Paleontologists have discovered a new dinosaur with small arms like T. Rex | Science

Meraksi had a large skull and short arms, in the same proportions as Tarbosaurusrelative of T. rex.
Charles Papolio

If T-Rex is known for more than just bone-crushing bites, it’s the dinosaur’s tiny hands. Paleontologists have been puzzled by the anatomical oddity of dinosaurs for more than a century. But Tyrannosaurus it was hardly the only dinosaur to develop tiny little appendages. An international team of paleontologists has just described Meraxes gigasa huge carnivorous dinosaur with surprisingly short forelimbs, today c Current Biology.

“It was a big surprise,” said National University of Rio Negro paleontologist Juan Canale, starting the day Meraksi was discovered. When paleontologists search for new dinosaur fossils, it often takes many days or weeks in a field season to make a remarkable find. That the best find of a trip will be on the last afternoon of the last day is a common paleo superstition. But the bones of Meraksi were found on the first day of a field trip in 2012. The team found a large carnivorous dinosaur with four long-necked herbivorous dinosaurs near Canale, all in the same rock layer.

Just how unique the theropod dinosaur would have been, however, didn’t become clear until the bones were later exhumed from the rock that encased them. Not only is this theropod a new species, but it is also among the most complete specimens of its family ever discovered. The dinosaur belonged to a group of shark-toothed predators called carcharodontosaurs – the same family as Giganotosaurus this summer Jurassic World: Dominion. The pop culture connection doesn’t stop there. The name Meraksi comes from the nickname of a dragon in the famous A Song of Fire and Ice series by George RR Martin, an apt name for a sharp-toothed dinosaur that weighed more than four tons.

Although Meraksi is known from only a partial skeleton, the specimen includes some telltale parts. Canale and co-authors found almost the entire skull of Meraksia significant discovery, considering that many other carcharodontosaurs – such as the famous one Giganotosaurus— are not fully known. Based on the proportions of Meraksifor example, Canale and co-authors appreciate that Giganotosaurus had a skull about 5.3 feet long—slightly shorter than previous maximum estimates. But while the skulls and jaws get all the attention, the limbs of Meraksi may be the most informative parts of the new dinosaur.

“There’s a lot of new information that helps us reconstruct the appearance of other giant, related carcharodontosaurs,” says Canale, “like their arms and legs, which were almost unknown in Giganotosaurus, Mapusaurus, Carcharodontosaurus or Tyranotitan.” Thighs, legs, shoulders and arms of Meraksi, by comparison, are nearly complete and show that carcharodontosaurs were copies. Just like the tyrannosaurs that roamed the Northern Hemisphere, Meraksi and the family had their hands full. The whole hand of Meraksi is less than half the length of a dinosaur’s femur. These are roughly the same proportions as Tarbosaurusa close relative of the famous T. rex.

Knowing that some of the last carcharodontosaurs had short arms will help paleontologists track when these dinosaurs began to change. Earlier carcharodontosaurs, such as the pinnipeds Acrocanthosaurus had longer forelimbs, while Meraksi shows that later members of this group had much shorter limbs, says SUNY paleontologist Sarah Birch, who was not involved in the new study. Tyrannosaurs underwent a similar change over time, so the shortening of the forelimb appears to be a shared response to their evolution into a large carnivorous dinosaur with a powerful bite.

So why Meraksi and its relatives evolved short arms just like tyrannosaurs? “We found a relationship between skull size and arm length in some groups of large theropod dinosaurs,” says Canale, where carnivorous dinosaurs with larger skulls tend to have shorter arms. This relationship doesn’t always hold, Birch notes, and some small theropod dinosaurs have very short arms for their body size, but there does seem to be a correlation between heavier heads and smaller arms in large carnivores. This hints at that Meraksi, Tyrannosaurus and similar dinosaurs were for their bites, with hands largely irrelevant for capturing prey. This makes anatomical sense. Even among theropods with more prominent arms, such as Allosaurus, carnivores had a relatively limited range of motion and would not have been able to see what they were grabbing. For dinosaurs that primarily bit with their jaws like a hyena—rather than clawing like a cat—the little hands that were out to the side may have prevented them from being broken, torn off, or otherwise injured.

The model suggests that there is some benefit to shortening the arms, Birch notes, but more disturbingly, the arms of Meraksi and large tyrannosaurs still served some function in the lives of these dinosaurs. “What we don’t see is evidence that these forelimbs were dysfunctional,” Birch says, because “you can’t hold muscles like that so you don’t do anything with them.” The question for paleontologists is what the large carnivorous dinosaurs might have been doing with their arms as those appendages grew shorter, a mystery that may have been lost to the Cretaceous world.

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