Why did T.rex have such tiny arms?

When you think of a Tyrannosaurus rex, the first thing that comes to mind is its gigantic, bone-crushing jaws, sharp teeth and fearsome stature. But then, immediately after that, a slightly more comical image pops up: those ridiculously short, tiny front arms. For decades, scientists and dinosaur enthusiasts have chuckled at the incongruity of the apex predator's viciousness and its seemingly useless little claws.Turns out the T. rex wasn’t the only carnivore with an upper body layout that skipped arm day. At least five lineages of two-legged, meat-eating dinosaurs lost the use of their forelimbs and reduced them to comic proportions over millions of years. Now, a new study has found the answer to this evolutionary mystery, and the answer turns out to be a very simple rule of nature: use it or lose it.In a fascinating piece of research published in the Proceedings of the Royal Society B: Biological Sciences, scientists have discovered that as these giant predators evolved, their heads took over the heavy lifting entirely. The team of scientists studied the physical data of 82 different species of meat-eating dinosaurs to find out why their arms shrank. They discovered that the secret was hidden right in the creatures' massive, heavily built skulls.The head assumes the huntThe team of researchers created an elaborate system to assess the true strength and sturdiness of a dinosaur skull. They checked the heads for their overall measurements, the amount of fusion in the bones of the skull, and the overall bite force. When they compared these heavy-duty heads to the sizes of the dinosaurs’ limbs, one clear pattern emerged. There was a direct correlation between tiny arms and super-strong, powerful skulls.As these carnivores got bigger and began to pursue larger prey, trying to grab onto a shifting giant that was 100 feet long with relatively small claws was asking for trouble. Evolution favoured dinosaurs that could catch and hold their dinner with just their jaws instead. Their heads, over generations, became their main weapons of attack. Arms were no longer needed to hold down struggling prey, so they naturally shrank over time.Throughout the entire study, the well-known T. rex scored the highest points for bite force. Following close behind was the Tyrannotitan, a giant predator that roamed Argentina some 30 million years before its more famous North American relative. The two giants depended on their crushing bites for hunting, and so their front limbs had little to do. Different shrinking stylesInterestingly, the study noted, this arm-shrinking trend happened even in smaller carnivores. Majungasaurus was a fifth of the size of a T. rex, and weighed in at the time in Madagascar 70 million years ago. But it had an incredible head and a mighty bite, and its arms were equally as underdeveloped.The way these limbs shrank was entirely determined by the family tree of dinosaurs. For example, the Majungasaurus belongs to a group of abelisaurids which had a significant reduction below the elbow, leaving them with incredibly tiny forearms and hands. Meanwhile, the arms of the tyrannosaurid family were getting more and more stunted from shoulder to fingertips.If you’re wondering which ancient carnivore takes the prize for the absolute shortest limbs, the winner is the Carnotaurus. This odd-horned predator had front limbs that were even more ridiculously tiny and stumpy than those of the T. rex. In the end, these ancient predators were the masters of the killing bite, and their bodies simply sacrificed arm strength for ultimate power in the braincase.