Where the Dinosaurs Are


TYRANNOSAURS: A CALL FOR ARMS


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Whither Arms, or is it Wither ?


 


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)



Lindberg's JP T-rex



This is a display of the old tail-dragging style Tyrannosaurus..  I've had this ITC  skeleton since I was six.  

 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!

                                           




(contributed by Barry Galef)
This is my brother's view on the short-arm dilemma

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