The T. Rex may have evolved its tiny arms to help it stand up again after rolling over, a study suggests.
Despite being perhaps the most ferocious land carnivore of all time, equipped with 60 serrated eight-inch teeth and a crushing bite, Tyrannosaurus Rex is the object of ridicule in the modern world for its small forelimbs, which are only three feet long and serve without a clear goal.
But the discovery of a new dinosaur that also had disproportionately small arms suggests that T. Rex’s tiny limbs may actually have been useful and played a very important role.
Meraxes gigas is a species that resembles T. Rex, although it went extinct 20 million years before it existed. The discovery of a new fossil of M. gigas in Patagonia reveals that both species have a huge head, sharp teeth, a long tail, weigh several tons and, as it turns out, small hands.
Convergent evolution
But the two creatures are not even distant relatives, occupying opposite branches of the dinosaur evolutionary tree.
Experts say this is an example of convergent evolution, where two species independently develop the same trait, and this only happens if there is a clear and significant survival advantage.
“I’m convinced that those proportionally small hands had some function. The skeleton shows large muscle insertions and fully developed pectoral girdle, so the arm had strong muscles,” says Juan Canale, lead researcher on the M. gigas excavation project at the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina.
One thing they almost certainly weren’t used for was hunting, he said, since the huge, muscle-covered head filled with dozens of dagger-like teeth was more than enough for catching food.
They may have used weapons for reproductive behavior
“I tend to think that their weapons were used for other types of activities,” Mr. Canale said.
“They may have used the arms for reproductive behavior, such as holding the female during mating or supporting themselves to stand up after a break or fall.”
The authors, writing in the journal Cell Press, also suggest that short limbs may be part of an evolutionary trade-off. Resources can be invested in either long limbs or a large head, but not both.
So there are dinosaurs with long limbs and small heads and creatures with big heads and short arms. However, it cannot have one with both, the researchers suggest.
“The presence of multi-ton theropods with long forelimbs but small skulls further confirms that forelimb reduction is not a simple function of body size in theropods, but rather that it traces some other trait that for large carnivores probably the size of the skull,” they write.
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