T. rex's big tail was its key to speed and hunting prowess

Nov 15, 2010
Tyrannosaurus rex , T rex
T. rex, credit: Natural History Museum.

Tyrannosaurus rex was far from a plodding Cretaceous era scavenger whose long tail only served to counterbalance the up-front weight of its freakishly big head.

T. rex's athleticism (and its rear end) has been given a makeover by University of Alberta graduate student Scott Persons. His extensive research shows that powerful tail muscles made the giant carnivore one of the fastest moving hunters of its time.

As Persons says, "contrary to earlier theories, T. rex had more than just junk in its trunk."

The U of A paleontology student began his research by comparing the tails of modern-day reptiles like and Komodo dragons to T.rex's tail. Persons found for that all animals in his study, the biggest muscles in the tail are attached to upper leg bones. These caudofemoralis muscles provide the power stroke allowing fast forward movement.

But Persons found T.rex had one crucial difference in its tail structure.

The tails of both T.rex and modern animals are given their shape and strength by rib bones attached to the vertebrae. Persons found that the ribs in the tail of T. rex are located much higher on the tail. That leaves much more room along the lower end of the tail for the caudofemoralis muscles to bulk-up and expand. Without rib bones to limit the size of the caudofemoralis muscles, they became a robust power-plant enabling T.rex to run.

Persons extensive measurements of T.rex bones and computer modeling shows previous estimates of the in the dinosaur's tall were underestimated by as much as 45 per cent.

That led many earlier T. rex researchers to believe the animal lacked the necessary mass for running which in turn limited its hunting skills. That lack of speed cast T. rex in the role of a scavenger only able to survive by feeding on animals killed by other predators.

As for an T. rex's exact speed, researchers say that is hard to measure, but Persons says it could likely run down any other animal in its ecosystem.

Explore further: Short-necked Triassic marine reptile discovered in China

More information: Persons' research was published in the journal The Anatomical Record.

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A_Paradox
not rated yet Nov 15, 2010
The article makes me glad they are no longer around!

But I have a question: T. rex had very small front limbs compared to the rest of their body structures.
My idea used to be that this came about because many of the creatures they attacked had powerful tails which could damage T. rex forelimbs. I still think that is part of the reason but I have seen articles which point to energy conservation as important.

The energy conservation theory points our that four legged creatures must expend energy holding up their torso/midsection and their heads. In contrast we save energy by striding around in a constant balancing act where head and body weight is carried on our double pendulum legs.

For energy conservation to have been a significant factor with T. rex, [ie in losing their forelimbs] I think their heads and tails would have needed to be almost rigidly attached to their midsection.
trekgeek1
5 / 5 (2) Nov 15, 2010
Or possibly they just don't look as menacing swinging their arms whilst running.

If their primary hunting mechanism was their enormous bite, then arm length may not have been a trait that was actively selected by survival. Though given the time span of their existence, it's odd that their arms would not have become vestigial to the point of vanishing completely. It's conceivable they were used during mating and their length served their purpose just fine (Cretaceous spanking?), or balancing in an awkward position for a few moments. Despite their relatively tiny size, a recent TV program investigated their strength and found them to be very strong. I believe they could curl 500lb's each.
Husky
not rated yet Nov 15, 2010
i've seen videos of fighting kangaroos (also short forearms) sitting on their tail, so that they could use the powerfull legs for destructive kicks forward, i wonder if the T-rex might have been able to do such a thing
scidog
not rated yet Nov 16, 2010
also possible that Rex hatchlings were quad walkers and became biped as they matured.the small front legs are a vestige of that development.
Donutz
not rated yet Nov 16, 2010
A_Paradox almost brings up an interesting idea. What if the reason for the small arms has to do with the balancing act? The T-Rex, as any bipedal dinosaur, has to balance mass behind the legs against mass forward of the legs in order to avoid falling over. The article alludes to the fact that the T-Rex head is freakishly large (and therefore unusually heavy). Presumably there was a good reason for needing a big bite, so the animal would have had to either evolve more mass in the back to compensate, or evolve less mass elsewhere in the front. If the arms weren't particularly useful in an adult, they'd be a natural to atrophy.
A_Paradox
not rated yet Nov 16, 2010
Well, after further thought, I have to admit that the energy conservation idea [for the bipedalism] fits well with dinosaurs being a fair bit less exothermic than mammals and birds. My thinking is that warm blooded mammal predators are pretty much all quadrupeds [apart from orca and dolphins] and the forelimbs are very effective for grappling with and subduing prey.

As for kangaroos, the evidence supports energy conservation as the pay off. It is particularly interesting because with kangaroos and wallabies the biggest payoff is for females with joeys where the extra weight of the baby does not require them to expend any extra energy to get around. Unlike with placental mammals where energy usage is proportional to mass so a heavy fetus makes life much harder for the placental mother.

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