T. rex wasn't the only dinosaur with those weird little arms

Quick! Make like a T. rex.

What is the first step to mimicking the famous, fearsome dinosaur? After roaring, a person probably tucks both arms in, contorting them to make them tiny relative to the rest of the body, mashing the five fingers together to have just two digits on each hand. One of the most characteristic features of the iconic tyrant lizard dinosaur is its strange, seemingly uselessly small forelimbs. 

But Tyrannosaurus rex wasn't the only two-legged carnivorous dinosaur to sport such teeny, two-fingered arms.

"Theropods in general do this quite often," Lindsay Zanno, head of the Paleontology Research Lab at the North Carolina Museum of Natural Sciences, tells The Christian Science Monitor in a phone interview. "There are a lot of different groups of theropods that tend to reduce the size of their hands and their arms or change the way that they're used."

And another one is joining the bunch. 

Gualicho shinyae, discovered in Argentina in 2007, is named and described in a paper published Wednesday in the journal PLOS ONE.

This new dinosaur's "arms are short – about 2 ft long – which is less than the length of the thigh bone, and they have weak muscle attachments and poorly developed articulations indicating they had little strength," Peter Makovicky, associate curator of dinosaurs at the Field Museum of Natural History in Chicago who co-led the team that discovered Gualicho, describes in an email to the Monitor.

The fingers on the 90-million-year-old fossil are similar to those of tyrannosaurs. The thumb has a large claw while the second finger is more slender. A third finger has become so reduced that it is just a tiny bone in the flesh of the animal's hand.

"It's really cool that the arms of this particular dinosaur are reduced down to basically two fingers like we see in another very famous well-adapted predator that we know of as Tyrannosaurus rex," says Dr. Zanno, who was not part of the research team.

Could this actually be a new kind of tyrannosaur, a T. rex cousin of some sort?

Gualicho has weak little arms with just two functional fingers like T. rex, but the similarities pretty much stop there.

"This animal has a kind of mosaic of features. There are aspects of its skeleton that show some affinities with some groups of dinosaurs and some affinities with other groups of dinosaurs, although none of those are really tyrannosaurs," study co-author Nathan Smith, associate curator in the Dinosaur Institute at the Natural History Museum of Los Angeles County, tells the Monitor in a phone interview. 

But the "oddball" dinosaur, as Dr. Smith describes it, could help researchers figure out why so many diverse theropod dinosaurs have evolved similar, reduced forelimbs.

Why such tiny arms?

"Gualicho gives us valuable insights into the 'how' of digit reduction," in that it adds another example of the odd evolutionary feature in the fossil record, Dr. Makovicky says. "But we still aren't sure about the 'why'."

"The 'whys' are the hard part to answer in paleo," Zanno says. "But we can speculate on what's going on here."

Some scientists have suggested that humongous predatory dinosaurs would have evolved smaller arms because their skulls were used more readily to wrangle prey, she says.

There seems to be a pattern among tyrannosaurs, for example, in which the arms became shorter and the fingers fewer as the animals' skulls and bodies became larger over generations, says Stephen Brusatte, a paleontologist at the University of Edinburgh who was not part of the study, in an email to the Monitor. This would suggest that "the head was taking over many of the duties that the arms once had, like procuring and processing food."

"Most theropods with reduced forelimbs, like tyrannosaurs, ceratosaurs, and carcharodontosaurs are clearly macropredators that rely on their massive skulls for hunting, so it seems likely that the same was true of Gualicho," Makovicky says.

These diverse dinosaurs were likely under similar evolutionary pressures that lead to similarly reduced forelimbs. The feature would have evolved independently in the different groups, in a process called convergent evolution.

"It's like if two teenagers from the same town start greasing up their hair to impress girls. It just means they are behaving similarly for similar reasons, not that they are brothers," Dr. Brusatte explains.

Oddballs across continents

The mosaic features of Gualicho "makes figuring out the evolutionary placement of this animal a little difficult," Smith says.

Weighing an estimated 1,000 pounds, Gualicho appears to fit into the family neovenatoridae, a large-bodied branch of carnivorous theropod dinosaurs, Smith says, but it also seems to bear the closest resemblance to Deltadromeus, a large theropod from Africa.

But could a South American dinosaur be closely related to an African one?

Possibly. Scientists have previously noted a lot of similarities between dinosaurs unearthed in the Kem Kem Beds on the border of Morocco and Algeria, where Deltadromeus has been found, and the Huincul Formation in Argentina, where Guialicho was discovered, Smith says. "So it may not be surprising that these two carnivorous dinosaurs are close relatives."

And at the time when Guialicho roamed the Earth, the two continents had only recently, geologically speaking, begun to separate as the supercontinent Gondwana broke up.

With some unique traits, the African theropod has been difficult to place on a dinosaur lineage too. "Deltadromeus itself is one of these evolutionary problem taxa," Smith says. 

"It's illustrating to us how little we really know about this part of the dinosaur family tree," Zanno says.

But these fossil oddballs could give scientists insights into long-gone diversity on Earth.

"We can only get so much detail about Earth's history from the animals and the organisms and the plants that are alive today," Smith says. For example, for researchers curious about the evolution of reduced forelimbs, the fossil record contains many more examples to piece together the history of such a trait. 

"It tells us more about the history of Earth and the history of life on it," Smith says.