Tiny T. rex arms were built for combat | Science News for Students

Tiny T. rex arms were built for combat

Far from lame and puny, they would have been powerful slashers
Nov 29, 2017 — 6:45 am EST
T. rex

This dino may have been one of the ultimate slashers — if any beast was unfortunate enough to get too close.


SEATTLE, Wash. — No question, Tyrannosaurus rex had small arms. Still, this dino was no pushover.

It is best known for its giant head, powerful jaws and overall fearsome appearance. And then there were those comical-looking arms. One scientist now argues that they weren’t funny when it came to combat. Those roughly meter- (39-inch-) long limbs weren’t just sad reminders of a longer-armed past, concludes Steven Stanley. He’s a paleontologist at the University of Hawaii at Manoa. Those forelimbs were well-adapted for vicious slashing at close quarters, he says.

Stanley shared his assessment October 23, here, at the Geological Society of America’s annual meeting.

T. rex ancestors had longer arms, which they used for grasping. But at some point, T. rex and other tyrannosaurs began to rely on their giant jaws for grasping. Over time, their forelimbs evolved into shorter arms.

Many scientists had suggested the tinier arms were, at best, useful in mating or perhaps for pushing the dino up off the ground. Others suspected they might at this point have had no role at all.

Those arms remained, however, quite strong. With robust bones, they would have been able to slash out with forceful power, Stanley notes.

What’s more, he points out, each arm ended in two sharp claws about 10 centimeters (4 inches) long. Two claws give more slashing power than three, he notes, because each one could apply more pressure. Their edges also were beveled and sharp. That makes them more like the claws of a bear rather than the flat, grasping claws of an eagle. Such traits support the slasher hypothesis, Stanley argues.

But not all scientists buy his claim. While an interesting idea, it’s still unlikely that an adult T. rex would have used its arms as a primary weapon, says Thomas Holtz. He’s a vertebrate paleontologist at the University of Maryland in College Park. Although the arm of an adult T. rex was strong, it would barely have reached past its chest. That would have severely limited the size of its potential strike zone.

Still, fossils show that the arms on a T. rex grew more slowly than its body. So the arms would have been relatively longer in juveniles. And that, Holtz says, may have helped the young predators slash their prey.

Power Words

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annual     Adjective for something that happens every year.

evolve     (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, such an evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs).

forelimb     The arms, wings, fins or legs in what might be thought of as the top half of the body. It’s the opposite of a hindlimb.

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

geological     Adjective to describe things related to Earth’s physical structure and substance, its history and the processes that act on it.

hypothesis     (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

juvenile     Young, sub-adult animals. These are older than “babies” or larvae, but not yet mature enough to be considered an adult.

limb     (in physiology) An arm or leg.

paleontologist     A scientist who specializes in studying fossils, the remains of ancient organisms.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

prey     (n.) Animal species eaten by others. (v.) To attack and eat another species.

primary     An adjective meaning major, first or most important.

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

tyrannosaur     A line of meat-eating dinosaurs that began during the late Jurassic Period, about 150 million years ago. These species persisted into the late Cretaceous Period, about 65 million years ago. The best known member of these species: the late Cretaceous’ Tyrannosaurus rex, a 12-meter (40 foot) long top predator of its time.

vertebrate     The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.


Journal:​ ​​S.M. Stanley.  Evidence that the arms of Tyrannosaurus rex were not functionless but adapted for vicious slashing. Geological Society of America annual meeting, Seattle, October 23, 2017.