New dinosaur species reveals short arms evolved repeatedly in theropods

The tiny arms of Tyrannosaurus rex may bring ridicule, but a new dinosaur reveals they were part of a much larger trend.  

A new species of carnivorous dinosaur, Meraxes gigas, may shed light on the reduced forelimbs that continually reappear throughout Cretaceous carnivores. The researchers behind the species' description believe it may point to the presence of a hitherto unappreciated use.

Lead author Dr Juan Canale says, 'I'm convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, suggesting the arm had strong muscles.' 

'While they weren't used for hunting, 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 up after a break or a fall.'

Museum dinosaur expert Professor Paul Barrett, however, notes that any role of short arms in theropod dinosaurs is still yet to be convincingly proven. 

'We didn't know very much about the arm anatomy of carcharodontosaurids before this discovery was made and it demonstrates an interesting example of evolutionary convergence between large predatory theropods,' Paul explains.

'It represents an interesting pattern that occurs in similar ways across different times and different dinosaur groups, but the reasons for this trend remain unclear. This paper opens up new questions on this topic, but does not provide many answers.'

The findings of the study were published in Current Biology. 

What do we know about Meraxes gigas?

The new species was discovered in the Huincul Formation of Argentina, a layer of rocks that were laid down during the Cretaceous Period between 90 and 100 million years ago. Among the dinosaurs discovered in this area is Argentinosaurus, one of the largest dinosaurs which ever lived.

While it wasn't quite on the same scale as the sauropods, M. gigas was a prehistoric giant. It would have weighed more than four tonnes and measured around 11 metres from nose to tail. 

By the standards of its relatives, however, this is relatively average. Some carcharodontosaurids could grow to over 13 metres long, which is around one and a half times the length of a London bus.

These large predatory dinosaurs roamed the world for a period of around 70 million years from the Late Jurassic into the Late Cretaceous. They preceded both the abelisaurids and tyrannosaurids; groups of dinosaurs whose arms also evolved to become significantly shorter. 

The scientists behind the recent study compared measures of evolution across these short-limbed dinosaurs and found that their arms showed more evidence of convergence than would be expected by chance.

The study suggests a link to changes in skull size, with powerful jaws able to overcome some of the disadvantages offered by short arms. A recent theory put forward by Professor Kevin Padian suggests that Tyrannosaurus limbs may have become small to avoid the risk of infection or damage from damaging bites during group feeding on carcasses, which could also apply to this group.

The preservation of M. gigas's forelimbs will offer scientists an opportunity to investigate the potential uses of these arms further in the coming years

The dinosaur's skull, meanwhile, offers hints as to how the species might have behaved. Ridges, furrows and bumps in the bone are found in other theropods, with the researchers suggesting they might have been used for social signalling.

'Those ornamentations appear late in the development when the individuals became adults and may have been used to attract potential mates,' Juan says. 'Sexual selection is a powerful evolutionary force. But given that we cannot directly observe their behaviour, it is impossible to be certain about this.' 

Are many dinosaurs named after pop culture icons?

After realising the bones represented a new species of dinosaur, the researchers named the dinosaur 'giant Meraxes', referring both to its size and a dragon of the Targaryen house in fantasy TV series Game of Thrones.

The scientists are not the first to name a dinosaur after a fictional character. In 1997, Gojirasaurus quayi was named in part after the kaiju Godzilla, while nine years later, Dracorex hogwartsia was named 'the dragon king of Hogwarts' in honour of the Harry Potter series of novels.

Sauroniops pachytholus is one of the most recent species to be given a name referencing a fictional character. Known only from a bone above the eye socket, the species was named in 2012 in honour of the villainous Sauron from the Lord of the Rings, whose giant eye looks out over Middle Earth.

However, the status of many of these dinosaurs is uncertain. Since being named, further research suggests that many of these species may be younger specimens of already existing ones. This can account for differences in size and structure that can make the specimens appear different.

One dinosaur that has, so far, stood the test of time is Bambiraptor feinbergi, named after the deer from the eponymous Disney film. However, such names are generally uncommon in the palaeontological world.

'There aren't many examples of dinosaurs that are named after fictional characters,' Paul says. 'There are one or two named after celebrities, such as Masiakasaurus knopfleri which is named after the Dire Straits musician.'

'In general, however, dinosaurs aren't named after celebrities or fictional characters. This is partly because there are fewer dinosaurs to name than in other groups such as flies or beetles. Naming a new dinosaur doesn't happen nearly as often, so pop culture references have tended to be passed over in favour of names that describe a species' characteristics or where it was found.'

While the specimen itself offers the potential to learn more about Cretaceous predators, only time will tell if M. gigas remains a separate species.