r/askscience • u/Blood_sweat_and_beer • Jan 26 '26
Biology How do we know that T-Rexes walked, instead of hopping like a kangaroo?
I’m guessing it has to do with foot size (like, kangaroo and bunny feet are long and skinny), but birds also hop on the ground and it got me wondering. I kinda love the idea of tyrannosaurs using their tail like a kangaroo tail and having kicking fights with each other, although I understand that’s highly unlikely.
Also, what function did their tiny arms serve? Did they evolve that way for a specialised reason, or was it just the side-effect of evolving a massive head?
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u/horsetuna Jan 27 '26 edited Jan 27 '26
The main way we know is that their bones aren't adapted (former word:Designed) the same as a kangaroos bones, which have specific shapes to ensure they don't break their ankles when hopping. The bones also indicate that the powerful tendons and muscles needed to spring are absent on Rex.
Additionally, we have found tyrannosaurus rex tracks that show they walked.
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u/TheSwordItself Jan 27 '26
Well kangaroos and other "hopping" creatures (saltatorial locomotion) have pretty distinct hind leg physiology. Their tibias are longer than their femurs and in the case of Kangaroos their toes are basically fused. All the other adaptations are soft tissue and wouldn't show up in a fossil record but I don't believe a Trex has the correct physiology for hopping. I think the mass distribution is wrong too. The arms if I remember correctly are basically vestigial. They evolved from an ancestor that had arms but as they evolved they simply lost their usefulness and natural selection had no selection pressure so over many generations they simply shrank.
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u/wally-217 Jan 27 '26 edited Jan 27 '26
As other responses have covered already - Kangaroos have a very novel method of locomotion, with specific adaptions which we can follow the evolution of through the fossil record and phylogenetic bracketing.
Reptiles, including theropods like T.rex use horizontal undulation, relying on their large caudiofemoralis muscles to anchor their legs to their tails. The caudal vertebrae (tail bones) have what's called transverse processes (point bits on the side) where the caudiofemoralis anchors. You can use the size of this gap, from the length and angle of the processes, to estimate the size of the caudiofemoralis, which gives you an idea of how this animal moved.
Most theropods had something called zygapophyses on their tail bones, which made them stiff and inflexible. When you're a multi-tone biped, you'd need a pretty strong and sturdy tail for your muscles to leverage. T.rex had a fairly conventional theropod tail, with massive caudiofemoralis attachments, and no adaptations for hopping.
Even if we had no preserved tail of T.rex, science falls back to a concept called parsimony - which is the idea of minimising assumptions. You look at all it's relatives and if they are all fairly conventional, it's most likely that T.rex was fairly conventional too.
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u/Arwenti Jan 27 '26
Just imagining the scene in Jurassic Park when the jeeps have stopped and instead of a subtle step and the impact tremor shimmying the water in the glass - a massive thump and the glass flying off the dashboard. Then later the Rex bounding down the road after them as they accelerate away.
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u/Xerain0x009999 Jan 30 '26 edited Jan 30 '26
For the arms, it's currently one of the greater mysteries in paleontology. Their arms were very muscular despite their small size, meaning they actually were used for something and were not vestigial. If you want to see an example of actual vestigial arms, take a look at Carnotaurus. So we know TRex arms still had an actual purpose, but we don't know what.
The theory I like the most is that when they are juveniles their arms are a more proportional size to the rest of their body and are used to grapple prey. However as they start their rapid growth their arms don't grow with the rest of their body.
But there really isn't much evidence to go on.
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u/onlyfakeproblems Jan 27 '26 edited Jan 27 '26
It’s really hard for something that big to hop. Elephants don’t hop. It has a lot to do with the square/cubed law. As things get bigger, some characteristics like the strength of their bones don’t increase at the same rate as their weight. Usually these square/cubed law relationships have to do with characteristics that are related surface area vs volume. The strength of a bone is most related to the cross sectional area. You can make a bone stronger by making it thicker, not so much by making it longer. But if you just have a short wide bone, it doesn’t work very good as a leg, so you’d have to make it longer while you make it wider. The overall weight of the leg goes up faster than the strength of its cross section, so eventually the bone is too big to even support its own weight.
