New Giant Predatory Dinosaur Suggests T. Rex's Tiny Arms Were Mating Apparatus

Those lil' twigs may have been what peak sexual performance looked like 66 million+ years ago.


Rachael Funnell

Digital Content Producer

clockJul 7 2022, 15:05 UTC
meraxes gigas small arms
Meraxes gigas had tiny arms before it was cool. Image credit: Carlos Papolio,

Hunting with your face was the done thing back when Tyrannosaurs (like T. rex) were going ham sammich face-first on their prey. This meant that arms weren’t all that necessary for predation, which could lead you to think that their tiny limbs were useless, but the discovery of an unrelated dinosaur with similarly small appendages has led researchers to think otherwise.

A Meraxes gigas specimen is to thank for the fresh insight, a new-to-science species retrieved from Patagonia in excellent condition. It belonged to the Carcharodontosaurids and is the most complete of the group of dinosaurs ever found in the Southern Hemisphere.


“The fossil of M. gigas shows never seen before, complete regions of the skeleton, like the arms and legs that helped us to understand some evolutionary trends and the anatomy of Carcharodontosaurids,” said project lead Juan Canale of the Ernesto Bachmann Paleontological Museum in Neuquén, Argentina, in a statement.

Convergent evolution is a process through which unrelated organisms evolve to have similar traits, owing to the fact that those features serve some kind of beneficial purpose. Carcinization is perhaps one of the best examples, a phenomenon that has led many to believe that becoming a crab could be inevitable.

M. gigas and T. rex were not closely related, and the former went extinct 20 million years before the latter even existed. As such, researchers on a new paper published in Current Biology conclude that their shared feature of tiny arms is an example of convergent evolution, and therefore could indicate that having mini limbs garnered these animals some benefit.

“I’m convinced that those proportionally tiny arms had some sort of function,” said Canale. “The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles.”

meraxes gigas small arms
M. gigas had a very ornamental head. Image credit: Jorge A Gonzalez, CC BY SA

Carnotaurus is another famously small-armed dinosaur, and Prehistoric Planet recently did a heck of a job demonstrating how they may have used their highly-muscular, miniature arms as a means of attracting a mate. However, Carnotaurus was unique in having a ball socket at its joint made for twirling (like our shoulders). So, could M. gigas and T. rex’s arms also be useful in the prehistoric bedroom?

“They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall,” Canale argued. “Sexual selection is a powerful evolutionary force. But given that we cannot directly observe their behavior, it is impossible to be certain about this.”

T. rex’s tiny arms have been puzzling palaetonologists for some time, and previous suggestions for their benefit have included simply keeping themselves out of the way during group feeding sessions. This could perhaps link up with Canale’s movement theory as hungry T. rex may have needed to duck and maneuver while trying to get their share of the bloody spoils.

As for M. gigas, beyond being tiny-armed dinosaurs, these babies were also pretty ornamental in the skull department. Studying the skeleton discovered by Canale and colleagues revealed crests, furrows, bumps, and small hornlets on its head. You fancy, M. gigas.


“The fossil has a lot of novel information, and it is in superb shape,” said Canale, who is planning to continue working with their fortuitous find. “We found the perfect spot on the first day of searching, and M. gigas was found… It was probably one of the most exciting points of my career.”

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