New theory explains why the mighty T. Rex had such tiny arms
It was one of Earth’s most fearsome creatures – a 12m long lizard king with jaws that could crushed a car and feet fit to chase prey at 40km/hr. Yet, for some reason, the T. Rex had teensy, tiny arms
READING LEVEL: ORANGE
Ever wondered why the mighty T. Rex had such tiny arms?
A British team of palaeontologists* has been studying the bones and sinew marks* of T. Rex fossils to figure out how they ticked, including why the apex predator had such puny upper arms and comically small claws.
It turns out, the “tyrant lizard king*” wasn’t only about brawn.
It also had to use its considerable head.
EVOLUTIONARY ARMS RACE
“Why would it have arms so short that it couldn’t even reach its own mouth? How did it use them?” asked Binghamton University palaeontologist and Assistant Professor Sara Sheffield in an article for media publication the Conversation.
The T. Rex had arms about 30 per cent the length of its legs, she explained. If this were applied to a 1.8m (6ft) tall human, their arms would be just 30cm long.
Previous theories have offered a wide range of reasons for this anatomical anomaly.
Dr Sheffield was not part of the British research team, but her questions have been asked time and time again by many researchers in the field.
“You can see the shortening of T. Rex’s arms as a pattern in its family tree, as earlier relatives had proportionally longer arms,” the tyrannosaur specialist said.
Perhaps the T. Rex needed small arms to attract the attention of a mate – but mate attraction in the animal kingdom usually involves distinct differences between males and females.
Or perhaps they were needed to grasp prey in a particular way, to guide the carnivore’s* attack, though palaeontologists have struggled to find evidence supporting that scenario.
Was it a response to accidental bites among a pack feeding frenzy? Or about redistributing weight to ensure the T. Rex could keep its balance?
University College London (UCL) and University of Cambridge palaeontologists now believe T. Rex arms shrank simply because they were useless.
“We sought to understand what was driving this change and found a strong relationship between short arms and large, powerfully built heads,” said lead researcher, UCL palaeontologist Charlie Scherer. “The head took over from the arms as the method of attack.”
It was an evolutionary* way of dealing with larger, more difficult prey.
Sauropods* (such as brontosaurus) got bigger. Ceratopians* (such as triceratops) expanded their horned crests. Ankylosaurus* armour grew thicker. And duck-billed hadrosaurs* were increasingly fast.
Grappling arms could no longer cut it.
But a high-velocity, battering-ram-like head and crushing bite could.
“Trying to pull and grab at a 100ft-long (30m) sauropod with your claws is not ideal,” Mr Scherer said. “Attacking and holding on with the jaws might have been more effective.”
LONG REIGN THE KING
The tyrannosaurus family, whose members became the largest known land predators*, were so successful that they lived right up until an asteroid strike killed off all the dinosaurs some 66 million years ago.
We know roughly when they appeared. But how long individuals lived has been something of a mystery.
New research, however, suggests it may have taken up to 40 years for a T. Rex to reach its full adult size, not the previously estimated 25 years.
Analysis of 17 tyrannosaur specimens has detected previously overlooked growth rings*.
“Unlike the full sequence of rings visible in a tree trunk, a cross section of T. Rex bone typically captures only the final 10 to 20 years of the dinosaur’s life,” researchers stated in a new study published by the journal PeerJ.
But by overlapping the growth rings of young T. Rex with those of older specimens, the team said they have constructed a composite* – but complete – growth curve.
Biomechanics* researchers have also come up with a new theory about the way the T. Rex moved.
Cinematic and silver-screen depictions of the T. Rex generally show it stomping through the forest tracking its prey.
But the new study suggests it moved with catlike poise – on its toes.
The researchers reached this theory by examining the monster’s foot structure and the shape of preserved footprints. They then modelled how a moving foot could create such impressions.
“(Tests) reveal a complex and birdlike function of the foot,” the study concludes.
“This includes a very birdlike gait* defined by higher stride frequencies*, proportionally short stride lengths and elevated speeds.”
The research suggests the tyrannosaur family could run at between 18 and 40km/hr – that’s 20 per cent faster than previously thought.
EVOLUTION’S EMBRACE
“Everyone knows the T. Rex had tiny arms, but other giant theropod* dinosaurs also evolved relatively small forelimbs,” Mr Scherer said. “These adaptations often occurred in areas with gigantic prey.”
