Tyrannosaurus Rex: Armed and Dangerous | Science | Smithsonian
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Tyrannosaurus Rex: Armed and Dangerous

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 f...

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Arms measure only three feet


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 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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