Researchers believe that as Meraxes gigas’s head grew, the front arms shrank.
Tyrannosaurs rex gets all the fanfare, but after a recent discovery in Argentina’s northern Patagonia region, the terrible lizard may just be one of at least three different dinosaurs to show off those abnormally large heads coupled with puny little front limbs.
Now, scientists believe there’s a correlation between the two body parts.
Researchers from Argentina and the United States have spent the last decade extracting, preparing, and analyzing a new dinosaur of the carcharodontosauridae family. Last week, the team published a new study in the journal Current Biology that showcases how the Meraxes gigas resembles both the T. rex and abelisauridae families with a distinct head-to-arm ratio.
M. gigas (named after a Game of Thrones dragon, of course) predates T. rex, leading researchers to believe that as the skulls and heads of the meat-eating dinosaurs continued to grow, the front limbs only shrank more and more.
The most recent find, thanks to Juan Canale, lead author of the new paper, is believed to be from a 9,000-pound, 36-foot-long dinosaur and marks the most complete carcharodontosauridae skeleton found in the southern hemisphere. Archaeologists first unearthed the M. gigas specimen in 2012.
This relatively unheard of, half-complete skeleton allowed researchers to piece together the parallels between dinosaur families, along with understanding how this family reached “peak diversity shortly before their extinction with high rates of trait evolution in facial ornamentation possibly linked to a social signaling role,” the authors write. Notably, M. gigas went extinct millions of years before T. rex. But that’s not all.
“The neat thing is that we found the body plan is surprisingly similar to tyrannosaurs like T. rex,” Peter Makovicky, a principal author and professor at the University of Minnesota N.H. Winchell School of Earth and Environmental Sciences, says in a news release. “But they’re not particularly closely related to T. rex. They’re from very different branches of the meat-eating dinosaur family tree. So, having this new discovery allowed us to prove the question of, ‘Why do these meat-eating dinosaurs get so big and have these dinky little arms?’”
Using statistical data mined from M. gigas, compared to other fossils previously discovered, researchers found that the mega-predatory dinosaurs in all three families grew in similar ways, with the skulls getting larger as the arms progressively shortened.
“What we’re suggesting is that there’s a different take on this,” Makovicky says. “We shouldn’t worry so much about what the arms are being used for, because the arms are actually being reduced as a consequence of the skulls becoming massive. Whatever the arms may or may not have been used for, they’re taking on a secondary function since the skull is being optimized to handle larger prey.”
The limbs never dropped below a forelimb-to-femur ratio of 0.4 and remained functional. Of course, the larger the skull—and the jaw that goes with it—the bigger the prey any of the three families of dinosaurs could go after. Plus, what’s more intimidating, large arms or a ginormous head?