Tyrannosaurus Rex's small arms are what is most well-known about the dinosaur. For over 100 years, paleontologists have struggled to understand the dinosaur's anatomical peculiarities. Tyrannosaurus wasn't the only dinosaur with tiny appendages. Today, Current Biology has published the description of Meraxes gigas by an international team paleontologists. This was a large meat-eating dinosaur that had surprisingly short forelimbs.
Juan Canale, Universidad Nacional de Rio Negro paleontologist, says that it was a surprise. He starts with the discovery of Meraxes. It can take paleontologists many days or even weeks to find a new fossil from a dinosaur. A common paleo superstition states that the best finds of a trip are made on the last afternoon of the day. The bones of Meraxes were discovered on the first day in 2012 during a field trip. Canale said that the team discovered a large, carnivorous dinosaur and four herbivorous dinosaurs in the same rock layer.
It wasn't until the bones were removed from the encasing rocks that it became clear how unique this theropod dinosaur was. This theropod is not only a new species but also one of the best known specimens in its family. 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. This is just the beginning of the pop culture connection. Meraxes is a moniker of a dragon from George R.R.'s Song of Ice and Fire series. Martin is a name that suits a sharp-toothed dinosaur weighing more than four tonnes.
Meraxes is only one part of a partial skeleton. However, there are some distinguishable 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. Canale and coauthors have estimated that Giganotosaurus' skull measured approximately 5.3 feet in length, which is a bit shorter than the previous maximum estimates. The limbs of Meraxes, however, might be the most interesting part of the new dinosaur.
Canale states that there is "a lot of information" which helps us reconstruct the appearances of other giant, related carcharodontosaurs. "The hips, legs and shoulders of Meraxes are almost complete and indicate that carcharodontosaurs were copiedcats." Meraxes, like other tyrannosaurs who roamed the Northern Hemisphere had stubby arms. Meraxes' entire arm is half the length of the dinosaurs' thigh bone. This is about the same size as Tarbosaurus (a distant relative of T. rex).
Paleontologists will be able to track the evolution of these dinosaurs by knowing that some of the last carcharodontosaurs had shorter arms. SUNY paleontologist Sara Burch said that earlier carcharodontosaurs, such as Acrocanthosaurus with fins, had shorter forelimbs. However Meraxes showed that later members of the group had shorter limbs. Tyrannosaurs experienced a similar change over time. Therefore, shortening the forelimb may have been a common response to becoming large-eating dinosaurs with powerful bites.
Why did Meraxes and its cousins develop short arms like tyrannosaurs? Canale states that there was a correlation between the skull size and the length of arms in large-sized groups of theropod dinosaurs. Carnivorous dinosaurs with bigger skulls had shorter arms. Burch points out that this connection may not always be true. Some small theropod dinosaurs are smaller than their bodies, but there is a correlation between larger skulls and shorter arms in large carnivores. This suggests that Tyrannosaurus, Meraxes and other dinosaurs focused on their bites. The arms were largely irrelevant in catching prey. This makes perfect anatomical sense. Even with theropods that had more prominent arms like Allosaurus', carnivores still had a limited range of motion. They wouldn't be able to see what the other dinosaurs were grasping. Dinosaurians that bit down with their jaws more like a hyena than with their claws like cats may have been protected from injury by having little arms that could be thrown out of reach.
Burch says that this pattern suggests that there is some benefit to reducing the length of the arms. But what's more confusing is the fact that large tyrannosaurs and Meraxes still had some function in these dinosaurs' lives. Burch states that there is no evidence that these forelimbs are functionally useless. This is a puzzle that paleontologists have to answer.