New dinosaur helps scientists understand why meat-eating dinosaurs had short arms

An international team of researchers have discovered a new huge meat-eating dinosaur called Meraxes gigas. The newly-discovered species gives scientists clues about the evolution and biology of similar massive meat-eating dinosaurs including the Tyrannosaurus rex (T.rex) and why these animals had very big skulls and tiny arms. The study, co-led by Peter Makovicky, Juan Canale and Sebastian Apesteguia has been published in Current Biology.

The Meraxes was initially discovered in the Patagonia region of South America in 2012 by the researchers and since then, they have been extracting, preparing and analysing the specimen. The Meraxes gigas is part of the Carcharodontosauridae group of giant carnivorous dinosaurs that includes the Giganotosaurus, one of the largest known meat-eating dinosaurs.

Even though it is not as large in the group, Meraxes was not a small animal by any measure. From snout to tail tip, it measured almost 11 metres and must have weighed around 4,000 kilograms. Its fossils were discovered from rocks that are estimated to be between 90-95 million years old, along with those of other dinosaurs. Its skeleton is one of the most complete carcharodontosaurid skeletons ever found in the southern hemisphere and includes the animal’s whole skull, hips, and all arms and legs.

“The neat thing is that we found the body plan is surprisingly similar to tyrannosaurs like T rex. But, they’re not particularly closely related to T.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?,” said Makovicky in a press statement.

Meraxes provided the researchers with statistical data that led them to the conclusion that large predatory dinosaurs in all three families of therapods grew in similar ways. Therapods are a group of dinosaurs characterised by hollow bones and three-toed limbs. There has been much speculation and debate about how T.rex and other large carnivorous dinosaurs used their tiny forelimbs.

“What we’re suggesting is that there’s a different take on this. 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,” said Makovicky.

Further, the researchers also found that carcharodontosaurids evolved very quickly but then disappeared suddenly from fossil records. This is peculiar because species go extinct because their evolutionary rate isn’t fast enough for them to adapt to their environment. In the case of this dinosaur group, it looks like they went extinct despite evolving pretty fast.

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