A team of palaeontologists led by researchers at University College London (UCL) and Cambridge University may have just found the answer as to why large theropods evolved such tiny arms.
The study, published in the journal Proceedings of the Royal Society B, examined 82 species of theropods and found that shortening of forelimbs occurred across five groups. These groups included tyrannosaurids, the family to which T.rex, Tarbosaurus, Albertosaurus, Gorgorsaurus, and several other iconic Late Cretaceous-aged predators belong.
An adult T.rex had arms that were roughly the size of our own, measuring 1m from the shoulder to the finger tips. While they were comically small compared to the rest of their bodies, they were surprisingly robust and operated by thick, powerful muscles. Carnotaurus - a distant, South American cousin - had even smaller arms than T.rex, measuring just 45cm in length.
By studying such a large diversity of theropods, the researchers were able to identify correlations in certain morphologies. They found that smaller arms were closely linked to the development of large, powerful skulls and jaws, more so than to larger body size. This suggested that small arms were not just an evolutionary leftover from their bodies getting bigger.
“We sought to understand what was driving this change and found a strong relationship between short arms and large, powerfully built heads,” said lead author Charlie Roger Scherer, a PhD student in UCL’s Earth Sciences department. “The head took over from the arms as the method of attack. It’s a case of ‘use it or lose it’ - the arms are no longer useful and reduce in size over time.”
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Based on their findings, the researchers suggested that an increase in the size of theropods’ prey may have driven this change in the style of hunting. As sauropods and ceratopsians got bigger, theropods shifted from using their claws to their jaws. This interpretation is backed up by the fact that theropods with larger, more powerful skulls evolved in areas with giant prey.
The researchers think strongly built skulls likely evolved before shorter forelimbs. “It would not make evolutionary sense for it to occur the other way round, and for these predators to give up their attack mechanism without having a back-up,” said Roger Scherer.
As part of this latest study, Roger Scherer and colleagues developed a new way to quantify skull robustness, using factors such as: how tightly connected the bones of the head were, the dimensions of the skull (a compact shape is stronger than an elongated shape), and bite force.
Using this method, the researchers found T.rex had the most robust skull - to not much surprise - followed by Tyrannotitan, a lesser-known cousin that was similarly large. Tyrannotitan belonged to a different group of theropods known as carcharodontosaurids and lived more than 30 million years before T.rex in what is now Argentina.
The fact that small arms evolved in five different groups of theropods suggests they were subject to similar selection pressures - the biggest factor likely being the predator-prey arms race between themselves and giant herbivores.
While tyrannosaurids, abelisaurids, carcharodontosaurids, megalosaurids, and ceratosaurids all evolved small arms, the way they did so differed.
The researchers noted the forelimbs of these theropods reduced at different rates. The hands and the lower part of the arm (past the elbow) shortened most in abelisaurids. Majungasaurus, a relatively small abelisaurid that lived in Madagascar, had particularly tiny hands - so small that they’re hardly visible in most reconstructions. The forelimbs of T.rex, on the other hand, are thought to have reduced proportionately.
Considering this evolutionary trend of increasingly smaller arms amongst large theropods, it’s reasonable to think that - if non-avian dinosaurs weren’t wiped out 66 million years ago - giant, armless theropods may have eventually evolved. These hypothetical theropods may have looked a lot like sharks, but with legs.
Top image credit: Roger Harris/Science Photo Library/Getty Images
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