While it was arguably the most ferocious land predator of all time, equipped with 60 eight-inch serrated teeth and a crushing bite, Tyrannosaurus Rex has been derided in the modern world for its short forelimbs, which are only three feet long and serve . without a clear purpose. But the discovery of a new dinosaur, which also had disproportionately small arms, suggests that T. Rex’s diminutive limbs may have actually been beneficial and played a very important role. Meraxes gigas is a T. Rex-like species, even though it went extinct 20 million years before it existed. The discovery of a new M. gigas fossil in Patagonia reveals that both species have huge heads, sharp teeth, long tails, weigh several tons, and what appear to be tiny hands.
Convergent evolution
But the two creatures aren’t even related, occupying opposite branches of the dinosaur evolutionary tree. This, experts say, is an example of convergent evolution, where two species independently evolve the same trait, and it only happens if it has a clear and significant survival advantage. “I’m convinced that those proportionally tiny arms had some kind of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the hand had strong muscles,” says Juan Canale, the lead researcher on the M. gigas excavation project from the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina. One thing they almost certainly weren’t used for was hunting, he said, as the huge, muscle-clad head filled with dozens of dagger-like teeth was more than enough to catch food.
He may have used weapons for reproductive behavior
“I’m inclined to believe that their weapons were used in other kinds of activities,” Mr. Canale said. “They may have used the arms for reproductive behavior, such as holding the female during mating or supporting herself to get back up after a break or a fall.” The authors, writing in the journal Cell Press, also suggest that the short ends could be part of an evolutionary trade-off. Resources can be invested in either long ends or big head, but not both. So there are long-limbed, microcephalic, and big-headed, short-armed creatures. However, it cannot be 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 tracks some other trait, which for large predatory species is likely skull size,” they write.
