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Tyrannosaurs are famous for their teeth, but possibly their most distinguishing feature is their incredibly small arms. Almost all other carnivorous dinosaurs have large, powerful forearms and impressive claws. Some theropods, such as the Baryonyx are famous mostly because of their forearmament. Deinocheirus is known only for its arms. Yet, Tyrannosaur arms were so small that it has only been recently that they were really identified with certainty, and their true articulation and position is still not totally clear. Through the years since these diminuitive limbs have been described their actual function has remained a mystery. It makes sense for a carnivore to have powerful weapons at its disposal for subduing and dismembering prey or for battling rivals, but why would such a large and obviously deadly creature have such absurdly tiny arms that really couldn't have been used for anything? I have seen a variety of theories advanced to explain a plausible function. These have included bracing themselves when they get up from a prone position, spreading the ribs of their victims, cleaning their teeth, grasping their mate during intercourse, or for building a nest afterwards. It has been suggested that even though they were small in comparison to the rest of the tyrannosaur, they were actually powerful enough to be used for carrying food. None of these ideas is in real danger of being proven one way or the other, so I'll toss out a few more: Maybe they used them to juggle eggs or gastroliths for entertainment. Perhaps they were really omnivores and used them to pick fruit. I'll admit that most of these functions are pretty far-fetched, but let's stop and consider this from an evolutionary standpoint. Tyrannosaurs are known to have evolved from ancestors with longer, more "proportionate" arms. In order to evolve from a rather gangly progenitor there must have been a significant selective advantage to the smaller structures. Which, if any, of the functions listed above can be done more efficiently with short arms than long ones? Longer arms would have done a better job no matter how strong the short versions were, or how flexible the shoulder may have been. It seems that the selective advantage needs to be looked at from an alternative point of view as it is unlikely they evolved tiny arms because they worked any better than big ones. Perhaps they evolved to become small precisely because they really weren't good for anything at all!
It seems reasonable that in an animal reliant on its hind legs
for propulsion and a massive set of jaws filled with banana-sized
teeth for everything else, arms carried no useful baggage. 
Any
mutation that coded for smaller arms resulted in less protoplasm
being wasted on non-essential structures, hence a selective
advantage in tough times. How genes actually operate to code
for size and structure is not well understood, but certain genes
are often closely linked to each other. The gene (or genes)
for decreased arm size may have been associated with the genes for
larger legs and increased speed. a definite advantage in a
competitive Cretaceous landscape. This selective advantage
may have caused female Tyrannosaurs to be attracted to males with
big legs and impressive short-arms. (Ahem) If this was true,
why didn't their arms vanish completely, given the millions of
years of the rexes' reign? Who knows? Some snakes
still have vestigial leg structures and look how long they've
had. Perhaps a no-front-limbs-gene never occurred, or it may
have been linked to fatal flaws in other organ systems.
I have
seen speculation that the size of the arms decreased in
order to improve balance, but I believe this line of reasoning is
flawed. Even fairly long arms could always be positioned so
that stability wouldn't be impaired, and consider the advantage of
using them for stability while crossing rough terrain.
Actually, forelimbs would have to be huge in order to have an
impact on balance close to what one of their meals must have
had. Imagine a giant theropod tipped forward with a massive
load of meat in its foregut, unable to right itself. Perhaps
chronic heartburn or terrible sciatica was the real cause for
their extinction.
I can think of at least one area where smaller may have been better. Nesting and parental behavior is a big rage in dinosaur studies these days. If Tyrannosaurs endeavored to be good parents, some of the nurturing may have involved transporting their young. Now a long-limbed theropod was capable of reaching its mouth and whatever its intentions may have been, its intense predatory instinct could have been triggered by an armload of squirming infant. "Honey, I ate the kids!" Obviously there would have been strong selective pressure against such long-armed beasts while arms too short to do anything but clutch a baby to the chest would have led to the successful production of gene-sustaining progeny.
There
is one final scenario that merits consideration as much as any
other I have advanced. Perhaps most Tyrannosaurs actually
had huge arms. Those unlucky few with length-challenged
limbs could have been murdered by organized gangs of highly
intelligent and brachially buffed but intolerant and
politically incorrect rexes that otherwise cremated
their dead. This would explain the paucity of the tiny-armed
Tyrannosaurs in the fossil record and the complete absence of the
common long-armed variety. Interestingly, this also fits
with evidence of serious injuries found in Tyrannosaur skeletons
apparently inflicted by their brethren. Does this seem
heretical? Prove me wrong.
(portions of this were originally submitted to the Archosaurian Archive and later published in Dinosaur World)
It is standing on a piece of petrified wood and shares the display case with a classic Knight sketch.
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This is a newer version of the same skeleton model, reissued by Glencoe.
Practically every piece in the kit was modified in bringing it in line with current thinking
regarding theropod posture as well as increasing the detail in rib and joint articulation.
The tooth is Skulduggery's cast and the paleontologist is made from the driver of a
1:24 scale model car. The skeleton itself is supposed to
be 1:25.
There are lot more detailed views of these kits and many others
on the new Old
Bone Odori pages in this website!
Lascha Tschondia's 1:10 sculpture of a Tyrannosaur skull. This was originally made for
Ants, but is now marketed by Echoes in Time. It came pre-finished in a somewhat
darker scheme, but I repainted it to resemble actual fossils I
have seen. More about this and
other models in this line at The Old Bone
Odori.
Over the Holidays, I received an e-mail from Mike Wagner, looking for this kit. He was anxious to bury it in "matrix" and present it to his daughter for Christmas. He recalled the excitement of building it when he was her age and wanted her to feel the same joy of discovery. Unfortunately, Glencoe is not producing it now, so the T-rex has come to the same kind of extinction suffered by the Ants and Wiccart dinosaurs. There are no commercial sources for them right now, but we finally found one on Ebay. Here is what his daughter found under the Christmas Tree:
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Here is the young paleontologist herself, hard at work with the tools of the trade.
Apparently, a lot of other gifts didn't get much play time until she was finished.
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Final results. Merry Christmas 2003!
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