If Tyrannosaurus rex is known for anything more than bone-crushing bites, it’s the dinosaur’s tiny arms. Paleontologists have been stumped about the dinosaur’s anatomical quirk for over a century now. But Tyrannosaurus was hardly the only dinosaur to evolve itty bitty appendages. An international team of paleontologists have just described Meraxes gigas, a huge meat-eating dinosaur with surprisingly short forelimbs, today in Current Biology.
“It was a big surprise,” says Universidad Nacional de Río Negro paleontologist Juan Canale, starting with the day Meraxes was discovered. When paleontologists search for new dinosaur fossils, it often takes many days or weeks in a field season to locate a noteworthy 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 Meraxes were found on the first day of a 2012 field trip. The team found a large carnivorous dinosaur with four long-necked, herbivorous dinosaurs nearby Canale says, all in the same rock layer.
Precisely how unique the theropod dinosaur would be, however, didn’t become clear until the bones were later removed from their encasing rock. Not only was this theropod a new species, but it is among the most complete specimens of its family yet uncovered. The dinosaur belongs to a group of “shark toothed” predators called carcharodontosaurs—the same family as the Giganotosaurus in this summer’s Jurassic World: Dominion. The pop culture connection doesn’t stop there. The name Meraxes comes from the moniker of a dragon in the famous Song of Ice and Fire series by George R.R. Martin, a suitable name for a sharp-toothed dinosaur that weighed more than four tons.
Even though Meraxes is only known from one partial skeleton, the specimen includes some telltale parts. Canale and coauthors found almost the entire skull of Meraxes, a significant find given that many other carcharodontosaurs—like the famous Giganotosaurus—aren’t as-completely known. Based on the proportions of Meraxes, for example, Canale and coauthors estimate that Giganotosaurus had a skull that was about 5.3 feet long—a little shorter than previous maximum estimates. But even though skulls and jaws get all the attention, the limbs of Meraxes might be the most informative parts of the new dinosaur.
“There is a lot of new information which is helping us to reconstruct the appearance of other giant, related carcharodontosaurs,” Canale says, “such as their arms and feet which were almost unknown in Giganotosaurus, Mapusaurus, Carcharodontosaurus or Tyrannotitan.” The hips, legs, shoulders and arms of Meraxes, by comparison, are nearly complete, and they indicate that carcharodontosaurs were copycats. Just like the tyrannosaurs that roamed the Northern Hemisphere, Meraxes and family had stubby arms. The entire arm of Meraxes is less than half the length of the dinosaur’s thigh bone. That’s about the same proportions as Tarbosaurus, a close relative of the famed T. rex.
Knowing that some of the last carcharodontosaurs had short arms will help paleontologists track when these dinosaurs started to change. Earlier carcharodontosaurs such as the fin-backed Acrocanthosaurus had longer forelimbs whereas Meraxes shows that the later members of this group had much shorter limbs says SUNY paleontologist Sara Burch, who was not involved in the new study. Tyrannosaurs went through a similar shift over time, and so shortening the forelimb seems to be a shared response to becoming a large meat-eating dinosaur with a powerful bite.
So why did Meraxes and its relatives evolve short arms just like tyrannosaurs did? “We found a correlation between the size of the skull and the arm length in some groups of large-sized theropod dinosaurs,” Canale says, where carnivorous dinosaurs with larger skulls tended to have shorter arms. This connection doesn’t always hold, Burch notes, and some small theropod dinosaurs have very short arms compared to their body size, but a link does seem to exist between heavier heads and smaller arms in the large carnivores. This hints that Meraxes, Tyrannosaurus and similar dinosaurs were all about their bites, with the arms being largely irrelevant to catching prey. That makes anatomical sense. Even among theropods with more prominent arms, such as Allosaurus, the carnivores had a relatively limited range of motion and wouldn’t have been able to see what they were grabbing. For dinosaurs that principally bit down with their jaws like a hyena—rather than catching with claws like a cat—having little arms that were out of the way may have prevented them from being broken, torn off or otherwise injured.
The pattern suggests that some benefit exists to shortening the arms, Burch notes, but what’s more confounding is that the arms of Meraxes and large tyrannosaurs still performed some sort of function in the lives of these dinosaurs. “What we’re not seeing is evidence that these forelimbs were functionless,” Burch says, as “you don’t retain muscles like that to do nothing with them.” The question before paleontologists is what big carnivorous dinosaurs might have been doing with their arms as those appendages became shorter, a mystery that may very well have been lost to the world of the Cretaceous.