feet. We're looking at evolution where the mouth/intake is the focal point for a carnivores survival. Consider the crocodiles and sharks. Apex predators with no need for kitchen utensils.

Anonymous said...

So a summary of theories:
-Help the dinosaur stand up
-Balance or aid in sex
-Meat hooks while T. rex ate
-Counterbalance to running
-Parental care - moving eggs around in the nest
-Nothing - vestigal body part.

Any more ideas? I have heard some creative theories, do you scientist guys have any other ideas that seem credible?

Netzer said...

What about just regular balance?
I always found it weird that a two legged creature would be able to keep himself up without the aid of something to give more balance then just head and tail... even if his center of gravity was low..

Anonymous said...

From all of this, I (only an interested layman) can imagine an evolutionary sequence like this: T. Rex starts as a typical quadripedal reptilian creature. It begins to favor running on the hind legs. It begins to favor using the mouth as weapon over the front claws. So it begins to favor leaning forward for additional reach with the mouth.

That creates a balance problem, so it develops a heavy counter-balancing tail and less massive forelimbs. Less massive front limbs not only help avoid being front-heavy, but they also make the creature more agile. (It's seizing prey with it neck and mouth like a monitor lizard or snake, except from a biped stance. Long limbs in front are more of an encumbrance when moving quickly through cover than they are an advantage). By shortening the forelimbs, the creature can front reduce mass, and also "clear room" for quick, unimpeded seizing action by the neck and head.

But it still has small forelimbs that are useful for boosting itself upright, as meat hooks, for managing its nest. Or if it's a bird-like creature, for some limited grooming of its mate.

Anonymous said...

Maybe there was flesh & muscle that did not have bones, like an elephant's trunk. Could they have been cartilage wings used to hide under & capture prey?

Anonymous said...

Other suggestions:
Tactile Receptors
Perhaps they worked much like antennae for an insect providing additional sensory information or even like the tongue of a snake
Communication Devices
Perhaps the claws were used to make clicking noises to communicate with others; a clicking sound would be useful when hunting as the prey may mistake it for background noise rather than communication

Sarda Sahney said...

Sent to me via email by Daniel Robillard:
I have always thought T-Rex was a scavenger. Look at the way large scavenging birds such as Vultures feed. They have no need for arms. They have long necks and really dig into the carcass, all the way up to the shoulders. Picture T-Rex feeding like this. I see his arms held tight to the body so he can really dig in deep. Perhaps the long claws aided in pulling out the entrails. This may account for the broken limbs as this was likely a frenzied affair as it is with modern scavenging birds. Some of the other Ideas have merit as limbs are used for a variety of functions. I really think the primary adaptation was for feeding and hauling out guts.

The_Laughing_Seraphim said...

Why is it assumed they were vestigial as in, on their way out, rather than the precursors to longer arms?

Julie Ritchie said...

Great read! Thanks!

Anonymous said...

If the T.rex was able to reach his jaw with the forelimbs, he maybe could clean up his teeth

RandomPasserby said...

Here's a question, why don't we study the direction and type of break that occurred over arms? That would tell us the the opposing force applied to the bone and exact direction in relation to the body the force that was applied to the arm. Think forensics science, every break tells a story of how. This would at least put most if not all current theories to rest. Who knows it could possibly lead to a completely new surprising theory that has never been thought of before now (or even more mystery?). ;-)

J. Cipollina said...

This may be a little out of the box but I did send this idea to the curator at the AMNH.
Suppose T. rex, after a kill, would consume its prey as the large cats do today. After chasing down and killing their prey, the large cats go down in a prone position to eat. ( less energy expenditure)
If T.rex did go in a prone position (i.e. on stomach) to eat
and he did not have forelimbs, his body would roll side to side as he was ripping at his food from side to side.
His forelimbs, though small and had limited movement, would be perfect as stabilizers.
We see these use of side stabilizers today on firetrucks when they extend their ladders to reduce the chance of rollover.

Freddy Hodges said...

OOO! They were super strong right? So maybe they were for when the T Rex slept, or even when it fell. It would need some sort of leverage to get up right? This would explain why they'd break so often (to brace a fall). Did I just make the scientific discovery of the century? Who knows, but I'll put my name at the bottm just in case. ;)

Anonymous said...

Perhaps the forelimbs were vestiges of wings, the neck an s curve, the tail short, and feet directly beneath. The usual picture of t rex isnt balanced

Jason Argent said...

I it's a simple question of energy. It takes energy to grow anything, so if tyrannosaurs evolved a larger skull and longer tails and legs to compensate, they would have had less energy to expend on growing long 3-clawed arms. Think about their prey: hadrosaurs and ceratopsians. They were smaller than most tyrannosaurs so could be killed with an ambush attack and a quick bite to the neck or head area.

Now think about earlier theropods, allosaurs etc. They were targeting sauropods, potentially in packs, so they still needed relatively long arms for slashing and gripping, like dromaeosaurs (raptors). Even so, their arms weren't particularly long and subsequent cretaceous allosaurs (abelisaurs, Giganotosaurus etc.), correct me if i'm wrong, had even shorter arms.

In general, the bigger theropods got, the smaller their arms became, so I really think energy and prey type is the key. This theory does fall down with Carnotaurus though as it also lived alongside sauropods. Perhaps it had smaller prey?

P.S. the scavenger theory is interesting but if T. rex was sticking its head inside corpses it wouldn't evolve a bugger head....think of the long neck and small head of a vulture.

Anonymous said...

What if they were used for defense? If a T. Rex attacked a much smaller dinosaur, the other dinosaur may not be able to reach it's head/neck to fight back. Therefore, the dinosaur would be forced to attack something it could reach: the legs/belly of the T. Rex. If the smaller dinosaur got underneath the T. Rex in order to attack, then the T. Rex wouldn't be able to reach with its mouth. The arms of the T. Rex might be able to push the dinosaur back forward so it can be bit. It would also explain some of the broken arms.