Have scientists been wrong about the tyrannosaurus rex’s “tiny” arms?
Paleontologists have long speculated that T-Rex’s signature forelimbs, relatively small compared to its body as a whole, were not functional.
It was assumed that evolution gradually diminished the animal’s arms, and they served little purpose for the T-Rex. But scientists are rethinking the T-Rex as we know it.
We were wrong, according to one expert, and the arms were “not as tiny as often portrayed.”
The University of Hawaii’s Steven Stanley suggests the T-Rex’s arms were actually “quite robust” and equipped the animal for slashing at close quarters.
ZME Science reports:
It’s important to note that while T. Rex’s forelimbs were indeed small in relation to the dinosaur’s huge body and jaws, they were still one meter (three feet) in length and may have been capable of bench-pressing over 180 kg (400 pounds) each. Stanley has identified several clues that suggest T. Rex mini-arms were actually secret weapons capable of inflicting ferocious damage.
For starters, the arms are strong and robust, as indicated by the bones that make up the limbs themselves but also the coracoid, which is the shoulder bone that helps control arm movement. Being so short in relation to the dinosaur’s towering body, the arms would have been perfectly adapted to slashing at close quarters. Another hint is that T. Rex had two claws on each forelimb instead of the typical three most other theropods feature. Having just two claws would have helped T. Rex exert 50 percent more pressure when slashing and clawing. The (8-10 cm long) sickle-shaped claws would have caused deep wounds.
Stanley attacks the view that the T-Rex’s arms were vestigial, meaning that evolution gradually diminished the size.
He reminds us that each arm was three feet long, incredibly strong, and equipped with sharp claws that would have deeply wounded its victims.
Stanley spoke at the annual conference of the Geological Society of America about his research. He concluded that the T-Rexe’s arms were functional and “adapted for vicious slashing.”
The abstract for his conference talk reads:
For more than a century, many paleontologists have viewed the small arms of T. rex as having been vestigial. At ~1m long, these arms were not as tiny as often portrayed, and derived traits indicate that they were actually functional. The few previous suggestions of possible functions for the arms are all problematical. Six of the arms’ derived traits indicate that they were adapted for slashing at close quarters: (1) The shortness of the arms would actually have been advantageous for this activity. (2) A large coracoid indicates that the arms were very strong: not only slightly longer than the leg of a six-foot man but also of similar girth. 3) The arm bones were quite robust and would readily have sustained the impact of slashing. (4) The unusual reduction of the number of fingers from three to two would have resulted in 50% more pressure being applied to each claw. (5) The humoral head was part of an unusual quasi-ball-and-socket joint that would have provided considerable mobility for slashing. (6) The huge (8-10cm-long) sickle-shaped claws would have caused deep wounds.
Its short, strong forelimbs and large claws would have permitted T. rex, whether mounted on a victim’s back or grasping it with its jaws, to inflict four gashes a meter or more long and several centimeters deep within a few seconds — and it could have repeated this multiple times in rapid succession. Infliction of damage by slashing was widespread among other theropod taxa, so in light of its formidable weaponry, why should T. rex not have engaged in this activity?
Tyrannosaur ancestors used long arms primarily for grasping. These atrophied during the evolution that led to the tyrannosaurids because the jaws took over their 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. Then, as the arms approached their final size, natural selection kicked in opportunistically and put them to good use for slashing at close quarters.
In popular culture, the T-Rex’s forearms are portrayed as the least frightening element of this giant.
Were the T-Rex’s arms every bit as formidable as the rest of it?
Let us know what you think, and sound off in the comments below.