A newly discovered species of carnivorous dinosaur had disproportionately small arms, suggesting that this particular anatomical quirk – shared by the mighty but frail-armed T-Rex – may have been more common among large carnivorous dinosaurs than previously thought.
The newly described species, Meraxes gigas, is named after dragon Meraxes in the fantasy fiction series “A A Song of Fire and Ice’ (the inspiration for HBO’s Game of Thrones) by writer George RR Martin. Meraksi it belongs to a group of theropods—mostly bipedal carnivores—known as the Carcharodontosauridae, which includes others dinosaur titans like Giganotosaurus, Mapusaurus and Carcharodontosaurus. This group lived during the Cretaceous period (about 145 million to 66 million years ago) but became extinct before extinction event which killed all the non-avian dinosaurs and marked the end of the Cretaceous.
Paleontologists excavated the new one M. gigas specimen, which was in excellent condition, from the Huincul Formation of northern Patagonia, Argentina. The fossils date from the early Cretaceous period and are believed to be between 90 million and 100 million years old. Scientists found the bones, which include a nearly complete forelimb and parts of the skull, femur and pelvis, at a site that is rich in fossils; four sauropod dinosaurs were also buried in the same rock layer, said Juan Canale, a researcher at the Ernesto Bachmann Paleontological Museum in Neuquen, Argentina, and lead author of a study on the theropod called a dragon.
Canale and his colleagues dug up several tons of sandstone to get to the fossils, he told Live Science in an email. The authors of the study suspect that when the dinosaur died, its remains were quickly covered by sediment carried by flowing water, which prevented the body from rotting.
In its lifetime, the dinosaur would have weighed well over 4.4 tons (4 metric tons), the scientists calculated.
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However Meraksi and T. rex both had weak forelimbs, not close relatives; instead, this trait is an example of convergent evolution — when distantly related species develop similar characteristics, Canale said.
The evolution of miniature hands in these carnivorous cousins suggests that multiple lineages of large carnivorous theropods evolved to have reduced forelimbs to fill a specific ecological niche.
But not all large theropods had small hands. Some had long forelimbs, like Ornithomimosaurus Deinocheirus and the bird-like theropod Gigantoraptor. This suggests that forelimb reduction was not simply related to theropod body size. Rather, some other trait is being tracked in large carnivorous theropod species — possibly skull size, the study authors report.
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So why do some large theropods such as T. rex and Meraksi, have such small weapons? One explanation could be that some predatory functions in earlier species in Meraksi and T. rex lineages were done with the hands – but in species that evolved later in the group’s lineage, the large head with powerful jaws became a more effective tool for hunting prey.
Interestingly, preserved structures in MeraksiThe arm bones suggest that his small arms had relatively large muscles. Although proportionally small, these limbs may not have been completely useless, Canale said.
“I don’t think they were useful in predation, given that [that] most of the actions involved were most likely done by the head. I tend to think they were used in other types of activities, such as holding the female during mating or helping to raise the body from a lying position,” Canale told Live Science.
And Merax the small hands were not the only feature that caught the attention of paleontologists. The large dinosaur’s skull was surprisingly ornate, decorated with crests, furrows, ridges and miniature horns. This type of ornamentation usually appears late in development when animals become sexually mature, suggesting that complex ornamentation played a role in helping Meraksi find a mate.
“Given that sexual selection is a powerful evolutionary pressure, I think cranial ornamentation is related to some kind of display trait,” Canale said. “But given that we can’t directly observe their behavior, it’s impossible to be sure.”
The findings were published July 7 in the journal Current Biology (opens in new tab).
Originally published on Live Science.