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.
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.
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.
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.
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.
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.
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