The Tyrannosaurus rex (T. rex) is known for its bone-crushing bite, gigantic size, and famously small forelimbs. But why these large, carnivorous theropod dinosaurs evolved tiny arms has long been debated.
It now appears that the evolution of these shortened arms may be linked to the "tyrant lizard king's" skull and enormous size.
In a new study, published in Proceedings of the Royal Society B, researchers report that forelimbs shrank independently in at least five lineages of carnivorous dinosaurs, consistently alongside the evolution of more robust skulls and heavier bodies. The researchers suggest that as these animals evolved stronger jaws for tackling larger prey, their forelimbs became progressively redundant.
“It probably is how strong the skull is that's driving the forelimb to become smaller over time,” a paleontologist, Charlie Scherer at University College London, tells Refractor.
In an interview, Scherer told Refractor that he theorized that T. rex and its close relatives evolved smaller forelimbs by using their skulls more in hunting rather than using their forelimbs.
To understand the mechanisms behind forelimb reduction, Scherer and his colleagues compiled forelimb, cranial, and body mass data for 85 species of non-avian theropods (two-legged dinosaurs).
The researchers also tracked the evolutionary tree to check how the physiological traits of these theropods have changed across different lineages and geological time periods. Using this data, the team calculated the skull-forelimb ratio to standardize arm size across different species.
They demonstrated that forelimb reduction was not just a by-product of dinosaurs getting larger, but rather an evolutionary adaptation tied to a robust, stronger skull.
“It’s not necessarily how big it is [skull], but how strong it is, which is driving reduction,” says Scherer.
The team found the evolution of reduced forelimbs occurred independently in at least five distinct carnivorous lineages: Abelisauridae, Carcharodontosauridae, Ceratosauridae, Megalosaurinae, and Tyrannosauridae.
“The reason why the increasingly robust skull drove the forelimbs to become smaller is that these animals were hunting larger prey items,” Scherer told us. “It's probable that they couldn't really use their arms very much in order to bring down a massive furbivore.”
The dinosaurs “don't need a strong or very large arm when the head is doing most of the work,” he adds.
Some dinosaur species, like Spinosaurus and Megareparoids, did exhibit elongated arms.
“It's probably because they weren't eating the same prey as some of their co-occurring theropods,” Scherer explains. “If you're eating the same thing, probably you're going to become adapted in the same sort of way to eat them”
The paleontologist also hypothesizes that these dinosaur species used their arms more than their jaws.
Scherer says that a similar evolutionary pressure appears in toothed whales. Because larger prey are way too big to fit down their throats, toothed whales eat small to medium-sized prey.
“Prey choice or prey preference does have an impact on the overall morphology and strength of the skull,” Scherer concludes.
The study has been published in Proceedings B of the Royal Society.
Fact-checked by Mike McRae
