Tyrannosaurus rex and other giant dinosaurs with massive heads may have retained tiny arms to hold their partners during sex, scientists say.
Paleontologists publishing details of a recently discovered four-tonne species, Meraxes gigas, said their specimen shed new light on why some predatory dinosaurs evolved to have such huge heads and diminutive arms.
The M. gigas skeleton excavated in northern Patagonia is among the most complete carcharodontosaurids discovered in the southern hemisphere and includes almost all of the animal’s skull, hips, arms and legs.
With the statistical data it provides, the team found that large predatory dinosaurs in all three families of theropods developed in similar ways. As they evolved, their skulls grew larger and their arms progressively shortened until they were about half the length of their thigh bones.
Juan Canale, lead author of the study in Current Biology and a researcher at the National University of Río Negro, said: “The fossil of M. gigas shows never seen before, complete regions of the skeleton, like the arms and legs, that helped us to understand some evolutionary trends and the anatomy of carcharodontosaurids, the group that M. gigas belongs to.”
“I’m convinced that those proportionally tiny arms had some sort of function”Juan Canale
The authors stressed that T. rex did not derive their short arms from M. gigas or vice versa. Not only did M. gigas become extinct almost 20 million years before T. rex became a species, but they were also far apart on the evolutionary tree.
“There is no direct relationship between both,” Canale said. Instead, he believes that having tiny arms somehow gave different species a survival advantage. “I’m convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles.”
He said the arms were certainly not useful for hunting, as the huge heads and jaws of these species were optimised to grab large prey. “I’m inclined to think their arms were used in other kinds of activities. They may have used the arms for reproductive behaviour such as holding the female during mating or to support themselves to stand back up after a break or a fall.”
The researchers discovered M. gigas in Patagonia in 2012, on their first day of searching, and have spent the last few years analysing the specimen, which died aged 45 or so. Its family of giant carnivorous theropods also includes Giganotosaurus, one of the largest known meat-eating dinosaurs and one of the stars of the recently released film Jurassic World: Dominion.
Although not the largest of the carcharodontosaurids, Meraxes was an imposing animal measuring about 36ft from snout to tail tip, with a skull length over four feet and total weight over four tonnes. The researchers recovered the skeleton from rocks that are about 90-95 million years old, alongside other dinosaurs including several long-necked sauropod specimens.
They also found that carcharodontosaurids including species from Patagonia evolved very quickly, but then disappeared suddenly from the fossil record.
“Usually when animals are on the verge of extinction, it’s because their evolutionary rates are quite slow, meaning they aren’t adapting very quickly to their environment,” Canale said. “Here, we have evidence that Meraxes and its relatives were evolving quite fast and yet within a few million years of being around, they disappeared, and we don’t know why. It’s one of these finds where you answer some questions, but it generates more questions for the future.”