Meraxes gigas - giant carcharodontosaurid with small arms!

Meraxes gigas

(a video version of this article can be watched below) 

This giant carcharodontosaurid theropod dinosaur was named after a dragon in Game of Thrones, Meraxes.  The species name simply means "giant" and large it is!  Its skull is approximately 4' 2" long (127 cm) and it weighed over 9,000 lbs, or 4 1/2 tons in weight.  

Meraxes gigas carcharodontosaurid fossil crates

Meraxes skull reconstruction

Discovered in 2012, it took 4 separate field seasons to excavate the material.  It was found in mudstone covered by sandstone, suggesting the animal died alongside a quite stream or lake and then was buried by a high energy flash flood.  The bones were collected alongside vertebrae of titanosaur and rebbachisaur sauropods.  It lived in the Late Cretaceous of Neuquen province, Argentina, approximately 90 million years ago.

It lived in the Huincul Formation, the same formation that the carcharodontosaurid Taurovenator lives in, suggesting niche partitioning was happening among these large theropods, much like we see in the Late Jurassic Morrison Formation of the western United States where at one locality we have AllosaurusCeratosaurusMarshosaurus, Stokesosaurus, and Torvosaurus all living side by side!

How big of a skull?

The skull is longer than that of Acrocanthosaurus by a few inches but is significantly smaller than the giant Giganotosaurus.  Since "Giga"'s discovery the size of its head has been of great debate.  Meraxes bones are nearly complete and undistorted and allowed Juan Canale and his team to determine a more accurate estimate of the partial cranial material of the Argentine behemoth.  Their work indicates the skull was between 5'2" and 5' 6" long (158 - 169 cm)!  

Giganotosaurus Meraxes carcharodontosaur Argentina

Giganotosaurus skull bones.  Image from Coria and Salgado 1995

Interestingly, though they address the size of fragmentary Carcharodontosaurus and Spinosaurus specimens, suggesting the former reached 5'5" long and the latter's claims to be > 6' may not be true, they did not mention any of the giant Tyrannosaurus partial skulls, including possibly the longest of them all, "Scotty".

Meraxes lived in rocks younger than Giganotosaurus but older than Mapusaurus, all within a few million years of one another.  I am always amazed that the carcharodontosaurids, these dominant and giant predators, die out long before the Very Bad Day.  They were replaced by abelisaurs, super-short armed theropods and the enigmatic megaraptorids. Before they went extinct, however, the diversity of carcharodontosaurids is nothing short of amazing and I fully expect many new genera to be named in the coming years. However, why they went extinct is unknown, the last of their kind currently known was Shaochilong that lived in Asia around 80 million years ago.

How old?

The study conducted a histological analysis of the limb bones and determined Meraxes died somewhere between 39 and 53 years of age.  This is nearly double that of Tyrannosaurus!  It grew at a rapid rate for much longer than its distant Allosaurus cousins and reached maturity at a far later age than any other known theropod.  This specimen, MMCh-PV 65, is currently the oldest known dinosaur yet aged.  

Why the small arms?

Amazingly, it was found with a nearly complete forelimb, including most of the fingers and wrist bones.  This was a first for the carcharodontosaurids and, to the surprise of everyone, the arm was very short, only .4 the ratio of the femur, similar to that of tyrannosaurids and abelisaurids, which not probably coincidentally have giant heads as well.  The authors proposed a clever reason for why the arms didn't grow smaller over time, that being the large scapulocoracoids these three taxa possess.  Such a large shoulder blade means lots of muscle was attaching to it and, since muscles have two heads, the other end had to attach to the humerus.  As long as the shoulder was large the humerus would have to be a minimum size for all the musculature to attach to.  Until something in their evolution selected for smaller shoulder blades, they posit there is a minimum arm size, which seems to be .4 the length of the femur.  Though tiny, these small arms are all quite muscular, suggesting they were being used for something, either to better hold prey for a precise bite or having something to do with mating are the most likely reasons.

Meraxes gigas carcharodontosaur Argentina Cretaceous

Meraxes carcharodontosaurid arm from Canale et al. 2022
Daspletosaurus tyrannosaurid small arm
Daspletosaurus small arm at the Rocky Mountain Dinosaur Resource Center
majungasaurus abelisaur
Majungasaurus small arm at Eccles Dinosaur Park

Meraxes demonstrated that these giant theropods did not have the wickedly massive claws of the Jurassic giants such as TorvosaurusSaurophaganax, or even their distant cousin Allosaurus.  Surprisingly to me Meraxes has one very long, and sharp, toe claw that even has a sharp ridge on the bottom of its surface.  I suspect it was used like dromaeosaurids, for prey manipulation.  It would stomp on prey, possibly much smaller than itself, and while holding it underfoot it would use that massive mouth to bite and rip backwards, removing hundreds of pounds of flesh in one go.  This, to me, makes more sense than treating sauropod dinosaurs as walking meat lockers. :-)

The skeleton had a surprisingly fused, and pneumatic, sacral and proximal caudal region.  We see this in many sauropods as a way for a giant animal to have a modicum of lightening while still providing for attachment points of powerful muscles.  I wonder if we'll see such proximal tail pneumatization in other large carcharodontosaurids that will be discovered in the future?

Meraxes gigas carcharodontosaur Argentina Cretaceous

Skeleton of Meraxes from Canale et al. 2022

What an incredible find this animal was and I hope they are able to find not only additional specimens of Meraxes but also most of Taurovenator, for as of today it is known from a single skull bone. 

 

Information from Canale et al. 2022 

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