I thought I’d take a look at Tyrannosaurus Rex. We all remember Tyra, right? Always represented as having a mighty roar and sharp, pointy teeth, and teeny, tiny forearms that wouldn’t even reach its mouth. Why would it evolve with such useless arms?
The first thing I discovered was that those tiny arms were quite strong, and each ‘hand’ had 2 sharp claws. So in a fight, if its mouth was already full of opponent, or it was still looking for the chance to sink its teeth into an opponent, those claws could be used to protect its belly, maybe? Well, not its abdomen, but the chest area. Any attack below that would call for leg action, either to stab or slice with its bigger foot claws, or to back up and get those fearsome teeth involved.
Other uses for these arms have been suggested; that they were used to grasp the female during sex, or that they assisted Tyra in rising from resting on the ground. Or from falling down, or being knocked down, or whatever. But one suggestion is actually supported by biomechanical analysis, and that is that the arms held Tyra’s struggling prey as the teeth did the work of killing it. Those arms are almost always shown bent at the elbow and held close to the body. And there’s a reason for that; Tyra’s shoulders could only move 40°, and its elbow only moved a maximum of 45°. So, no charades or sign language for this creature! To help you think about that, a healthy human shoulder can move 360°, while the elbow allows 165° of movement.
I had trouble picturing these restrictions. If you want, try this: Hold your arm down along your body and bend your elbow to make the forearm perpendicular to your body. This is your starting position. Now, keeping the elbow stiff in that position, raise your upper arm to not quite half-way to being perpendicular to your body. That is about how much Tyra’s shoulder could move. Now, extend your forearm to halfway between where it is and it being straight at the elbow. Imagine all the things you and I would not be able to do if that was all the further we could move those joints!
Okay, so these tiny arms may have been somewhat useful, but why did they evolve that way? What were Tyra’s ancestors like? And are there any descendants still around?
It was hard to find anything definitive about ancestors. The family tree that includes Tyra has many branches in that same time period, and they all seemed to have ‘stunted’ arms. A recent discovery from an earlier epoch held an almost complete skeleton of a very similar creature, possibly an ancestor of Tyra and/or other branches of that family tree. That article did not include much description - only that it was ‘horse-sized’ compared to Tyra’s ‘elephant-size - but the ‘artist’s rendering’ showed that ancestor as a skinnier Tyra, with somewhat longer and looser arms. That article stated that the ancestor already had a big brain, keen eye-sight, and sharp hearing at lower frequencies, and deduced that the Tyra family had developed these ‘smarts’ before it developed the brawn.
And of course, when the meteor hit and killed almost all the herbivores, a few Tyras - out of sheer desperation - shed over 99% of their weight, sprouted feathers and became birds. No, not really. When the herbivores died, Tyra’s family tree died, too. But some distant relatives - the maniraptoriformes family - did live on, and some of those did develop into modern birds. Which is good, because those tiny, practically frozen arms of the Tyra family were not going to launch a Tyra into the air, no matter how many feathers it had!
And now I’ll be shoving all this information into the grist mill that is my day-dreaming mind. Perhaps, on another planet, the end of the dinosaurs did not happen quite so fast, and the Tyras did manage to slim down and learn to fly. What do you think? Some kind of bird? Or dragon? Or something else entirely?