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What did the T. rex use its little arms for? – Aurora, age 11, Pemberton Township, New Jersey
One of the most famous dinosaurs to ever roam across Earth, Tyrannosaurus rex, has filled people’s minds with wonder since the first skeleton was discovered in the early 1900s.
Scientists believe T. rex, or King of the Tyrant Lizards, as its name translates, was a fearsome predator. An adult T. rex was massive in size – approximately 40 feet (12 meters) long and 20 feet (6 meters) tall, weighing as much as an African elephant. Each of its enormous sharp teeth could be near a foot (0.3 meters) in length from the root to the tip.
I’m a paleontologist, and I use fossils to study how animals lived and evolved over long periods of time. One of the coolest things about being a paleontologist is that there are always new questions to ask and new things to learn – even about a super-well-known dino like T.rex, which went extinct just over 65 million years ago.
One T. rex mystery has to do with this giant predator’s relatively tiny arms. Why would it have arms so short that it couldn’t even reach its own mouth? How did it use them?
How ‘short’ is short?
First, let’s define what we mean by “short.”
The biggest T. rex could measure 45 feet (14 meters) from the snout to the tip of the tail, but their arms were only about 3 feet (1 meter) long. On average, a T. rex’s arms were just about 30% of the length of its legs.
In comparison, humans have, on average, arms around 66% of the length of their legs. If people had the same arm proportions as a T.rex, a 6-foot (1.8 meters) tall person would have arms only 10 or 12 inches (25 to 30 centimeters) long!
T. rex isn’t the only dinosaur with such short arms. The evolutionary trend toward shorter arms in theropods – the larger group of meat-eating, two-legged dinosaurs that T. rex belongs to – happened multiple times. Similar to how wings separately evolved in different animals – like birds and bats – traits can emerge many times in evolutionary history.
You can see the shortening of T. rex arms as a pattern in its family tree, as earlier relatives had proportionally longer arms.
Fossil skeletons of Tyrannosaurus rex make clear that the dinosaur itself was very big, even if its arms were proportionally small.
John Zich/AFP via Getty Images
How did they use their mini-arms?
Short arms don’t seem to have been a problem for these mighty dinosaurs. T. rex was a successful carnivorous species that existed for over a million years. They only went extinct when an asteroid hit the Earth, causing a global mass extinction.
Scientists have suggested a few ideas to possibly explain how T. rex used their arms. Maybe they were used as some kind…
