Why did T. Rex have such small arms?

While the dinosaur Tyrannosaurus Rex is huge, its arms are surprisingly small and this has been a puzzle for scientists. This article describes the search for an explanation ever since the discovery of fossil remains of the dinosaur in 1902 by a team of paleontologists led by Barnum Brown from the American Museum of Natural History, an institution that was headed by Henry Fairfield Osborn.

But right from the beginning, one aspect of these kings of the “tyrant lizards” was deeply mysterious: their puny arms. Brown’s T. rex skeleton was missing all its fingers and both its forearms, which were drawn on early portraits using surprisingly accurate guesswork – prompting speculation that they surely couldn’t really be that stumpy. What could have been their purpose? And how did they end up being so small?

Bizarrely, they were. Today T. rex is almost as famous for its withered little arms as for its enormous teeth – they’re so totally out of proportion, they almost look like they’ve been plucked from another species and simply stuck on, in a throwback to the hilarious blunders of bone assembly from the 19th Century (such as the time Stegosaurus’ signature diamond-shaped back plates were added to its tail instead).

With arms that might measure just 3ft (0.9m) long on a 45-ft (13.7m) individual, this formidable carnivore’s hilariously small appendages have been a source of intense speculation ever since they were discovered – despite decades of studying them, to this day no one has any idea what they’re for.

The article goes on to describe the slow accumulation of fossil discoveries that accompanied the theories about the size of the arms.

One early idea came from Osborn, who gave T. rex its name. “He saw these very small, curiously tiny arms and made a comparison with small fins that that are present on modern-day sharks, says Scott Persons, head curator of the Mace Brown Museum of Natural History, South Carolina.

Another, potentially comical, possibility is that T. rexes may have used their little arms to get up off the ground. With up to 15,500lb (7,031kg) bodies – equivalent to the weight of a large African elephant – they may not have found it easy to manoeuvre out of a resting position, or get back on their feet in the event that they fell over. (Many living animals struggle with this to this day, such as tortoises, which often rock themselves upright when they end up on their backs.)

Another controversial idea, put forward by a single scientist in 2017, is that adults like the Wankel Rex may have used their stubby arms as weapons – perhaps holding their victim in their jaws or pinning it down with their bodyweight, before ripping and slashing at it. Underpinning the idea is that though they’re tiny, T. rex’s arms are surprisingly muscular. He calculated that even with its 3ft (0.9m) limbs, these eviscerating actions could have done some serious damage, creating gashes several centimetres deep and at least a metre long in a matter of mere seconds.

However, there is also the possibility that they had no function whatsoever – T. rex’s tiny arms were the last vestiges of once-useful appendages that had long ceased to be necessary.

An alternative is to use basic physics.

Intriguingly, one of the latest ideas is that the dinosaur king’s arms may have withered to their shrunken form to serve an important purpose – there was a reason they needed to be so puny. It’s possible their small size helped them to have the most enormous head and powerful bite possible: the bloodcurdling silhouette of the average T. rex was no accident.

It is possible that we may never fully understand the reason for the small arms but that the fascination with the question will continue.

Alas, it’s also possible that we may never know the true function of T. rex’s arms. Like discovering the long-tubed passion flowers of North and South America without finding the hummingbirds that dip their long beaks into them, sometimes the context you’d need to understand a feature has been lost to the fossil record. After more than 66 million years – in which time volcanoes have sprung up and gone extinct, islands have been formed and lost, and tens of thousands of species have come and gone – the nuances needed to understand certain behaviours may be too far gone to unravel.

All this interest in the odd arms of an animal that went extinct 66 million years ago may seem odd. But apart from curiosity about something so intrinsically interesting, Persons thinks he knows why. “We human beings are probably a little bit too preoccupied with the importance of our arms and our hands, because they’re so gosh-darn critical to our survival,” he says.

As the primary way we interact with our environment, it’s hard to imagine giving them up on purpose. “And here we’ve got this incredibly successful, very scary-looking, animal, and it seems to want very little to do with them,” says Persons.

The idea that the arms were becoming vestigial and that if the dinosaurs had not gone extinct, may have eventually evolved away so that these huge animals became like land sharks, is fascinating.


  1. Oggie: Mathom says

    Considering the number of Tyrannosaurid arm fossils that have been found with evulsion scarring on the bones, the arms were definitely used for something. Something that required a fair amount of exertion.

  2. moarscienceplz says

    I’m actually more bewildered by quadruped predators that DO use their forelimbs to both run and catch their prey, such as cheetahs. They have to rely on inertia while they switch from running mode to catching mode, and if they miscalculate, they could easily do a faceplant which could cause a lot of damage to their eyes, jaw, and brain.

  3. moarscienceplz says

    the action of plucking something out by force; violent or forcible extraction.
    I don’t understand your post. What was causing the scarring? Stresses on the T. Rex’s forelimbs while they were plucking something out?

  4. Oggie: Mathom says

    What was causing the scarring? Stresses on the T. Rex’s forelimbs while they were plucking something out?

    My bad. Avulsion, not evulsion:

    Avulsion injuries or fractures occur where the joint capsule, ligament, tendon or muscle attachment site is pulled off from the bone, usually taking a fragment of cortical bone. Avulsion fractures are commonly distracted due to the high tensile forces involved.

  5. Tethys says

    I imagine those short arms were useful to reach any smaller creatures that might try to attack its belly in the process of being prey.

    Another possibility is holding on to a female T-Rex’s head during mating, which is a good survival strategy if you need to get intimate enough to entwine your tails. Modern birds hang on with their beaks.

  6. Holms says

    Ultimately, tyrannosaurid arms were puny simply because they weren’t detrimental -- or perhaps, detrimental enough -- to be selected against.

  7. dangerousbeans says

    Given that carnotaurus had even small arms proportionally, it seems that thicc legs with massive mouth was a valid dinosaur body plan. and leads to the correlated question: why weren’t T Rex’s arms smaller?
    The fact they weren’t completely vestigial suggests they did something.

  8. lanir says

    I would tend to guess it’s a mix of three things. They’re vestigial but on the way to vanishing entirely some use for them appeared that made a difference. I’d guess something to do with mating. And they probably aren’t well designed for the purpose they serve, which would account for the evidence of extreme stresses.

    They seem ridiculous and they probably are. Nature sometimes has extreme results that aren’t very practical for an individual but work well enough for a species to survive. Hyena birthing for example reads like a body horror story. But the species still exists so it apparently isn’t enough of a factor to write them off as an evolutionary failure.

    Quick caveat: This isn’t my field so my views may be incorrect or ill informed. They’re based on little more than curiosity and pop science.

  9. moarscienceplz says

    I learned two new words. Thanks!
    Yeah, I agree with lanir @#10. Ostrich wings are kinda similar. They are vestiges from their flying ancestors, and they are used, IIRC, to shade their eggs and in courtship displays, but an utterly wingless ostrich could still probably survive.

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