The thing is that they seem to have been quite strong, and didn't really diminish further over time (insofar the fossils we have can tell us), which doesn't really fit with being purely vestigial.
I wonder if the strength was to help pin down struggling prey. The small arms would then be designed for aerodynamics -- to not slow the T. rex down when running toward its prey -- but also help out when capturing it. That would prevent them diminishing further over time and find that optimal length for both competing requirements.
I find it fascinating how in terms of morphology, T. rex's tiny arms somewhat resemble those of a modern dinosaur: the kiwi bird, which has wings so small they're practically invisible (and its genus name, Apteryx, means "no wings"). That may give a clue as to how T. rex might have actually looked; after all, we can't prove they weren't huge fatbirds: https://imgur.com/gallery/rmad4
For not bleeding out when multiple T-rexs were feeding on a creature. A larger appendage could be eaten by mistake and then they would bleed out as health care was still a future idea for a future species.
Don’t know why you’re getting downvoted, this is literally a theory discussed in TFA:
> "This kind of quirky idea was that their arms were small enough that they didn't get in the way of all these guys fighting over food with giant mouths, so they weren't basically biting their own arms," says Krumenacker.
Another possibility/question I was idly wondering about is whether they were proportionately larger for juveniles and served some purpose that full-grown adults did not need. Maybe digging out of nests? - though I think specializations only for hatching (e.g. egg teeth) tend to disappear shortly after.
Thanks for the paper! It seems that, while the juvenile limbs were proportionately longer than they were in adults, they were still tiny (figures 2 and 3.)
If you've ever been a T-Rex, you'd know the (principle) answer to what those little arms are for.
Two-legged beings fall, sometimes. Then you have to get up. If you're a Rex without arms, the problem is that you can't get your legs tucked under you before you rise. You can try to get your big fat ass up in the air some in order to get at least one leg under, but as your ass rises, you inevitably begin to roll to one side or the other. Putting your head sideways keeps you fron skidding forward in the dirt as much, and even helps a bit with lateral stability, but not enough. So then you try to shoot up before you're leaning too damn far to the side, but it's too late, you're tilted and off you go at a sidewise angle. You try to sidestep to get a foot under your centre of gravity but you can't step far enough, much less double step, so down you go again, only harder this time. Ooof.
What you need is just a bit of lateral stability that's just good enough to keep you stable with your massive fat butt stuck up in the air, while your head is still on the ground. Spreading your "little" arms out wide is just what the doctor ordered. That lets you get both legs a ways under you (not without a little skidding forward) without rolling to either side so that, after a brief moment of rest, you can heave a bit and pop your head fully forward again (but still on the ground) and then almost immediately shoot nearly straight up while taking a few really big steps forward. (Because now you're shooting forward at a good speed.) You're UP! A few big forward strides is definitely something you're made to do. Sidestepping like a Broadway trouper, only a capella, not so much. You don't die where you fell.
Some people have great internal-to-the-brain physics-and-world simulators. This is what my simulator tells me. But try to get up from face down with you arms tucked back and not relying on wide human shoulders if you're in doubt. I think you'll find it's hard to do.
A predator following migratory preys could need a way to carry eggs and hatchlings, yup.
Another possibility is using them as a safeguard to feel or gather eggs and small t-rexes when laying the heavy body to incubate so they don't crush them.
The article mentions this as an early hypothesis, and then notes that both male and female specimens have the same arm structure, as opposed to sharks which exhibit sexual dimorphism in that facility.
I'm curious about the energy expenditure in keeping the tail raised the whole time - since the 'tripod' theory of T.Rex posture has dropped in favour of the the bipedal theory, it's keeping a presumably very heavy tail lifted up most of the time - for what purpose? Are there any other examples of animals that do this?
One experiment attached an artificial tail onto a chicken and the chicken shifted its stance and was able to walk similarly to how we expect dinosaurs walked.
I believe that the idea is that the tails of T.rex and other dinosaurs were held up by non-stretching tendons. It would not require active effort. By counterbalancing the head, the high tail would reduce the energy needed to hold the head up.
Can you not make a perfect 3D model by now and see how they would move? By enlarging the arms slightly you could at least test if that would tip the weight scale. And it of course would be pretty cool to see.
There's way too much we don't know, like where exactly their muscles were, how strong/dense/fast their muscles were, the mass distribution of their internal organs, the elasticity of their tendons, how flexible their joints/spine/tail were in which directions, how elastic their skin was, the density/strength/flexibility of their bones, etc., etc.
Pretty much all we have to go on is mineralised bones, which tell us quite a bit but leave out the large majority of information about them.
