Tyrannosaurus rex was one of the mightiest predators of its time. This theropod grew to be about 45 feet long, and with its large, sharp teeth and wide, powerful jaws, it must have been a sight to behold. The only thing that didn’t feel so mighty about this predator was its small arms. According to a report from the University of California, T. rex's arms were only about 3 feet long. For comparison, this would be like a 6-foot-tall adult human with 5-inch arms.
Though these smaller arms may not have provided much predatory advantage, T. rex and other theropods didn't need them as they evolved larger skulls and jaws.
Publishing their findings in the Proceedings of the Royal Society B, researchers from University College London (UCL) and Cambridge University may have identified one of the driving factors behind such small arms.
Theropods with Big Heads and Little Arms
For the study, the research team, which included Elizabeth Steell at Cambridge and Paul Upchurch at UCL, analyzed data on 82 theropod species and looked for signs of limb shortening. The results indicated that forelimb shortening occurred in five theropod groups, including: tyrannosaurids, abelisaurids, carcharodontosaurids (including the Tyrannotitan), megalosaurids, and ceratosaurids.
“Everyone knows the T. rex had tiny arms, but other giant theropod dinosaurs also evolved relatively small forelimbs. The Carnotaurus had ridiculously tiny arms, smaller than the T. rex,” said Charlie Roger Scherer, a Ph.D student at UCL Earth Sciences, in a press release.
The team noted that smaller arms were closely associated with a species developing larger, more powerful skulls and jaws rather than overall larger body size. This led the team to believe that smaller arms were not only a by-product of larger bodies.
“We sought to understand what was driving this change and found a strong relationship between short arms and large, powerfully built heads. 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,’’ Roger Scherer added in the press release.
Read More: Why These 4 Massive Dinosaurs Also Had the Tiniest Arms
What Drove Theropods to Grow
From the data, the research team was likely able to discern what led to theropods like T. rex to grow so large — their prey was getting larger.
For meat-eating theropods, their prey usually consisted of the giant herbivores. The larger the prey grew, the less useful the claws became, making larger heads and jaws more necessary.
“These adaptations often occurred in areas with gigantic prey. Trying to pull and grab at a 100-foot-long sauropod with your claws is not ideal. Attacking and holding on with the jaws might have been more effective,” Roger Scherer said.
According to the research team, larger prey may have triggered an “evolutionary arms race,” leading theropods to evolve larger body sizes as well.
Comparing Skull Shapes and Sizes
During their research, the team developed a novel method to rate skull robustness, incorporating factors such as the tightness of bone connections, skull dimensions, and bite force. Based on these factors, the team ranked T. rex highest, with Tyrannotitan — nearly as large as T. rex — coming in second.
Upon comparing skull length with arm length, the team noted that smaller forelimbs were more strongly associated with skull robustness than with skull and body size. Some theropods, like the Majungasaurus, had robust skulls and small arms, but were not very large compared to T. rex, indicating again that forearm size does not correlate with overall body size.
While there is evidence of this smaller-limbed evolution, it does not mean that it happened all at once.
“While our study identifies correlations and so cannot establish cause and effect, it is highly likely that strongly built skulls came 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 backup,” Roger Scherer said.
Read More: Why Did T. Rex Have Such Tiny Arms? To Keep Them Out of the Way
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article includes information from a study published in Proceedings of the Royal Society B
- This article includes information from The University of California: Why T. rex has tiny arms
