Meraxes Gigas.

Image Credit: Meraxes Gigas.

We have a new dinosaur! A theropod with short limbs like Tyrannosaurus Rex. Tyrannosaurus is not the only famous giant carnivorous dinosaur; meet Meraxes Gigas.

During the summer of 2022, researchers uncovered a previously unknown dinosaur from the Late Cretaceous, Meraxes Gigas. Meraxes belongs to the genus carcharodontosaurid theropods. The name, “Meraxes”, honors a dragon from the George R. R. Martin novel, A Song of Ice and Fire – Wikipedia The specific name, “Gigas”, derived from the Greek word meaning “giant”, in reference to the theropod’s large size.

Found in the Huincul Formation of Argentine Patagonia was a nearly complete skeleton. Meraxes Gigas is of pivotal importance because this discovery constitute a complete skull, partial forelimbs, complete hindlimbs, fragmentary ribs, and cervical and complete caudal vertebrae. 

The hand of theropods is imperative to determine whether the specimen is primitive or derived. Primitive in dinosaur terminology means an early stage in evolutionary development. Derived means most recent in dinosaurian evolution. To help this make sense, Coelophysis is a primitive ancestor of the derived Tyrannosaurus Rex.

Image Credit: Gignotosaurus skeleton, notice the longer arms and three digits on each hand. Giganotosaurus is more derived than the primitive theropod Coelophysis.

Primitive theropod dinosaurs have longer arms and digital formula I-IV. Derived theropods will have shorter arms and a reduced number of digits.

Image Credit: Coelophysis primitive theropod dinosaur. Notice that Ceolophysis has longer arms and 4 digits on each hand.

Coelophysis, the oldest known theropod, defined as a primitive Late Triassic theropod had longer arms and 4 digits on each hand.

Image Credit: Allosaurus

Allosaurus, more derived lived during the Middle-Late Jurassic and possess shorter arms and only three digits on each hand. Tyrannosaurus the Cretaceous apex predator had very short arms and only 2 digits on each hand.

Image Credit: Noelle K. Moser. Walter, Tyrannosaurus Rex located in Washington, D.C. has very short arms and only 2 digits on each hand.

As observed by these images, the skull, forearms, hands, and digits are gold in theropod discoveries. The hands of theropod dinosaurs tell us so much about the evolution of dinosaurs during the Mesozoic.

In addition to the forearms and digits, complete or near complete skulls found in new dinosaurs’ discoveries help us understand more about the creatures.

At first glance, the skulls of these giant theropods appear similar; massive jaws with numerous long serrated teeth. To the trained eye, the skulls of Giganotosaurus and Tyrannosaurus Rex are remarkably different.

Giganotosaurus has a long narrow skull that comes to a sharp point. The skull of Tyrannosaurus Rex is boxier and comes to a lateral line rather than a point. See the images below.

Another example of primitive and derived evolution is theropods, Allosaurus, Ceratosaurus, and, Tyrannosaurus. See the images below.

While these three theropods appear similar, Allosaurus and contemporary Ceratosaurus predate Tyrannosaurus by 80 million years.

Head Creasts:

Another feature present in theropod dinosaurs, specifically primitive species, is that of head crests. Many Late Triassic and Early Jurassic theropod species possessed head crests. It is thought that these crests were used as display features for mating and sexual dimorphism. In later derived theropod species, these crests disappear.

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The image above depicts Ceratosaurus nasicornis, the apex predator of the early Jurassic, displaying crests on the head can be seen. Ceratosaurus was driven to extinction by the Late Jurassic, succeeded by the more successful Allosaurus.

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Allosaurus, as mentioned above, is the most famous predecessor to Tyrannosaurus Rex. Living through the greater part of the Jurassic, Allosaurus was the apex predator of its time and possessed primitive features – three-digit hands, long forearms, and head crests. It is now known why derived theropods lose the head crests.

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By the time we traverse, the expansive amount of time between Ceratosaurus and Tyrannosaurus Rex (80 million years) head crests disappear from theropod anatomy.


