Meraxes gigas, a huge meat-eating dinosaur that lived in Argentina some 94 million years ago, had short arms like Tyrannosaurus rex.
Meraxes gigas lived in what is now the Patagonia region of Argentina during the Late Cretaceous epoch, about 94 million years ago.
The ancient predator measured about 11 m (36 feet) long and weighed more than 4 tons.
Meraxes gigas was a member of Carcharodontosauridae, a family of giant carnivorous theropod dinosaurs.
“The group flourished and reached a peak of diversity shortly before became extinct,” said Dr. Juan Canale, a paleontologist at the Ernesto Bachmann Paleontological Museum, the Universidad Nacional de Río Negro and CONICET.
“They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall.”
The nearly complete skull and partial skeleton of Meraxes gigas were discovered in the Huincul Formation in Las Campanas Canyon, 25 km southwest of Villa El Chocón, Neuquén Province, Argentina.
The specimen is among the most complete carcharodontosaurid skeletons paleontologists have found yet in the southern hemisphere.
“The neat thing is that we found the body plan is surprisingly similar to tyrannosaurs like Tyrannosaurus rex,” said Professor Peter Makovicky, a paleontologist at the University of Minnesota.
“But, they’re not particularly closely related to Tyrannosaurus rex. They’re from very different branches of the meat-eating dinosaur family tree.”
“So, having this new discovery allowed us to probe the question of, Why do these meat-eating dinosaurs get so big and have these dinky little arms?”
“The discovery of this new carcharodontosaurid, the most complete up to now, gives us an outstanding opportunity to learn about their systematics, paleobiology, and true size like never before,” said Dr. Sebastian Apesteguía, a paleontologist at the Maimónides University.
The researchers found that large, mega-predatory dinosaurs in all three families of therapods grew in similar ways. As they evolved, their skulls grew larger and their arms progressively shortened.
“What we’re suggesting is that there’s a different take on this,” Professor Makovicky said.
“We shouldn’t worry so much about what the arms are being used for, because the arms are actually being reduced as a consequence of the skulls becoming massive.”
“Whatever the arms may or may not have been used for, they’re taking on a secondary function since the skull is being optimized to handle larger prey.”
The scientists found that the skull of Meraxes gigas was decorated with crests, furrows, bumps and small hornlets.
“Those ornamentations appear late in the development when the individuals became adults,” Dr. Canale said.
“These features were probably used to attract potential mates.”
“Sexual selection is a powerful evolutionary force. But given that we cannot directly observe their behavior, it is impossible to be certain about this.”
The authors also found that carcharodontosaurid dinosaurs evolved very quickly, but then disappeared suddenly from the fossil record very soon after.
“Usually when animals are on the verge of extinction, it’s because they’re evolutionary rates are quite slow, meaning they aren’t adapting very quickly to their environment,” Dr. Canale said.
“Here, we have evidence that Meraxes gigas and its relatives were evolving quite fast and yet within a few million years of being around, they disappeared, and we don’t know why.”
“It’s one of these finds where you answer some questions, but it generates more questions for the future.”
The findings appear today in the journal Current Biology.
Juan I. Canale et al. New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction. Current Biology, published online July 7, 2022; doi: 10.1016/j.cub.2022.05.057
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