New dinosaur species discovered that had tiny arms like T. rex

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Paleontologists in Argentina have discovered a new species of dinosaur, with disproportionately short arms like those of Tyrannosaurus rex.

A fossil of Meraxes gigas, as the new dinosaur has been called, was found in what is now the northern Patagonia region of Argentina, revealing that the creature was 11 metres (36 feet) long and weighed more than four tons, according to a study published in the journal Current Biology on Thursday.

At the time the area would have been hot and humid with lots of waterways and vegetation including large trees, Juan Canale, the project lead at Ernesto Bachmann Paleontological Museum in Neuquén, Argentina, told CNN.

The carnivore is from the Carcharodontosauridae group of dinosaurs, which lived in the Cretaceous period, 145 to 66 million years ago, according to the study.

A raft of Carcharodontosauridae fossils have been found in the past 30 years, but little was known about their skull, forearms or feet.

That has changed with the discovery of M. gigas, thanks to the remarkably complete fossil.

"For the first time we know, in a lot of detail, about certain parts of the anatomy of these giant carnivorous dinosaurs," Canale said.

Researchers found an almost complete forelimb, which allowed them to conclude that M. gigas had tiny arms for such a large dinosaur, a physical feature shared with T. rex that has long puzzled paleontologists.

They also found an almost complete skull and foot, allowing them to shed light on how this group of dinosaurs evolved, said Canale, explaining that there was a trend towards larger body sizes, larger skulls and smaller arms in proportion to the body.

'THERE WAS A KIND OF ARMS RACE'

The fossil was found in the Huincul Formation, where the study said remains of one of the largest known land animals of all time, Argentinosaurus huinculensis, have been found which date from the same period as the M. gigas fossil.

The area is also known to have been home to other carnivorous dinosaurs, albeit smaller ones than M. gigas, as well as other species of long-necked herbivores.

Canale said that it is generally very difficult to establish what dinosaurs ate, but loose teeth found at excavation sites where fossils of herbivorous dinosaurs have been discovered have been matched to carnivorous dinosaurs.

This means we can say that M. gigas would have preyed, at least in part, on these long-necked herbivores such as Argentinosaurus huinculensis, he added.

"It's not a coincidence that giant herbivorous dinosaurs and giant carnivorous dinosaurs lived in the same environments," said Canale, who explained that as herbivores evolved larger bodies as a form of defense, carnivores did too in order to be able to prey on them.

"There was a kind of arms race," he said.

'No direct relation' to T. rex

But the team says that M. gigas evolved separately to T. rex and became extinct almost 20 million years before T. rex walked the earth.

Canale said that, while the two dinosaurs both had large heads and small arms, their bone structure is very different.

"There is no direct relation," said Canale.

The ancestors of M. gigas had longer arms and smaller heads and their arms would have been important in hunting, said Canale, but this changed over time.

Previous research found that dinosaur species such as M. gigas and T. rex developed smaller arms as their heads became larger.

This shows that the arms were not used for hunting, and instead they used their head to kill their prey, said Canale.

"What I think is that, in the more evolved forms ... activities related to predation, such as grabbing or holding the prey, would have been performed straight away with the head," he said.

However, the fossil shows that, although the arms were short, they were muscular, and the chest muscles were also well developed, said Canale.

"This is not consistent with an extremity that doesn't have any function," he said, adding that they could have been used to help get up from the floor, or as support on the female when mating. The researchers do not know whether this fossil belonged to a male or female dinosaur.

The team also found that M. gigas had ornamentations such as crests, furrows, bumps and small hornlets on its skull, which were likely used to attract potential mates.

There is still more work to be done on M. gigas, said Canale, and one colleague at the museum is writing a thesis on its feet and arms.

In addition, there are plenty of fossils that still need to be excavated in the area, as well as dinosaur footprints to analyze, he said.

"We have a lot of work ahead," added Canale.