Tyrannosaurus--what's with the arms?

After watching a fascinating documentary on the Tyrannosaurus and the possibility that it was a scavenger, I got to thinking–what was with those arms?

If Tyrannosaurus ran at a full sprint, assuming it could run at all, and tripped, it would die. A similarly-sized dinosaur with better arms would possibly be able to break its fall–any word on that dinosaur discovered in Africa that was bigger than old Rexy?

But even if Tyrannosaurus were a scavenger, what purpose could possibly have been served by pitifully small arms like that? I understand that it wouldn’t have to run very fast if it just ate things already dead, thereby eliminating that nasty fall-down-go-boom problem, but surely the arms could still be useful. How’d this beastie get up in the morning, assuming it slept on its belly?

Well, I’m not sure the arms could have been very useful. The head is so very big and with the dinosaur leaning horizontal, the arms would probably have to be very long in order to hold something for the T-Rex to bite. Also, the Rex probably didn’t have to hold anything down too often given the teeth and the strength of its jaws. Also, I’m not sure why you think a T-Rex falling would be fatal to it.

As for the Tyrannosaurus being a scavenger, I think it’s likely that the older, bigger ones just scavenged but that the younger ones had to hunt.

Ever see a chicken asleep? Birds are modern dinosaurs. The dont usually prop themselves up with their wings and a t-rex has this long counterbalance (tail) that is not rigid like with a apatasaurus. It may have used that more than its arms which is why they became vestigial.

Of course, my WAG is reall that the T-rex had great big dragon like wings but only a few of the carpal bones were recovered.

I believe that the arms now are pretty much believed to be vestigial, like X~Slayer(ALE) said.

As for the running and dying bit, a fall at a moderate speed could very well be lethal. If you think about it a several ton creature going at 20 m/h (Which is believed to be their top speed based on advanced modelling using skeletal elements and chicken (really!)). That is a lot of energy, and when it hits the ground at once, you’ve got a problem. I’ve got to go, but when I get back, I’ll post some actually details (If no other doper beats me to the punch).

P.S. I was talking with some one about this question (I think it may have been Phil Curry), but apparently an allosaurs skeleton had been found with massive chest fractures that had actually healed up. The fractures are believed to be the result of a fall.

And that’s why paleontologists shouldn’t smoke pot :smiley:

What use where T. rex’s arms? They were likely used for whatever was within reach of them. There would not have been a use for them, just like there is not a use for yours. Keep in mind that they were about as long as an adult human’s arms, and more heavily muscled.

As for getting up, likely most theropods would have stood up rather like birds do. Having large tails to aid in balancing certainly wouldn’t have hurt in that regard. No biggee.

Yeah, I thought about that. But you know, there are ways for humans to fall with a minimization of injury, and I have to think that the T-Rex would instinctually know how to do that. Plus it had really big bones, a healthy amount of muscle and a large area with which to absorb the shock. I’m not saying that some falls weren’t fatal, but I can’t see making it a general rule or something.

It asn’t made clear in the OP, but I assume you’re alluding to the suggestion made several years ago that T. Rex used hose pitifully tiny arms to help it get up. The idea was that the arms kept it from sliding forwards as the legs tried to push forward. Presumably, without the arms to dig in and keep the upper body from moving, the T. Rex would just sort of trolley itself forward with its hind legs until it hit a tree, or went off a cliff, or something.

It’s not terribly persuasive, but it’s the best suggestion for the use of those tiny arms I’ve ever heard. At least its better than the old standby suggested for practically vestigial limbs – “some sexual purpose”.

2 legged dinosaurs balanced all their weight at the hips, bigger arms means a bigger tail to counterbalance the weight making it heavier and needing thicker legs for support increasing the weight and slowing it down. For the T-rex sacrificing the arms for a greater overall body size works better.

Sock Munkey covered why they’re so small; Cal Meacham the major probable use. BTW, muscle-attachment scars show that while they were little compared to the beast, they were strong – very well muscled. The general idea is that T. rex slept on its stomach, got up by anchoring itself with its forearms and then bringing its powerful legs forward underneath the rear of the torso, then tilting upward to its normal stance. They probably had other, not-so-obvious uses too.

I’m pretty sure that the consensus is that the forearms were mainly vestigal, but I read somewhere (wish I could remember where) that they could have been used to hold open the abdominal cavity of a large dinosaur while the Tyrannosaur reached inside with its head.

