October 8, 2008
Tyrannosaurus Rex: Armed and Dangerous
Allan Tooley / iStockPhoto
Consider the Tyrannosaurus Rex. Length: 42 feet. Weight: 7 tons. Length of teeth: 6 inches.
Length of arms: 3 feet.
Even as a child, I marveled at this indignity—that one of the fiercest predators ever to stalk the earth, the “King of the Tyrant Lizards,” should be endowed with such comical, stubby forearms.
Nor was I alone in this observation. In 1906, Henry F. Osborn, the former president of the American Museum of Natural History, noted that the fossil remains of T. rex’s forearms were “so small” that “grave doubts were [initially] entertained as to its association with this animal.” T. rex’s arms were too short to reach its mouth (and, by default, too short to scratch its nose)—so what, in the words of the late biologist Stephen Jay Gould, “did Tyrannosaurus do with its puny front legs anyway?”
For decades, paleontologists have thrown around theories. Osborn speculated that the arms could have been a “grasping organ” to aid “in copulation.” In 1970, British paleontologist Barney Newman argued that the forearms helped T. rex stand upright if it ever found itself in a prone position (thus avoiding the dinosaur equivalent of “Help, I’ve fallen and I can’t get up!”). Freelance paleontologist and illustrator Gregory S. Paul suggested in 1988 that the forearms might be vestigial organs. Indeed, a number of researchers have argued that, because of its puny forearms, T. rex was more likely a scavenger than a hunter.
One promising theory was published in 2001 by paleontologists Kenneth Carpenter and Matt Smith. Their investigations suggested that while T. rex’s forearms were stubby, they were buff: the muscles in the upper forearms, for instance, were 3.5 times more powerful than the same muscles in humans. Their conclusion: Tyrannosaurus Rex relied upon its formidable jaws to grasp its prey, then used its forearms to clutch the struggling animal against T. rex’s body to prevent its escape.
As such Carpenter and Smith conclude that T. rex did indeed stalk and ambush prey—although it was not above scavenging if the opportunity presented itself. When you gotta eat, you gotta eat.
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The main problem in dealing with this question is that barring some really spectacular and unlikely fossil evidence all we’ve got is speculation.
One thing we do know is that Tyrannosaurus used its arms vigorously — there are a number of injuries recorded that demonstrate this pretty clearly.
Since all we’ve got is speculation, here’ my two bits. Arms three times as strong as mine aren’t going to be much use in dealing with animals who’s weight is measured in tons. This goes for both breeding and predation. I’m not saying the arms never came into play in these scenarios — I just doubt they were of much use.
My guess (and since it’s not testable don’t think it’s anything more than a guess) is that they may have been used for nest building. Holding down a struggling Edmontosaurus seems unlikely. Gathering and carrying branches makes more sense to me.
The armes were obiviously used to move chess pieces! The tail and hide legs were much to clumsy.
Actually, the small limb size solves itself exactly because of weight issues. Imagine, with arms ANYTHING LESS than as massive as the T-Rex legs, trying to wrestle with prey of a smiliar size. Any arms much smaller than Rex’s legs would literally be torn off! And, massive arms simply weigh too much, and require vast skeletal structures to support. So, for safety, Rex’s arms dwindled into little things that were not in the way and could not be hazards to his own health. I suspect they were proportionately larger in the very young, and little Rex’s would have lower mass and be less likely to tear themselves limb-from-body. As for adults,, nature always finds something to do with limbs. Conducting orchestras, for example. Or, blogging.
Isn’t it possible that T Rex’s arms are smaller due to evolution? If they weren’t used or if they were a problem than over the thousands of years that they roamed nature may have shortened the arms similar to what humans experience in their little toes? Supposedly in a few more hundred years we won’t have them at all. If it is possible that the arms were getting smaller and we have only uneartherd the most recent of the T Rex’s then it might be possible that at one time the arms were longer but not used enough to maintain their length.
With that said, I am also realizing that I can’t think of one carnivorous dino that did have proportionally large arms. Some of the smaller ones did but even they weren’t truly proportionate to the body size. Nothing like a human or ape arm. The reptiles of today all seem to have similar arm and legs. by that I mean they front and back legs are about the same size. We don’t have many reptiles that stand on two legs and have short arms. We do have a few lizards that run on rear legs but the front ones are the same size. Kind of strange.
Birds, which are descended from dinosaurs, turned their arms into wings. Wing muscles are powerful, but not well adapted to grasping. T-reges probably were too large to fly, but just like birds, they didn’t need more than two feet. The small arms were probably useful for clutching in certain situations; every extremity in every animal does not have to be useful in combat.
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