Why did ‘Tyrannosaurus rex’ have such short arms?
- Written by Sarah Sheffield, Assistant Professor of Earth Sciences, Binghamton University, State University of New York
How did they use their mini-arms?
Short arms don’t seem to have been a problem for these mighty dinosaurs. T. rex was a successful carnivorous species that existed for over a million years. They only went extinct when an asteroid hit the Earth[11], causing a global mass extinction.
Scientists have suggested a few ideas to possibly explain how T. rex used their arms. Maybe they were used as some kind of social display that could impress other T. rex – kind of like the bright feathers of a peacock that can attract potential mates.
But male and female T. rex skeletons don’t show the major differences[12] that paleontologists would take as clues that they relied on social displays to attract mates. And while animal behavior can sometimes be preserved, such as in bite marks[13] or fossilized footprints[14], it’s rare to have enough fossil data to draw clear conclusions.
Maybe T. rex used their arms as weapons to attack or hold down prey. But these options seem unlikely since T. rex’s huge jaws would have made contact with an enemy or prey before the short arms would have been able to reach it.
Some scientists have recently hypothesized that T. rex‘s short arms were an adaptation to competition with other carnivores[15]. If multiple predators were feeding on a carcass, one could get hurt by accidental bites or even intentional warning bites for getting too close. Shorter arms would be less likely to get chomped. Similar things occur today with territorial carnivores, like Komodo dragons[16].
Maybe the arms didn’t have a purpose
Another possibility is that the arms served little or no purpose at all, so over time, they became vestigial[18]. That’s the scientific term for body parts that don’t have clear purposes anymore, but are still passed down through evolution.
One example is a whale’s hindlimbs[19]. Whales evolved from mammals that lived on land that had large legs to move around. The bones are still present in today’s whales, but have gotten much smaller over millions of years and have no function.
Some scientists have suggested a different idea: T. rex’s arms may have evolved to be smaller as another body part grew larger[20]. The fossil record reveals that arms got shorter as theropod skulls got larger[21] across many different dinosaur groups, including T. rex. Larger skulls likely would have made it easier to hunt and eat larger prey.
Researchers can use mathematical equations to accurately predict theropod arm length if they know the animal’s skull size and length of its upper leg bone, the femur. It turns out that larger skulls are strongly linked to shorter arms in theropods.
The reason for the change in arms, however, isn’t as clear. Some scientists have argued that the smaller arms could have helped with balance[22] as the head got larger, but others aren’t so sure. In evolution, there isn’t always a reason why a change occurs – sometimes, changes just happen. In this case, we don’t yet know if there was a benefit for the arms to get smaller as heads got larger.
However they got that way, small arms don’t seem to have been an issue for these big predators.
Orla/iStock via Getty Images Plus[23]
So for now, we don’t really know how T. rex used its arms or why they evolved to be so small, proportionally. As scientists find new data, we will continue to test hypotheses to better understand why this tiny-arm trend occurred so many times in theropod evolution. That’s what makes science so exciting – a future fossil discovery could be the missing puzzle piece that helps us answer these questions.
Sarah Sheffield describes – and her students act out – some of scientists’ hypotheses about T. rex arms.Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com[24]. Please tell us your name, age and the city where you live.
And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.
References
- ^ Curious Kids (theconversation.com)
- ^ CuriousKidsUS@theconversation.com (theconversation.com)
- ^ massive in size (www.amnh.org)
- ^ I’m a paleontologist (scholar.google.com)
- ^ extinct just over 65 million years ago (www.amnh.org)
- ^ couldn’t even reach its own mouth (www.universityofcalifornia.edu)
- ^ theropods (www.britannica.com)
- ^ happened multiple times (www.discovermagazine.com)
- ^ emerge many times in evolutionary history (www.nhm.ac.uk)
- ^ John Zich/AFP via Getty Images (www.gettyimages.com)
- ^ asteroid hit the Earth (theconversation.com)
- ^ don’t show the major differences (www.sciencealert.com)
- ^ bite marks (www.floridamuseum.ufl.edu)
- ^ fossilized footprints (www.nhm.ac.uk)
- ^ competition with other carnivores (www.universityofcalifornia.edu)
- ^ like Komodo dragons (nationalzoo.si.edu)
- ^ Mark Garlick/Science Photo Library via Getty Images (www.gettyimages.com)
- ^ vestigial (theconversation.com)
- ^ whale’s hindlimbs (ncse.ngo)
- ^ another body part grew larger (cse.umn.edu)
- ^ arms got shorter as theropod skulls got larger (doi.org)
- ^ could have helped with balance (www.bbc.com)
- ^ Orla/iStock via Getty Images Plus (www.gettyimages.com)
- ^ CuriousKidsUS@theconversation.com (theconversation.com)
Authors: Sarah Sheffield, Assistant Professor of Earth Sciences, Binghamton University, State University of New York
Read more https://theconversation.com/why-did-tyrannosaurus-rex-have-such-short-arms-273438