It is, in my opinion, unlikely that braking while standing up was a major function of the forelimbs. As has already been mentioned, the animal was nicely counterbalanced above the hips, obviating the need for such a braking mechanism to stand. Were that, in fact, a major function of the forelimbs, then it could hardly be argued that they were ripe for degeneration – indeed, larger forelimbs would surely have aided the animal if they were required for such a purpose.

It seems likely (to me, anyway) that the limbs were reduced because their relative importance to the tyrannosaur was decreasing as more adaptive emphasis (so to speak) was placed in the head and hind limbs (as Sock Munkey mentioned). They were almost certainly not reduced to vestigial status, as evidenced by the relative proportions of the humerus and the manus, as well as the strong indications for heavy musculature. As such, I still contend they they had no major function, and were simply used for whatever they could have been used for: scratching, holding onto whatever was in reach, possibly aiding in standing on occassion, etc.

Actually, to correct myself, the balance point for tyrannosaurs was actually slightly forward of and above the hips, more in a vertical line with where the ball of the foot would be while standing.

I saw that program and it wasn’t all that persuasive. I’ve seen lots of chickens and turkeys and I don’t recall seeing one fall down and not be able to get up. Just as a WAG, I would assume that the Trex’s nose and neck would be strong enough to use as a prop to assist in rising if it ever did fall down. And, as a matter of fact, I’ve never seen a bird fall down while either walking or running. True, you can’t scale bird mechanics up to Trex without going through the equivalent of what the Reynold’s number is to fluid mechanics (I forget what the factor is).

In addition, I don’t think the argument about falling down makes it clear that Trex must have been a scavenger. The scenes of present day scavengers I see on TV show a lot of fighting, pushing and general turmoil, including having the predator return and try to chase the scavengers away. A scavenger looks just as likely to fall as a predator.

The fact is Trex survived a rough and tumble world for a long time whether as a scavenger or a predator. The arms may or may not have been useful, but so what?

This could be one of the stupidest questions around… Are the arms long enough for the bloke to pick his teeth with? Would that be a fit use for the arms?


To continue on merrily with this hijack…

I looked up some numbers, and according to a study by James Fallow, any fall had the potential to be lethal (of course, the same could be said for anything really, including people). Running was a different story. At speeds at or over 20 m/s (72 km/h or ~45 mph), a fall would have been bad news (cite).

Here is another article stating that in a fall (I’m not sure if it is at speed or standing still), the body would experience a deceleration of ~6 g, and the head would decelerate at ~14 g. Even with a heavy bone structure, that it still a mighty hit to the noggin (cite).

Here is that article from nature talking about how they scaled for the mass and top speed of a T. rex. (Mods: I’m not too sure if this is an acceptable link or not, as it links to a google cache of an article that you would normally have to be a member to view. Remove at your discretion).

Okay, I think I’m done geeking out for a while.

T Rex’s arms were too short to pick its teeth.

What if the arm bones were short, but actually had long extended boneless tentacles. He could just sit and wait, whip those babies out and WHAAAAH-TYE-YOWWWW–reels in his prey like a frog’s tongue!

I know, I know–but wouldn’t that have been a much more interesting critter.

I think possibly the reverse is true.

Any fall serious enough to fatally injure a beast the size and build of a tyrannosaur would cause serious damage to any reasonable-sized limbs that got in the way; the only way to overcome this would be to make the forelimbs so large and sturdy that they would be a bigger burden and hindrance in lots of other ways.

Given that the forelimbs were small, a fall at speed (as long as it didn’t involve collision with something hard or pointy - lets say a headlong trip at full speed in grassy scrubland) and the creature would just slide along the ground; the forelimbs would be pressd flat against the body and would not be seriously injured, whereas slightly larger forelimbs (extended to break the fall) would be broken.

Keep in mind, however, that even if a fall were to be fatal, that does not imply that such falls were commonplace. In general, larger animals are likely to be more difficult to trip than smaller ones, and tyrannosaurs may well have been able to recover from most stumbles. Even if the occassional fall were fatal, that does not rule out potentially risky behavior - particularly if the rewards (e.g., a meal) outweighed the danger (e.g., death).

Keep in mind, after all, that tyrannosaurs were fairly successful (Tyrannosauridae persisted for some 18 million years). If these animals fell every time they tried to move faster than a walk, and such falls were always fatal, we certainly wouldn’t expect them to have stuck around for very long.