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The Purpose of T-Rex's Tiny Arms

Mamerto Adan is a feature writer back in college for a school paper. Science is one of his many interests, and his favorite topic.

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I never really thought about it at first. During my younger years I was too busy admiring the Tyrannosaurus Rex’s monumental size to notice something odd about the animal. Definitely dinosaurs are odd, that’s why they are cool and interesting. Maybe that’s why I never noticed it since it’s an accepted part of their oddity. Or maybe I had too much cartoons back then. I’m seeing monsters with laughable proportions every day that I got used to it. So much so that I thought it’s also normal for dinosaurs.

Then I grew up and finally realized it.

A T-Rex had all the makings of a monster. Large head with jaws full of teeth, and monumental but powerful frame. It had everything but those tiny arms.

When compared to modern bipeds, the T-Rex arm is badly proportioned to its size. Smaller therapods, like raptors sport longer reach while spinosaurids boasts robust forelimbs. This left most of us wondering why Mother Nature granted the tyrant lizard king such laughable limbs. We knew perfectly that animals look the way they are for a reason. They evolve according to their function, and sure those arms could have served anything.

Those Arms Are Really Small

A colored T-Rex reconstruction.

A colored T-Rex reconstruction.

I’ll say it again; T-Rex arms are really small. The largest T-Rex, Sue towers above many therapods at 12 ft tall. And at 40 ft long, she will easily dwarf many modern day land critters. With that said a T-rex arms is only 3.3 feet long, pit squeak compared to the overall size. Now small arms are typical for most therapods, with the exception for a few like the spinosaurids which are known for robust arms. Raptors also display longer arms ending in large claws and adorned with feathers. In the case of abelisaurids, where the Carnotaurus belongs, their arms are so small that it was basically vestigial body parts.

The shorter arm in relation to hind legs of the therapods can simply be explained by their bipedal gait. Nevertheless the size of the arms will depend on the dinosaur’s lifestyle. Spinosaurids probably need large arms to hold fish. Raptors on the other hand could have used their forelimbs for climbing or as rudimentary flapping as they leap on their prey. With that said, the large and terrifying head of T-Rex meant it had little use for its arms. When the animal attacked, the head went first to unleash a powerful bite. Simply the jaws and teeth are enough of a weapon, and the arm became vestigial.

But recent analysis simply doesn’t support such idea.

The Carnotaurus with vestigial arms.

The Carnotaurus with vestigial arms.

Compare to the vestigial arm of Carnotarus, T-Rex arms are not that degraded. In fact it is still very much functional. Now there’s not much in the forelimbs of the Carnotaurus. The hand lacks carpalia and the claws may be absent. In fact with not much nerve for stimulus, Carnotaurus arms maybe comparable to the non-functional wings of emus and kiwis.

T-rex arms are different. The overall arm structure of the tyrant lizard king will be discussed later on, but to give you an idea, it had stronger arm for its size. The bones are robust while the fingers bear large claws, something that vestigial arms don’t have. These indicate that the arms might be used for something after all. What it could be will be mentioned below.

Possibly Used For Mating

T-Rex skeletons in a mating position.

T-Rex skeletons in a mating position.

We mentioned earlier that T-Rex arms are not exactly wimpy. It is disproportionately small, but disproportionately powerful also. The cortical bone is extremely thick, an evidence that the arms could withstand heavy load. And the arms exhibits impressive guns. The biceps could lift 439 lbs by itself. And that’s just the biceps. When we lift, multiple muscles work together, multiplying the load. It is safe to say that a T-Rex could toss an average person with one arm, though the arm is pretty stiff. The motion is limited, with the shoulder and joints allowing only 40 and 45 degrees of motion.

A T-Rex arm structure.

A T-Rex arm structure.

So, what’s the use for a pair of powerful arms with limited range of motion?

The limited range indicates that the two arms are ideal for clamping tight on something. It was meant to hold. But what could be it?

Males could have used the arms during mating. We see it all the time in modern creatures. Animals do their thing as they hold on to the females. For a male T-Rex, a strong arm will come in handy to restrain a large female when it is his turn to pass his genes.

The theory sounds feasible indeed. It might make sense but it has flaws which will be discussed later on. Before that some scientists have another hunch on where those arms are used.

