They say that best place to start is the beginning, in relation to dinosaurs that is Coelophysis.
Dinosaurs are a very unique biological group, they were the most successful animals that evolution ever produced, their reign as dominant creatures spanned millions of years. Compared to humans who have only been here for 6 million years, dinosaurs were extremely successful. But dinosaurs did not spawn out of midair, they were the inheritors of the earth after the Permian Extinction or the Great Dying.
Approximately 251 million years ago, the Earth suffered the most severe extinction event known. It is thought 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 which breathe life into the oceans. As the oceans became stagnant, hydrogen sulfide began to accumulate. Because hydrogen sulfide is poisonous, life in the ocean died. As the gas escaped the ocean into the surrounding atmosphere, life on land began to die off. The oceans experienced increased anoxia (deplete of oxygen) and acidification (reduction of Ph). As a result, many of the plants and animals that inhabited the oceans and surrounding land was 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 interesting life forms emerged.
One of these life forms were the Stegocephalians. Stegocephalians are creatures that developed unique adaptions of bones, jaws, and lungs. Most importatly, Stegocephalians evolved specialized fins with wrists and ankles which helped them, 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.
These three clades are 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 fenstrae, where and how many fenestrae various vertebrate organisms have can be used to classify organisms into clades based on evolutionary lineage.
For example, below is a graphic of a Massospondylus Skull, labeled are the temporal fenestra and other fenestra that is present in this animal’s skull.
The anapsids, synapsids and diapsids are three different clades of animals all leading to various linages. 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 were represented by large 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 linages that later evolved into therapsids which had some 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.
That leaves Diapsids. Diapsids is the clade most important to our discussion on dinosaurs. Dinosaurs are diapsids! Diapsids split into two clades Lepidosauria and Archosauria. Lepidosauria is best represented by modern lizards such as geckos, iguanas and Komodo dragons. Archosauria are linages of animals that have special openings for air sacs in their skulls. Air sacs are one of the fundamental anatomical adaptations that birds possess aiding the ability to fly by efficiently using oxygen. Since birds are related to dinosaurs, theropod dinosaurs, this means that dinosaurs are descended from Archosaurs. Although all non-avian dinosaurs are extinct, Archosaurs are not extinct. Archosaurs still exist today; we call them alligators and crocodiles.
Postosuchus is the poster child for Archosaurs. While traits that would eventually give way to the Tyrannosaurus Rex can be seen, Postoscuchus is a reptile not a dinosaur. Postosuchus is a member of the clade Pseudosuchia, an Archosaur that include modern crocodilians and descendent of the dinosaurs we call birds. While it is commonly thought that dinosaurs are extinct, they are not, dinosaurs fly over our heads every day, these are the Avialians or birds.
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 true dinosaur. Coelophysis lived approximately 228 to 201.3 million years ago during the latter part of the Triassic Period. Coelophysis is not an Archosaur, he is a Diapsid which is on the same evolutionary line as Postosuchus, however Coelophysis is warm-blooded. Archosaurs are reptiles which are Ectothermic or cold blooded, these animals 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 which means “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 a very unique animal consisting of attributes that are unique to their 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. In this way I never grew up, 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 level. To achieve this, I travel to Natural History Museums, stand at the feet of these beasts, look up, and work to learn as much as I can 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.
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