The Tyrannosaurus Rex, for all its fearsomeness, has always been ridiculed a little bit for its teeny tiny arms. But a scientist has recently made the extraordinary claim that in fact, those arms were “vicious weapons,” and not to be mocked.
While the arms may be small and short, they were strong and had large claws that would have allowed the T-rex to mount the back of its victim, slashing huge gashes three feet or longer and several centimeters deep in a matter of seconds, University of Hawaii paleontologist Steven Stanley claims. Initially, scientists thought that the arms were meant to hold its partner close during sex, but that may not be the case.
However, not everyone is convinced, with some scientists reportedly criticizing the claim as illogical. But it shows we have a lot to learn about this truly terrifying beast.
The following is a statement from Stanley, as printed by the Geological Society of America.
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.
The following is an excerpt from Wikipedia on T-rex’s arms.
When Tyrannosaurus rex was first discovered, the humerus was the only element of the forelimb known. For the initial mounted skeleton as seen by the public in 1915, Osborn substituted longer, three-fingered forelimbs like those of Allosaurus. A year earlier, Lawrence Lambe described the short, two-fingered forelimbs of the closely related Gorgosaurus. This strongly suggested that Tyrannosaurus rex had similar forelimbs, but this hypothesis was not confirmed until the first complete Tyrannosaurus rex forelimbs were identified in 1989, belonging to MOR 555 (the “Wankel rex”). The remains of Sue also include complete forelimbs. Tyrannosaurus rex arms are very small relative to overall body size, measuring only 1 meter (3.3 ft) long, and some scholars have labelled them as vestigial. The bones show large areas for muscle attachment, indicating considerable strength. This was recognized as early as 1906 by Osborn, who speculated that the forelimbs may have been used to grasp a mate during copulation. It has also been suggested that the forelimbs were used to assist the animal in rising from a prone position.
Another possibility is that the forelimbs held struggling prey while it was killed by the tyrannosaur’s enormous jaws. This hypothesis may be supported by biomechanical analysis. Tyrannosaurus rex forelimb bones exhibit extremely thick cortical bone, which have been interpreted as evidence that they were developed to withstand heavy loads. The biceps brachii muscle of an adult Tyrannosaurus rex was capable of lifting 199 kilograms (439 lb) by itself; other muscles such as the brachialis would work along with the biceps to make elbow flexion even more powerful.
The M. biceps muscle of T. rex was 3.5 times as powerful as the human equivalent. A Tyrannosaurus rex forearm had a limited range of motion, with the shoulder and elbow joints allowing only 40 and 45 degrees of motion, respectively.
In contrast, the same two joints in Deinonychus allow up to 88 and 130 degrees of motion, respectively, while a human arm can rotate 360 degrees at the shoulder and move through 165 degrees at the elbow. The heavy build of the arm bones, strength of the muscles, and limited range of motion may indicate a system evolved to hold fast despite the stresses of a struggling prey animal. In the first detailed scientific description of Tyrannosaurus forelimbs, paleontologists Kenneth Carpenter and Matt Smith dismissed notions that the forelimbs were useless or that Tyrannosaurus rex was an obligate scavenger.
According to paleontologist Steven Stanley from the University of Hawaii, the roughly 1 meter long arms of a Tyrannosaurus rex were used for slashing prey. Especially by juvenile dinosaurs as their arms grow slower in proportion to their bodies and a younger Tyrannosaurus rex would have proportionally much longer arms than an adult one.
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