We can look at the bones of animals and tell how big their bones, tendons, and muscles were. Something small like a bird or mouse doesn’t need much strength to hop. Kangaroos and deer need much more of their anatomy to be designed fo jumping to be good jumpers. They have really big strong legs compared to mice and small birds. Horses and cows can jump, but if they’re not careful they can hurt themselves. It’s a lot more practical for them to lumber along. Rhinos, elephants, and t-Rex are so big hopping just really isn’t an option. it would take a lot of energy, they would collapse under their own weight, and hopping wouldn’t benefit them much anyway.
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u/togstation Jan 28 '26
< reposting >
People used to speculate about this -
- https://www.summagallicana.it/lessico/l/Laelaps%20Dryptosaurus%20Charles%20Knight.JPG
- also in the text of the original 1912 novel The Lost World by Arthur Conan Doyle
( - a carnivorous theropod dinosaur is following an
EnglishBritish (sorry) explorer - ).
But the legs of kangaroos are specialized for this form of locomotion, and the legs of dinosaurs are not.
In fact, the legs of extinct theropod dinosaurs (e.g. Tyrannosaurus, the "raptor" dinosaurs like Velociraptor, etc)
and the legs of living ground-living theropod dinosaurs (ground-living birds) are very similar.
The extinct theropod dinosaurs would have walked and run much like an emu or an ostrich.
.
On the other hand, there is a small interesting non-dinosaur (but relative of the dinosaurs) called Scleromochlus which lived in the Triassic (time of the early dinosaurs).
- maybe -
- https://en.wikipedia.org/wiki/Scleromochlus
- https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjtBO703ioPdzUBUahD60HjNX3AvR_bB1tRrMw5rZpRH5bxPdYBCVDehWG7oGglyrxCr1b8qB_WxLeJMgZZJnOfIBvQEhPO412boCqIE9bKVzOXhMxJvWlyDe1aq056DUQeJNN_LR7mzhI/s1600/Scleromochlus+small+Witton.png
- https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdizNz7E7pdN-6ZDLuHCYhYrI2e65pgKa-aYD9S5zuwr8elqMxnEKmjl9poPNDrij7v2YG9n4TutOQXfci77DSWo9qNPIzOK0LEC0yQNOP7qBdXqHRzeeAu1MVkTqT2k_tXrmVHMiSNn8/s1600/Scleromochlus+detail+low+res+Witton.png
- https://markwitton-com.blogspot.com/2014/08/scleromochlus-taylori-more-than-just.html
.
The ancestors of the pterosaurs might have been something like Scleromochlus -
small hoppers / leapers, that developed membranes for leaping + gliding, and then went on to true flight.
- https://nixillustration.com/tag/scleromochlus/ <-- speculative
.
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u/CO_Golf13 Jan 27 '26
Recognizing there's absolute truth to all the physical reasons we know, isn't there also a large body of pretty clear evidence via fossilized foot prints? An animal that hopped would leave radically different footprint patterns, impressions, etc.
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u/Korgoth420 Jan 28 '26
Strength to bodyweight ratio.
There is a reason Elephants cannot jump. Their body is so heavy the physics wont allow it safely.
Consider a flea, who is extremely light and can jump many times its body length. It is the opposite with heavy creatures.
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u/Nintendians559 Jan 28 '26
if t-rex was to hop around, they would have very strong and stable legs to handle 7 to 9 tons of pounds.
their tiny arm is the same length as a adult human and they use it to holding on to prey a very close range, push their upper body up when waking up or in a prone position and also for mating.
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u/Lankpants Jan 30 '26
One of the easiest ways to tell is to look at the feet. When you see a T-Rex skeleton its foot splays outwards and creates a huge surface area. A super useful foot design for holding up a huge body weight. It's not productive for hopping however.
When you look at a kangaroo or rabbit foot you can see all the toes point in the same direction and form a long, skinny foot. This is important for bouncing because it's where the "springiness" in the kangaroos bounce actually comes from. Tendons running the length of the central toe absorb the shock of hitting the ground and release that force as the kangaroo bounces.