Why The Tiny Arms, T. rex?

There are very few extinct creatures that cause so much curiosity as Tyrannosaurus rex, but their small arms have been a source of scientific debate for decades.

Dinosaurs and their Relatives was the last course I taught at university before transitioning from academia. It was a beautiful goodbye to a career that brought me many fun memories. I recently decided that I had to keep cultivating that part of my past and that I had to use my paleontological knowledge in my blogging adventures. This would not only help others understand this field better but also help me stay up-to-date with discoveries.

Today, I want to discuss an equally sweet, terrifying, and fascinating species: Tyrannosaurus rex! If you ask any dinosaur-loving kid (or even their parents), they’ll likely rattle off a few fun facts about this mighty dinosaur. It’s almost like dino common knowledge: T. rex was huge, incredibly fierce, and… sported some amusingly tiny arms. But have you ever wondered about the story behind those little limbs? Why did they evolve to be so tiny? Why did they stay so tiny? And why didn’t they completely lose them?

The T. rex roamed the western United States from 68 million to 66 million years ago during the Cretaceous Period, proudly standing as the largest carnivore on land. But despite its terrifying persona, those teeny arms are a source of many jokes and memes — no push-ups for T. rex, and no high-fives! What about some hugs?

It’s a bit of a puzzle from a biological standpoint, too. Imagine this colossal creature, weighing around 10,000 pounds (about 4,400 kilograms) and reaching up to 6 meters tall, yet its forelimbs were just a fraction of its total size. Sounds mismatched, right? But the evolution of T. rex and its pint-sized arms is quite the tale of the Tyranosaurus family tree.

Long ago, the ancestors of T. rex, dating back to the Triassic period, walked on four legs, but their forelimbs freed up as they evolved into bipeds (walking on two limbs). Initially, those arms, equipped with sharp claws, were handy for hunting. However, the family branch that wound up with Tyrannosaurus rex started evolving in a different direction about 150 million years ago. This lineage developed massive jaws with a terrifying bite force — think bone-crushing strength! With such a powerhouse of a mouth, long arms became unnecessary and, frankly, cumbersome.

Why did the arms evolve to be so tiny?

One of the main questions we — paleontologists and evolutionary biologists alike — tend to seek an answer to is why certain structures or characteristics appear (or disappear) in a family tree. For the T. rex, the answer might lie in a combination of lifestyle changes and sheer evolutionary necessity. It turns out that having shorter arms could have been more of a survival strategy than a disadvantage.

First off, let’s consider the T. rex’s hunting habits. With a bite that could shatter bones, this species likely didn’t need long arms to tackle its prey. The T. rex’s massive head and powerful jaws were enough to handle the hunt. So, in a sense, long forelimbs weren’t just unnecessary; they were potentially cumbersome. Think about it — long arms dangling and possibly getting in the way as this predator charged through the forests and plains.

Additionally, how T. rex moved may have played a role in shortening its arms. Unlike humans, who swing our arms for balance when we walk or run, the T. rex had a massive, rigid tail that served as a stabilizer. This tail allowed the T. rex to maintain its balance without arm-swinging. Thus, evolution could afford to trim down those forelimbs without affecting the dinosaur’s mobility.

Now, here’s where it gets really interesting. In 2022, Dr. Kevin Padian, from the University of California, Berkeley, proposed a fascinating new hypothesis: what if the shortening of the arms was an adaptation to avoid injuries during feeding frenzies? Imagine a group of T. rexes descending on a carcass. Space is tight, everyone’s hungry, and those massive jaws are snapping everywhere. In such a scenario, longer arms could easily become a liability — accidentally getting bitten off by a fellow diner isn’t conducive to survival!

Padian’s hypothesis suggests that as these dinosaurs evolved to hunt in packs, shorter arms became an advantage, reducing the risk of injury during group feeding sessions. This idea might also explain why similar patterns of limb reduction can be observed in other large carnivorous creatures of different eras and continents. For example, Indonesia’s giant Komodo Dragon lizard (Varanus komodoensis), which hunts in groups, has been observed accidentally hurting one another in a behavior like the one described here.

(Fun fact: I was lucky to see them in the wild when I visited Komodo Island in 2015. This is a story for another day!)

While proving such a hypothesis is challenging without direct fossil evidence of such interactions, it does offer another plausible narrative for why T. rex and its relatives might have evolved in this peculiar way. It shifts the focus from what the arms could do to what advantages their reduction offered the rest of the animal — primarily, keeping them out of harm’s way during meals.

So, next time you giggle at the thought of a T. rex’s tiny arms, remember: those little limbs might have been the key to its survival in a world where even a moment’s vulnerability could mean the difference between life and death.

Why didn’t they completely lose them?

But that doesn’t mean those little arms were useless. Recent studies suggest that T. rex might have used its strong, albeit short, forelimbs for activities like mating or even fighting.

While it’s tempting to think of T. rex’s arms as evolutionary leftovers without purpose, this may not have been the case. Paleontologists digging through fossils and performing some 3D reconstructions have found that T. rex’s arms were muscular and capable of movement. Maybe they weren’t central to the T. rex’s survival toolkit, but they had their moments — whether for hugging, holding or a bit of combat.

The big question is: if T. rex hadn’t met its untimely extinction, would its descendants have evolved to lose them entirely? It seems these arms were passed down through generations, shrinking over time but never disappearing. But since, as we mentioned, they were muscular and well-equipped with claws, it is hard to say whether their ultimate function would have kept them in the hypothetical descendants of Tyranosaurids.

However, dinosaur research depends on fossil discoveries and, as such, is always evolving. The mystery of the T. rex’s arms fascinates scientists and enthusiasts alike, so every new fossil find can potentially rewrite parts of this prehistoric puzzle. We organize new excavation campaigns every year! So, who knows? Perhaps one day, we’ll unearth a T. rex fossil — or one of its relatives — that gives us all the answers. But until then, we can enjoy the stories and theories.

Keep those curious minds wandering. Who knows what other dinosaur secrets we might dig up next!