Dinosaur Anatomy | Coffee And Coelophysis

Meraxes Gigas.

Posted on December 20 by Noelle K. Moser

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.

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.

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.

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.

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

Size:

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 ~

What is a Dinosaur?

Posted on December 7 by Noelle K. Moser

Before we advance in our discussion on Dinosaurs, it is imperative to define what a Dinosaur is. Dinosaurs are unique creatures that ascended from the ashes left by the Permian Extinction. To fully answer this question, we must understand from where dinosaurs come. We often think of Dinosaurs as giants, but the truth is they came from animals that are smaller than a house cat (Thomas R. Holtz. 2007). In the beginning, dinosaurs were small, bipedal lizard-like animals. Coelophysis is the oldest dinosaur. But before Coelophysis, another group inhabited the earth, the archosaurs.

Image Credit: ***Archosaurs Postosuchus and Desmatosuchus.***

Archosaurs, the dinosaurian ancestor, were small with one adaptation, open hip joints with hind limbs positioned directly under their bodies. This adaptation would prove vital during the Jurassic when the Titanosaurs roamed the Earth. Without this very primitive but necessary anatomical adaptation, dinosaurs could not grow to the giant sizes that mesmerize us. Without open hip joints and legs positioned directly under their bodies, dinosaurs (such as the giant Sauropods and Giant Tyrant dinosaurs) could not assume an upright posture.

Open hip joint with hind legs positioned directly under the body with three or more sacral vertebrae fused to the pelvis is the first requirement to classify an animal as a dinosaur.

Above are images of Walter, Tyrannosaurus Rex located at the National Museum of Natural History in Washington, D.C. If you look at his skeleton, the open hip joint of this large theropod is visible. Without a specialized hip joint and hind legs directly underneath his body, Walter could not support his massive 6-ton weight.

Additionally, open hip joints provided yet another essential mechanism that aided dinosaurs, smooth locomotion and running. As bipedal locomotion organisms, we can relate to this efficient mode of movement. Without open hip joints connecting our legs to our pelvis, we would not be able to perform many feats that humans accomplish. Sports that require running, jumping, or aerial maneuvers we could not do without this vital anatomical modification.

When looking at dinosaurs, there is another anatomical adaptation they possess. When you look at a Theropod such as Allosaurus, we notice “Big Al’s” hands. This anatomic trait is of much importance to the success of the dinosaurs. Hands complete with digits on the manas (hand) and pes (feet) allow the dinosaurs to grasp and carry objects. Bipedalism freed up their forelimbs to perform other functions that aided their survival. The incredibly tiny arms of Tyrannosaurs Rex are often the punch line of many jokes, but even T-Rex could use his hands. While small compared to the rest of the muscular bodies, it’s speculated that he could lift 100 lbs. with his forelimbs. But more importantly, forelimbs unnecessary for locomotion proved to be a great asset to the success of the dinosaurs.

As habitual bipedal locomotion specialists, we appreciate our ability to have our hands free to perform other functions besides movement from one place to another. Just like the dinosaurs, the adaption of bipedal locomotion proves essential to our success as Homo sapiens.

The second requirement to classify an animal as a Dinosaur is hands complete with digits, both manus (hand) and pes (feet).

**The drawing I did humorously** **demonstrate the possibilities available to T- Rex should he decide to take up berry picking. With adaptions of bipedal locomotion, T-rex and other dinosaurs had their hands free to carry and man*ipulate objects.***

We often think of Dinosaurs as being extinct; they are not. Dinosaurs fly over our heads every day, Avialians known as birds. Birds are not just related to dinosaurs; birds are dinosaurs, Coelurosaurs, a branch of theropod dinosaurs. I will revisit this in a later post. For now, understand that birds are dinosaurs, because they are dinosaurs and possess a vital anatomical adaptation that all dinosaurs share.

The skull of an animal has many bones. We often think of the skull as one big bone, the head bone or cranium. However, a skull is a collection of many smaller bones that fuse together, forming the cranium. In dinosaurs, one fundamental skull bone is missing, the postfrontal bone. In the image below, labeled are the bones of a dinosaur skull. Notice that the frontal bone (pink) is present, but the postfrontal bone is missing.

Anapsids (line of descension for reptiles) and synapsids (line of descension for mammals, including humans) possess postfrontal cranial bones. Diapsids (the evolutionary line which contains the dinosaurs) lack postfrontal cranial bones. Ancestors of the dinosaurs, including the Archosaurs, retain this bone. This missing cranial bone in all dinosaurs present in dinosaurian ancestors assists their definition (Martin 2006).

All birds (heir of the dinosaurs) do not have postfrontal cranial bones. The postfrontal cranial bone fuse into the frontal bone during the later stages of embryonic development.

The third requirement needed to classify an animal as a dinosaur; is the absence of a postfrontal cranial bone.

***Cnemial crest is visible on Walter’s right left. This adaptation is related directly to the evolution of dinosaurs from reptilian ancestors.***

The limb bones of dinosaurs, specifically if they are related to the appendicular skeleton (bones that attach or append to the axial skeleton). are vital because they provide information on how the dinosaur moved. In the posterior portion of a dinosaur, hind limbs are attached to the body by the pelvic girdle (hip socket) below the sacral vertebrae (fused vertebrae that attach the open hip joint to the spine).

Due to the evolutionary adaptation of an open hip socket, dinosaurs can support their weight directly over the hip joint. For this posture to be possible, dinosaurs require adaptions to facilitate this mode of locomotion. Attached to the hip joint by the acetabulum, the femur (thigh bone) terminates at the cnemial crest and the tibia. The cnemial crest is visible in the image above in Walter’s skeleton.

The tibia is key to two additional dinosaurian traits. The cnemial crest and the ascending process into the astragalus or anklebone. (Martin 2006). The astragalus (anklebone) and other small bones form a dinosaur’s tarsals (feet and toes). Dinosaurs walked with their metatarsals in contact with the ground. In other words, dinosaurs walked on thier toes. The digitigrade motion is visible in the image above. Walter walks on his toes, heals never touch the gound when walking. This digitigrade locomotin can also present in dogs.

Contrast this with human locomotion. Humans walk with a relaxing stance in which most of the body weight in on the heels in a plantigrade stance. Human can only achieve a digitigrade when wearing high-heeles shoes or tiptoe.

These adaptations relate directly to the evolution of dinosaurs from reptiles and archosaurs. Unlike reptiles which move with legs sprawled out to the side of the body like large lizards, dinosaurs walk upright with their legs directly underneath their bodies with digitigrade motion.

In addition to open hip joints and fused sacral vertebra, dinosuar locomotion requires the adaption of the cnemial crest, tibia, and astragalus to facilitate this upright smooth motion.

Finally, we come to the last requirement to classify an animal as a dinosaur; the femur that terminates into the cnemial crest, a tibia that ascends to the astragalus, and the astragalus which forms the metatarsals for digitigrade locomotion. Without these evolutionary adaptions, dinosaurs would not be able to support weight in an upward stance; dinosaurs would crawl in a sprawling posture like the reptiles that preceded them.

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