His team examined 82 species of theropods (two-legged, mostly carnivorous) dinosaurs and found a link between larger heads and smaller arms, with the findings published in the Royal Society Journal Proceedings B.
Victory in the fight between hunter and hunted was achieved through “killing bites” and dragging large prey to the ground, not by grappling and clawing them to death.
“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,” said Dr Sheffield.
“Similar to how wings separately evolved in different animals – like birds and bats – traits can emerge many times in evolutionary history.”
Tyrannosaurus Rex had the most massive, powerful skull and jaws.
A close second was Tyrannotitan, which roamed the plains of Argentina 30 million years before its famous great-grandchild; it too had strangely small arms.
POLL
GLOSSARY
- palaeontologists: scientists who study fossils of plants, dinosaurs and other prehistoric organisms
- sinew marks: marks or grooves left on fossil bones where the soft tissue, such as tendons, ligaments and muscles, used to be attached
- tyrant lizard king: Tyrannosaurus rex translates to “King of the Tyrant Lizards” or “Tyrant Lizard King, based on the Greek words tyrannos, meaning “tyrant” and sauros, meaning “lizard” and the Latin word rex, meaning “king”
- carnivore: meat eater
- evolutionary: when species adapt to their environment over time in order to survive
- Sauropods: very large herbivores that walked on four legs and had very long necks
- Ceratopians: herbivorous, beaked dinosaurs with bony frills that often had horns, similar to rhinos. They lived 70 to 66 million years ago and were thought to roam in herds
- Ankylosaurus: a type of armoured dinosaur that walked on four legs and lived in the Late Cretaceous period
- hadrosaurs: large, duck-billed dinosaurs that walked on two legs
- predators: animals that hunt other animals for food
- growth rings: concentric bands found in fossilised bone and wood that record seasonal change and pauses in growth, helping researchers to determine the age of the animal or tree they are studying
- composite: made up of various parts
- Biomechanics: how mechanical and physical principles apply together in living beings, such as the way bones, muscles, tendons and ligaments work together to produce movement
- gait: style of walking
- stride frequencies: number of steps taken per unit of time
- theropod: a group of two-legged dinosaurs that were meat eaters and known for being fierce predators, such as the T. Rex
EXTRA READING
How smart was the mighty T. rex?
Dino’s dinner studied from fossils
What to do if you find a dino bone
QUICK QUIZ
1. How big were the T. Rex’s arms compared to its legs?
2. What does the T. Rex’s head have to do with the size of its arms?
3. What new theory has emerged about the way the T. rex walked?
4. At what age did a T. Rex reach adulthood, according to scientists?
5. How long ago did the T. Rex become extinct?
LISTEN TO THIS STORY
CLASSROOM ACTIVITIES
1. Summarise and theorise
Show your understanding of this news story by writing a paragraph-long summary explaining the latest theory about why the T. Rex had unusually small arms compared to the rest of its body. Include information about how scientists believe this adaptation helped T. Rex.
Then, reread the paragraph describing evolutionary changes in other dinosaurs. For each example, write a simple explanation of how that adaptation may have helped the dinosaur survive.
Time: allow 25 minutes to complete this activity
Curriculum Links: English; Science
2. Extension
The T. Rex’s unusual body proportions, especially its tiny arms, have often made it a funny or exaggerated character in books, movies and cartoons.
Choose three different animals and create humorous sketches by giving them exaggerated or disproportionate body parts.
Time: allow 15 minutes to complete this activity
Curriculum Links: English; Visual Arts
VCOP ACTIVITY
Read this!
A headline on an article – or a title on your text – should capture the attention of the audience, telling them to read this now. So choosing the perfect words for a headline or title is very important.
Create three new headlines for the events that took place in this article. Remember, what you write and how you write it will set the pace for the whole text, so make sure it matches.
Read out your headlines to a partner and discuss what the article will be about based on the headline you created. Discuss the tone and mood you set in just your few, short words. Does it do the article justice? Will it capture the audience’s attention the way you hoped? Would you want to read more?
Consider how a headline or title is similar to using short, sharp sentences throughout your text. They can be just as important as complex ones. Go through the last text you wrote and highlight any short, sharp sentences that capture the audience.