SCIENCE

The tiny arms of a Tyrannosaurus rex may have been strong enough to slash prey at close quarters. (National Geographic)

What else are we learning about T. rex anatomy?

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Discussion Ideas

• The Nat Geo article says the general consensus is that Tyrannosaurus rex’s arms were a vestigial feature. What is a vestigial characteristic?
  • Vestigial describes a body part, or remnant of a body part, that no longer serves any useful function.
    • In a T. rex, those tiny arms are vestigial remnants of long, strong arms. “Tyrannosaur ancestors used long arms primarily for grasping. These atrophied during the evolution … because the jaws took over [the arms’] grasping function. No longer being selected for, the arms were selected against: the expansion of the head deprived them of nutrition in a zero-sum game.”
    • Vestigial characteristics in extant animals include the wings of flightless birds, such as ostriches and emus; the eyes of sightless animals such as blind mole rats; pelvic bones on snakes such as pythons; and even that small, weird triangular tissue at the inner corner of your own eye. (The plica semilunaris is a vestigial remnant of the nictitating membrane, the “third eyelid” that some animals can close over their eyes for protection.)

 

• What are some ways scientists think T. rex used its vestigial little arms?
  • n/a. A lot of scientists think T. rex didn’t use its arms at all. One speculates that the vestigial arms were a “necessary trade-off in order to support such huge and powerful head and neck muscles.”
  • sex. Some scientists think T. rex used its claws to clasp a partner during sex.
  • young guns. “During its lengthy juvenile years … a T. rex’s arms would have been larger relative to its body. ‘It might be that the arms were actually more functional in young T. rex, and became reduced in function as it became older … The strike zone would be proportionately larger in a young T. rex—and going after smaller prey would mean the force required to kill the victim would be less.’”

 

• What evidence suggests “the arms of Tyrannosaurus rex were not functionless but adapted for vicious slashing”?
  • The author examines six of the arms’ traits.
    1. short length. The shortness of the arms would actually have been advantageous for up-close slashing.
    2. large coracoid. The T. rex’s large coracoid indicates its arms were very strong. The coracoid is part of the the bone (scapula) that stabilizes the shoulder joint and is the attachment site for muscles such as the biceps and pectorals. A T. rex’s coracoid is not only slightly longer than the leg of a six-foot man, but also of similar girth.
    3. strong arm bones. T. rex’s arm bones were quite robust and would readily have sustained the impact of slashing.
    4. powerful claws. Over time, T. rex went from three fingers to two. The unusual reduction of the number of fingers would have resulted in 50% more pressure being applied to each claw.
    5. wide range of motion. The humoral head on a T. rex’s arm was part of an unusual “quasi-ball-and-socket joint” that would have provided considerable mobility for slashing. The humoral head describes the hemispheric “ball” of the upper humerus (arm bone).
    6. big claws. T. rex’s huge sickle-shaped claws would have caused deep wounds. T. rex claws were about 8-10 centimeters (3-4 inches) long.

 

• What are some criticisms about the new suggestion that T. rex arms were powerful weapons?
  • Those arms are just. so. tiny. “The chest is so broad on a mature T. rex, one expert noted, that the ‘effective strike zone’ of the swiping arm couldn’t be far from the animal’s torso. ‘I would expect it could cause some decent damage if it struck, but in order to deploy [the arm], Tyrannosaurus would basically have to push its chest up against the side of the victim. In such a position the tyrannosaur wouldn’t be able to use its far more powerful armament: its massively powerful jaws.’”

 

Teachers Toolkit

Nat Geo: T. Rex’s Tiny Arms May Have Been Vicious Weapons

Nat Geo: Pucker Up! New research suggests T. rex teeth were probably not protruding fangs, but kept hidden by thin, scaly lips.

Geological Society of America: EVIDENCE THAT THE ARMS OF TYRANNOSAURUS REX WERE NOT FUNCTIONLESS BUT ADAPTED FOR VICIOUS SLASHING