Then how do you explain Johnny Damon?
It’s still the wrong way of looking at it. Nature doesn’t have a mecghanism of “gradually disappearing over time” that requires some sort of half-life to disappear. In general, organs that are retained serve some purpose, which is why the aforementionedc “vestigial” external limbs have been explained in terms of reproductive usefulness.
It’s not a mechanistic process, but there are selective forces that will tend to cause organs that are not being used to disappear. Since the organs are not being used, mutations that interfere with their functionality will not be selected against. But it costs energy to make and maintain useless organs. Individuals that lose them will require less food and be at a competitive advantage against others that retain them. So it’s really a balance between how strong this selective effect is versus the developmental programs that make the organs.
Admittedly this process can take a long time. The eyes of blind cave fish become non-functional long long before the eyes actually disappear. The developmental program produces a semblance of the eye structure, but the structures don’t work. In this case, even if you lost the structure of the eye completely you would still need to fill the eye cavity with some other tissue, and the cost of making the eye structure may not be that much more than simply making other tissue.
Likewise with the hind limbs of whales. These have hung on for tens of millions of years, because the selective forces tending to reduce them are probably very slight.
Not necessarily true. It’s just that sometimes selection is so weak for eliminating them that they have persisted for a long time.
Even though his arms (hands) couldn’t reach its mouth they could still be used for eating as in holding prey. I would think.
Kind of a funny moment on King of Queens, when Doug wants access to a computer at the library, and drives away the girl using it by going on about T-rex’s difficulty in masturbating with such small arms.
They used them to flash gang signs. East Coast T-Rex in da HOUSE!
But really, they actually hunted with guns.
I see even other species have problems with all their young dudes.
If not useful for grabbing prey, they were probably handy for leaning on a sauropod carcass while munching on the top bits.
Because that’s where Jesus attached the saddle and the lasers (pew pew!)?
This is all a layman’s guess (of course, I would like partial credit if it gets published. 
)
Looking at Colibri’s pic (and the image in Wikipedia), the humerus attaches with a pretty sturdy ball and socket. There are large flanges on the scapula (clavicle?) which seem to imply that the arms could be swung forward with some force over a wide range of motion. That said, the curve of the claws seems to indicate that the arms were mainly used for ripping backwards and it seems that most artwork shows the latissimus as being well developed.
The arms may have been used for mating, but it seems like they are well positioned to protect the neck when fighting other tyrannosaurs. The neck seems to be the likely spot for a kill. The chest is protected by the ribs. The abdomen is relatively low to the ground, and it is protected by the legs and feet. By comparison, the neck is a large, soft, vital area that is out in the open.
Without the arms, there is no effective defense for the neck. A T-Rex would have to move their entire bodies to defend with their teeth, and using the feet to defend would likely result in their falling down in front of their opponent.
Balance, as shown in this highbrow documentary.
Ok, it’s Caveman starring Ringo Starr, but the T-Rex vs Gravity scene is one of my all time favorites.
How would those (relatively) small arms defend against gigantic T-rex jaws? Seems like there would end up being a lot of armless T-rexs*.
*what’s the plural of T-rex?
Y’know, folks talk a lot about the energy cost of useless limbs or organs, but I suspect that the entropy cost is a lot more significant. Organs become nonfunctional simply because it’s a lot easier for one random mutation to break them than it is for another random mutation to fix them once broken. Even if, say, blind fish have no direct benefit over sighted fish, all else being equal, you’d still get fish losing their vision in lightless environments, just by the statistics.
Actually I think balance is a pretty good reason for the size of the arms. It explains why the arms did not get any smaller or larger. If they helped to ‘get it on’, longer arms would not be a hindrance to that, nor would longer arms hinder eating, or rising from the ground. But I doubt the underlying sequence of events that eventually resulted in the short armed T Rex are all that simple. There’s always the possibility that T Rex’s long armed cousins were more fit, but were standing too close to an erupting volcano.
Yes, the entropy cost will cause them to become non-functional faster than the energy cost will cause them to disappear entirely, which is was part of my point about the eyes of blind cave fish.
One paleontologist that I know personally conjectured that they were used in caring for young. I don’t know (does anybody?) if T-Rex cared for its young or not; but, if it did, then the relatively tiny arms seem more suited to the task than the jaws or legs.
No tyrannosaur nest has ever been found, but lots of other theropods appear to have cared for their young, at least to the point of incubating the eggs.
Maybe so, but they look like they could have torn into the sinuses or taken out the eyes of other T-Rex (singular is the same as plural, but you probably already googled it just like me.) Also, they probably would have been used as part of a combined defense. They may have been defense against smaller predators. In any case, it does look like they were well developed for slashing.
If their only purpose was sexual reproduction, then like other secondary sexual characteristics I would expect that we would see greater development in one gender (e.g., the male T-Rex.)
Please set me straight if I am wrong about this. What I wonder about is why this happens over and over when these theropods get big. It’s not just tyrannosaurids, but also the South American abelisaurids (such as Carnotaurus). They’ve all got weeny arms, in my opinion.
Or maybe T Rex is a code word for all the giant carnivores in this discussion. Allosaurus has pretty short arms, but not so extreme as T Rex. Spinosaurus seems to have decent arms- maybe he needed them to fish? Of course, the much smaller raptors have great arms (but they needed them to evolve wings with;)).
I once read a suggestion that the arms may have been proportionally longer and more important to juveniles. I have never heard that since and I don’t recall any evidence being put forward. I really liked that idea, but I suppose juvenile skeletons must have been discovered which refuted the idea. (Also, Nanotyranosaurus also has teeny arms).
He had arms because they were useful to grab onto things. They were small because he was a large bipedal creature, making the ability to keep their balance a useful survival trait.