In addition to reduced digits and head crests, as theropods evolve they get larger. Coelophysis at maturity was about 10 feet long and weighed 100 pounds. Compare that to the Jurassic theropod Ceraosaurus which grew to 20 feet long and weighed about 2,000 pounds. Allosaurus grew to be 12 feet long and 10 feet tall weighing 2.3 metric tons. Tyrannosaurus Rex, the largest land theropod to ever live grew to 43 feet long and weighed 6-8 tons.

Meraxes Gigas:

Now, that you have a better understanding of theropod dinosaurs, let’s take another look at Meraxes gigas. Meraxes is a crucial find in the world of dinosaur studies because the remains included key anatomical features, a complete forelimb, hand, a skull, and various other bones. The most important being that of the hand and skull.

According to the research, Meraxes Gigas had tiny arms like Tyrannosaurus with three digits on each hand like Allosaurus. Tyrannosaurus and Meraxies are not related as they are separated by about 20 million years, but it does suggest that as theropods evolved larger heads, the arms shrunk, no longer useful for hunting. Regarding size, Meraxes Gigas was 36 feet long and weighed 4 tons. To put this into perspective, Meraxies is smaller than Tyrannosaurus Rex but much larger than Allosaurus. Further noted, Meraxies does not appear to have head crests.

In terms of theropod evolution, Meraxies is more derived than Allosaurus but primitive compared to Tyrannosaurus. In terms of theropod evolution, this put Meraxies Gigas living about 90 to 100 million years ago, firmly in the Late Cretaceous. Meraxes Gigas and its close relative, Giganotosaurus died out in the Late Cretaceous, succeeding their position at the top of the food chain to the tyrannosaurids, and its famous member, Tyrannosaurus Rex.

Meraxes Gigas and the newly discovered theropod dinosaur were one of the last groups of giant carnivores to walk the Earth. While we often think of Tyrannosaurus Rex as being the lone giant carnivore stalking the Cretaceous, other large theropods competed with the tyrant dinosaurs. Although Meraxes Gigas never laid witness to the meteor that would ultimately bring the reign of the dinosaurs to an end, Tyrannosaurus Rex did.

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As always, thanks for reading Coffee and Coelophysis. Next time, we will take an in-depth look at the most famous theropod, Tyrannosaurus Rex.

~ Noelle ~

Welcome to Coffee and Coelophysis.

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They say that the best place to start is the beginning, with dinosaurs that is Coelophysis.

Dinosaurs are a unique biological group and were the most successful animals that evolution ever produced, and their reign as dominant creatures spanned millions of years. Compared to humans who have only been here for 6 million years. But dinosaurs did not spawn out of midair; dinosaurs were the inheritors of the Earth after the Permian Extinction or the Great Dying.

Approximately 251 million years ago, Earth suffered the most severe extinction. It is theroized that the Great Dying was the cause of increased volcanic activity that released large amounts of carbon dioxide into the atmosphere leading to increased ocean temperatures. The increased ocean temperatures disrupted the ocean conveyor belt responsible for breathing life into the oceans. As the seas became stagnant, hydrogen sulfide began to accumulate. Hydrogen sulfide is poisonous thus, all life in the oceans perished. As the gas escaped into the surrounding atmosphere, life on land began to die. The oceans experienced increased anoxia (depletion of oxygen) and acidification (reduction of Ph). As a result, many plants and animals that inhabited the oceans and surrounding land were wiped out. But life is resilient; over millions of years, life on Earth recovered. Only this time, out of the ashes of the Permian Extinction, new and more exciting life forms emerged.

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One of these life forms was the Stegocephalians. Stegocephalians developed unique adaptions of bones, jaws, and lungs. Most importantly, Stegocephalians evolved specialized fins with wrists and ankles helping push through the dense vegetation in swampy waters. They could also use these specialized fins to push along land if they needed to travel from one water source to another. Stegocephalians are very important to our discussion on dinosaurs because they are the pivotal key to the evolution of the dinosaurs. 

In terms of cladistics (classifying animals based on ancestral descent) throughout the course of evolution, three clades of animals are of importance: Anapsida, Synapsida and Diapsida

These three clades recognized by the diversification of the arrangement of skull bones, specifically the presence and position of temporal fenestrae (holes in the skull), a latch where skull bones fuse. In humans, we can feel our temporal fenestrae at either side of our skull behind our eye sockets. Most vertebrates possess temporal fenestrae, where and how many fenestrae various vertebrate organisms have can classify organisms into clades based on evolutionary lineage.

