If there are three things any 6-year-old knows about dinosaurs, it’s that: Tyrannosaurus rex was big and vicious. But Tyrannosaurus rex had tiny arms! They dominated the Western United States during the Cretaceous Period, from 68 million to 66 million years ago. In its day was the largest carnivore land. But for such a fearsome predator, it certainly had its quirks like those teeny tiny arms. Even from a biological perspective, they look pretty small.
The Tyrannosaurus Rex is one of the most famous dinosaurs globally, originally uncovered in 1874 by Arthur lakes. After looking at the t-rex under an electron microscope, it reveals the structures that contain pigments. It allows seeing the color mapping of the dinosaur as it would have been. They determined that the t-rex would have had patches of black, brown, and possibly even brighter colors. They also found tiny arms and melanin. This would have been found on the brow of the Tyrannosaurus. And it could have been used as a sign of age and maturity to females.
These social cues are common amongst other members of the bird family. They also dug deep into the ancestry of the Tyrannosaurus and found it didn’t have traditional bird-like feathers. And it had a blanket of these feathers which prevented it from overheating. A dino Valley State Park in Texas glen Cuban has analyzed the footprints of a very close relative to the Tyrannosaurus. And it explained that the t-rex would have walked a lot like a bird bringing its foot back and then around.
How the T. Rex lost its arms?
The average T. rex weighed about 10,000 kilograms and stood between four and a half to 6 meters tall. But its two-fingered forelimbs were less than one-eighth as long as its hindlimbs, probably only a little bigger than arms. The story of how T-Rex lost its arms is itself pretty simple. But the story of why it kept those little limbs and how it used them? That’s a little more complicated.
The short answer as to why T. rex had tiny arms was, and they ran in the family. Early ancestors of the genus Tyrannosaurus included the prosauropods. Which first appeared way back in the Triassic and had forelimbs just as long as their hindlimbs. As time went on, more and more of these early dinosaurs became bipedal. Once they were standing on their hind legs, their front limbs were suddenly freed up to be used differently.
Many of the early carnivorous bipedal dinosaurs had sharp claws on their forelimbs, making them more useful for hunting than standing on. The group that produced Tyrannosaurus rex took an evolutionary right turn somewhere in the Jurassic, 150 million years ago. Instead of relying on their forelimbs to help them hunt, members of the superfamily known as Tyrannosauroidea. It includes Tyrannosaurus, and its relatives started to use their jaws. One of the earliest Tyrannosaurs is Guanlong, a small, 150-million-year-old dinosaur from China. Guanlong had a tiny head and long forelimbs long for a Tyrannosaur anyway, about half its legs.
Fast forward about 25 million years in China, and you find Raptorex, which had the same basic body plan, but with different proportions. Raptorex was still only about 1 meter tall, but its head was nearly one-third larger than Guanlong’s. And its forelimbs were almost half as long.
So, Raptorex looked a lot like a tiny, pocket-sized version of Tyrannosaurus rex. It shows up in the fossil record about 70 million years before T. rex did. It means that the characteristics we think of as defining T-rex, namely a huge head and tiny forelimbs, were part of a 90-million-year trend in that direction. That big head wasn’t just for looks.
Its enormous jaws gave T. rex a bite force up to 57,000 Newtons, which is enough power to pulverize bone. With a bite that nasty, having long forelimbs wasn’t necessary. Beyond that, long arms weren’t just “not needed” for some dinosaurs. They were kind of in the way. That’s because of how bipedal dinosaurs walked. We swing our arms when we walk and run to help us maintain balance to keep our upper body stable. Dinosaurs like tyrannosaurs had long, rigid tails, which acted as stabilizers.
So they didn’t need to swing their arms. If anything, having big forelimbs would have just slowed them down. The natural body posture for many tyrannosaurs was with their forelimbs tucked close to their body. Some specimens have even been found in this position as fossils! This type of research knows that Tyrannosaurus and other carnivorous dinosaurs held their forearms with their palms facing each other. They didn’t have the wrist mobility to hold them facing down.
So that’s how Tyrannosaurus rex, or Tyrannosauroidea in general, lost their big arms. Over 90 million years, their heads got bigger, as their jaws became their primary weapons. Their forelimbs got smaller because that probably made it easier for them to move around. But T-rex still had arms! If they were such a liability, why didn’t it just lose them entirely? Well, that’s a little more complicated. Initially, paleontologists thought that T. rex forelimbs were vestigial, just leftover relics of true arms that were so small as useless.
After lots of study of some well-preserved fossil finds, we’ve learned that their forelimbs were covered in muscle attachments. So, while they weren’t very strong, their arms could move. What could they have been used for? One of the most common theories is that T. rex used its forelimbs during mating. Studies on the biomechanics of its arms have shown that they were pretty powerful at adduction. It is the technical term for the movement when you hug something.
So, adduction could’ve been used either as part of courtship because who doesn’t like a good cuddle? Or to help hold male Tyrannosaurus in place during mating. However, other scientists think T. rex could have used its limbs, not for love but fighting. Its fingers, after all, were tipped with claws that were about a quarter the length of the whole forelimb. Still, that’s less than half as large as one of their teeth. Although the arms were muscular, they didn’t measure up to the force of its bone-crunching bite.
So if these arms were used to slash at prey, it was only a supplement for its jaws’ killing power. Others have suggested that T. rex used its hugging skills to kill, planting its forelimbs into its prey while biting down, dealing a death blow and a death hug simultaneously. It’s always possible that there wasn’t any use for these tiny forelimbs. Maybe T. rex arms were what we call a spandrel, a byproduct of other evolutionary changes. That doesn’t necessarily have a purpose or advantage of its own.
Over millions of years, Tyrannosauroidea followed an evolutionary course toward having smaller forelimbs and bigger heads. So maybe, if they hadn’t gone extinct, the descendants of T. rex would have had gone one step further and lost their forelimbs entirely.
If T. rex used its forelimbs for slashing, paleontologists would need to find a fossil that had slash marks that came from their claws. Even more challenging, to know whether they used their forelimbs during mating, two T. rexes preserved “in the act.” That’s part of what makes paleontology so interesting there are always more questions to be answered. To figure them out, we have to dig up the right fossils.
Breithaupt, B. H.; Southwell, E. H.; Matthews, N. A. (October 15, 2005).
Osborn, H. F. (1917). “Skeletal adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus.”
Larson, N. L. (2008). “One hundred years of Tyrannosaurus rex: the skeletons.” In Larson, P.; Carpenter, K. (eds.). Tyrannosaurus rex, The Tyrant King.