Scientists solve the mystery of T. rex's short arms

Researchers have revealed that the short arms of tyrannosaurs and other carnivorous dinosaurs shrank as their powerful skulls became the primary hunting tool.

Scraper SystemJune 30, 2026, 10:03 PM

Scientists solve the mystery of T. rex's short arms

Tyrannosaurus rex (T. rex), the giant carnivorous dinosaur that lived about 66 million years ago, is known for its disproportionately short arms despite its massive size and bone-crushing jaws. These forelimbs, measuring only 90 centimeters in length, were surprisingly small compared to a body exceeding 12 meters. Until now, many hypotheses had been proposed for this feature, such as grasping, mating, or preventing bites while scavenging.

A new study published in the journal Proceedings of the Royal Society B offers a compelling explanation to solve this mystery. Researchers from University College London and the University of Cambridge examined 82 theropod species and identified a direct link between skull strength and arm shortening.

Arms shrank as the skull became a weapon

Lead author of the study, Charlie Roger Scherer, states that they measured the skull's bite force, shape, and bone robustness using a new metric they developed. According to this, T. rex stands out as the species with the highest skull durability, followed by the giant carnivorous Tyrannotitan that lived in Argentina during the early Cretaceous period.

The study shows that the growth of prey—especially large herbivores like giant sauropods—over time created selective pressure on carnivorous dinosaurs. Scherer says, "As prey grew larger, dinosaurs had to develop more robust skulls and stronger jaws. Thus, the skull became the primary attack organ while the arms lost their function."

Not unique to T. rex

The research revealed that arm shortening evolved independently in at least four different theropod groups, in addition to tyrannosaurids. For example, Majungasaurus, which lived in Madagascar about 70 million years ago, had an extremely powerful skull and arms as small as those of T. rex, despite weighing only 1.6 tons. Similarly, Carnotaurus, notable for the horns above its eyes, also had exaggeratedly short arms.

It was observed that arm shortening occurred in different forms in these distinct lineages. In abelisaurids, the hands and lower arms shortened first, while in tyrannosaurids, all parts of the arm shrank proportionally. Scherer stated, "In both cases, the common point is that the skull increasingly became the primary tool for capturing and subduing prey."

The findings indicate that the short arms of giant carnivorous dinosaurs were not a flaw or an evolutionary remnant, but rather the result of a major adaptation in which the skull became the most efficient weapon.

Scientists solve the mystery of T. rex's short arms

Researchers have revealed that the short arms of tyrannosaurs and other carnivorous dinosaurs shrank as their powerful skulls became the primary hunting tool.