For example, below is a graphic of a Massospondylus Skull, labeled the temporal fenestra and another fenestra present in this animal’s skull.

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The anapsids, synapsids, and diapsids are three different clades of animals leading to various lineages. Anapsids are animals that lack temporal fenestrae. Fossil and modern turtles best represent this group of animals.

Synapsids and diapsids evolved from a common ancestor of anapsids during the later part of the Carboniferous period (Martin, pg.165). Synapsids during the Permian represented by herbivorous and carnivorous reptiles called Pelycosaurs. Dimetrodon is a perfect example of a Pelycosaur. Although not a dinosaur, Dimetrodon had a formidable appearance which causes many to assume it is a dinosaur.

Synapsids included lineages that later evolved into therapsids mammal-like characteristics and eventually into mammals. Dimetrodon is a very distant ancestor in our evolutionary lineage. Humans are Synapsids as are most mammals for which we share this evolutionary trait.

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That leaves Diapsids; dinosaurs are diapsids! Diapsids split into two clades; Lepidosauria and Archosauria. Lepidosauria are modern lizards such as geckos, iguanas, and Komodo dragons. Archosauria present adaption with special openings for air sacs in their skulls. Air sacs are one of the fundamental anatomical adaptations that birds possess. Air sacs aid the ability to fly by efficiently using oxygen. Since birds are related to dinosaurs, theropod dinosaurs, this means that dinosaurs descend from Archosaurs. Although all non-avian dinosaurs are extinct, Archosaurs are not. Archosaurs still exist today; we call them alligators and crocodiles. 

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Postosuchus is the poster child for Archosaurs. While traits that would eventually give way to the Tyrannosaurus Rex are apparent, Postoscuchus is a reptile, not a dinosaur. Postosuchus is a member of the clade Pseudosuchia, an Archosaur that includes modern crocodilians and descendent of the dinosaurs we call birds. While assumed that dinosaurs are extinct, they are not; they fly over our heads every day, and these are the Avialians or birds.

My backyard chicken flock. Chickens (as all birds) are theropods, descendents of Deinonychosaurs (better known as raptors) which are descendents of Archosaurs such as Postosuchus which are Diapsids.

How does this relate to Coelophysis?

You may be wondering how all of this relates to Coelophysis, the name of this dinosaur blog. Coelophysis is the first or oldest known dinosaur. Coelophysis lived approximately 228 to 201.3 million years ago during the latter part of the Triassic Period. Coelophysis is not an Archosaur. Coelophysis is a Diapsid on the same evolutionary line as Postosuchus except warm-blooded. Archosaurs are reptiles that are Ectothermic or cold-blooded animals that rely on the environment to regulate their body temperature. Dinosaurs are Endothermic or warm-blooded and maintain a constant body temperature independent of the environment. 

Dinosaurs are often associated with reptiles. The name dinosaur, coined by Richard Owen, meaning “Terrible Lizard” does not help this common misassociation. Dinosauria, the clade which contains all the dinosaurs, is a clade that is uniquely theirs. While dinosaurs have evolutionary relations to Archosaurs which are reptiles, Dinosaurs are in a class of their own. They are unique animals consisting of exclusive physiology. Coelophysis was simply the first, the beginning of the age of the dinosaurs.

I have been captivated by dinosaurs since I was a child. Astonished and memorized by these creatures, I still possess a child-like wonder about these animals. I never grew up, but today I am still fascinated by dinosaurs. More than just the immensity of their size, I seek to understand these amazing animals on a fundamental evolutionary level. To achieve this, I travel to Natural History Museums, stand at the feet of these beasts, look up, and work to learn about how they lived and died.

If you are like me, that is, you never grew out of your dinosaur phase from youth, grab a cup of coffee and let’s talk about dinosaurs. Coffee and Coelophysis is a journey through the Mesozoic, the great age of the Dinosaurs.

I am a published author, multi-disciplinary writer, and blog contributor. If you like this blog, please visit my writing portfolio.

The Works of Noelle Moser.

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