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We finally know why T. rex had small arms
26 May 2026

The mystery of Tyrannosaurus rex's disproportionately small arms may have finally been unraveled by a new study.

The research, published in the journal Proceedings of the Royal Society B, suggests that these tiny forelimbs were an evolutionary trade-off for a larger skull.

The study analyzed 82 dinosaur species and found a correlation between strong skulls and smaller forelimbs across five different groups of dinosaurs.

Evolutionary trade-off
Evolutionary insight

Charlie Roger Scherer, a doctoral student at University College London and the study's lead author, explained that evolution often prioritizes one feature over another.

He said, "If you want to focus on using your head to bring down large prey, you don't really want to be putting much effort in keeping your arms long and with claws."

This suggests that as dinosaurs evolved stronger skulls for hunting larger prey, their forelimbs became less important and eventually shrank.

Measuring limbs and skulls
Methodology

The researchers measured the forelimbs and skull bones of 82 dinosaur species, using fossils and data from existing scientific literature.

They also developed a new way of quantifying skull strength by looking at factors such as overall size, how the bones fit together, and bite force.

This allowed them to rank every skull on a scale.

Not surprisingly, T. rex topped the list with its massive head.

Shrinking limbs across species
Evolutionary pattern

The study's findings suggest that shrinking limbs were not a one-off occurrence, but an evolutionary trait observed across different, unrelated species over a long period.

The process varied among groups with some dinosaurs first reducing the size of their fingers while others focused on shortening the forearm.

Scherer said, "There's always a common driver of it," which was these dinosaurs preying on animals that required more force to bring down.

Other dinosaur groups with strong heads and small forelimbs
Trait prevalence

Beyond tyrannosaurids, the group that includes T. rex and its cousins, the researchers found a link between large, strong skulls and small forelimbs in four other dinosaur groups - ceratosaurids, megalosaurids, abelisaurids and carcharodontosaurids.

These were all large bipedal carnivores that lived across the globe from the start of the age of dinosaurs (Triassic) to the end of Cretaceous when a huge asteroid impact wiped out most non-avian dinosaurs.