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?
https://en.wikipedia.org/wiki/Tyrannosaurus
http://www.abc.net.au/news/science/2016-03-16/pregnant-t-rex-discovery-sheds-light-on-evolution-of-egg-laying/7251466
http://www.abc.net.au/news/2016-03-15/t-rex-tiny-ancestor-could-hold-clue-to-predator-dominance/7246928