For Support

A T-Rex skeleton resting.

A T-Rex skeleton resting.

When the tyrant lizard king finds the need to get up, it could have relied on the small arms. Those serve as braces so the front body won’t skid forward as it rose from the prone position. Again it might be small, but the arms are terrifyingly powerful. As it got up, the forelimbs will be extended in a push-up position, as an aid as the animal regains its footing.

But there are problems with this.

Remember how the arm motion is limited. And those arms might be strong, but the wrist is not powerful enough to support its massive bulk. And it was suggested that T-Rex never needed its forelimbs when rising from the ground. All it has to do is to place one leg below the centre of gravity before extending it. It also had this tail to aid with the balance. And we need to look at modern flightless birds to see how they get up without additional support from forelimbs.

For Combat

A T-Rex manus (hand) claw.

A T-Rex manus (hand) claw.

Going back to T-Rex’s arms being used for mating, the main problem here is that the arms might be too dangerous for procreation. It just doesn’t make sense why a body part for making love is armed with large claws. A T-Rex hand only had two fingers. But each finger bears curved bladed weapon that resembles that of a bear. And just to show you how large the claws are, a usual legal carry pocket knife has a blade less than three inches. And for most people, a three inch blade is scary enough. And when a folding knife blade reaches 3.5 inches, it is already considered as large. Now, a T-Rex claw is actually 4 inches long, bigger than most pocket knives and the size of bushcraft blades. And such claws make for serious slashing weapon. And that doesn’t mean that having only two fingers is a handicap. Having only two fingers meant there will be 50 percent more pressure being applied to the remaining claws. Hence the force of slashing multiplies.

The theory is the brainchild of Steven Stanley of University of Hawaii. He also believes that the joints of the arm, though limited in motion could permit the animal to slash with its angry fingernails. Some problems with this theory as T-Rex expert Thomas Holtz of University of Maryland College Park pointed out includes the problematic reach and the broad chest structure that inhibits effective striking. What’s more by using the arms the T-Rex can’t use its powerful bite, officially its primary weapon.

Nevertheless Holtz believes that juvenile T-Rex could have relied on their arms more when hunting. That it is longer in their younger years, but atrophied as they get older. Stanley agrees, but the arms could be brought into use when the situation calls for it, even in their mature years. Imagine how a fight with a prey culminated into close quarters, and the clawed hands makes for a good sidearm.

References

1. John Pickrell (November 2, 2017) "T. Rex's Tiny Arms May Have Been Vicious Weapons." National Geographic.

2. Jacqueline Ronson (November 9, 2017) "What's the Point of T-Rex's Tiny Arms?" The Daily Beast.

3. Helen Thompson (April 15, 2014) "Five Things We Don't Know About Tyrannosaurus Rex." Smithsonian.

Questions & Answers

Question: Is it possible that the Tyrannosaurus Rex's arms were involved in mating displays? I envision the arms either being adorned in vibrant feathers or if no feathers then bright red and engorged with blood. I am thinking along the lines of a Tom turkey's beard.

Answer: Since T-rex are basically gigantic birds, it is also possible that the arms could be some sort of display. And thanks for sharing ideas!

Question: Why does the T-Rex have small arms?

Answer: From the way I see it, the small arms resulted from the tyrannosaurid hunting style. They use their jaws a lot, and over time the arms became a "sidearm," or simply got smaller to give way to the powerful jaws.

Comments

Coelophysis on March 17, 2020:

Gharial from baryonyx line of spinosauridae the advance tyrannosaur line of theropod share none verticalized with t.rex it’s only modern crocodilian that has that feature it’s very rare in thecodont it’s found in pterosaur proterosuchus allso birds .gharial is fast like all spinosauridae and can do death roll like all spinosauridae allso can walk bipedal like all spinosauridae the Youtube video show that the video is sawtooth crocodile walking like a dinosaur by vamshikrishna sandela .gharial does not have fuse nasal like t.rex one dwarf caiman spinosaurus allso has neural arch on tail spinosaurus has no neural arch at all .gharial will allways land on his feet falling from a building like a cat because of the death roll system .the arm is nothing more than a speed booster like those Batman arm flops of crocodile

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