Earth before Humans

Reptiles belong to the phylum Chordata and kingdom Animalia. They are the cold blooded vertebrates having dry skin surrounded by epidermal scales. Reptiles are tetrapods that lack the ability to maintain a constant internal body temperature. They have internal and external fertilization, some reptiles lay eggs and some produce offspring through internal fertilization.

Paleozoic era started about 544 million years ago and ended in about 245 million years ago. Throughout the Paleozoic era there have been some great and remarkable events of evolution including the evolution of reptiles and some other allied animals such as fishes, insects and the development of the invertebrate groups. The start of the Paleozoic era was marked by the diverse explosion of life form. The great mass extinction event in the end of the Permian period put an end to the Paleozoic era and 95% of the marine life and 70% of the terrestrial life was completely wiped out. Through the Paleozoic era, the world was divided into several continents and by the end of this era they were all merged together forming one big supercontinent, Pangea.

By the end of the Paleozoic era, terrestrial life included fishes, arthropods, amphibians, and reptiles.

ANAPSIDS

Anapsids are the most primitive subclass of the reptiles present in the Paleozoic era and the ancestral stock from which the diapsids and synapsids had evolved. Anapsids are classified on the temporal holes present in their skulls. They have no temporal fenestrae (holes) in their skulls. However, their descendants the diapsids had two temporal fenestrae and synapsids had only one temporal fenestrae.

About 252 million years ago, when the era was Mesozoic, the reptiles were ruling the entire world. That’s why the Mesozoic era is also known as “the age of the ruling reptiles”. This era started and ended in two great mass extinction. The first mass extinction was in the end of the Paleogene era, the era before the Mesozoic. Throughout the Mesozoic era the temperature of the earth remained warm and humid, there were no polar ice caps. This climate was suitable and ideal for the reptiles and this is one of the major reason why they evolved in the best possible way throughout the Mesozoic era and eventually gave rise to huge reptiles weighing approximately 200 tons.

Later on in the first period of Mesozoic era, the Triassic period, Diapsids evolved into archosaurs and lepidosaurs further having four temporal fenestrae in their skulls. This evolution took place in the start of the next era, the Mesozoic era. Archosaurs are the predecessors of the pseudosuchians and the dinosaurs. The lepidosaurs are the predecessors of snakesand lizards.

Pseudosuchians had strong hind limbs that’s why they were able to stand on two legs and run fast however, they had unsteady breathing which lead to their extinction in the Triassic-Jurassic extinction event. Only one lineage was able to survive which was prone to live in the swamps and it resembles the modern day alligators. If they had strong breathing physiology, they could have dominated the Jurassic age instead of Dinosaurs.

Dinosaurs evolved from the Archosaurs as well and they were the most dominant branch of the Archosaurs, and became dominant terrestrial reptiles after Triassic – Jurassic Extinction. They were the highly intelligent reptiles of the Mesozoic era and the first noted dinosaur was the eoraptor which was an omnivore.

The number of dinosaurs increased exponentially in the Jurassic period. In fact, the earth was ruled by them for over 160 million years. The temperature was slightly warmer, both for the aquatic and terrestrial life, which was the ideal climate for them. Pangea started to split in two smaller continents, Gondwana and Laurasia. Two groups of Dinosaurs emerge as dominant in the Jurassic period, grass eating herbivore lineage of the omnivore eoraptor, the sauropods and flesh eating terrifying carnivore lineage of the eoraptor, the theropods. However, by the end of the Mesozoic era all these dominating reptiles were gone and once they were gone the mammals had a chance to take over the world, which is what happened in the Cenozoic era.

Therapsids were the Evolved form of Synapsids, most of them died out in the end of Triassic period. They had four-footed locomotion and they gave rise to mammal like reptiles and mammals having one temporal opening in their skull like mammals of today, including humans. That is a major proof of human evolution from Therapsids.

Ichthyosaurs (fish lizard) had evolved from unidentified reptilian group. They were the first land reptiles to become fully aquatic. They evolved a ‘fish’ shaped body from a four footed animal.

Pterosaurs (Winged lizard) had also evolved from Archosaurs. They are closely related to Dinosaurs. They had evolved the power of flight at the end of Triassic and survived the extinction. The current bird lineages are successors of Pterosaurs. The first bird was the archaeopteryx.

After the Mesozoic era ended the world was left with no dinosaurs. However, the few that were left behind evolved into today's reptiles such as Alligators, crocodiles, Iguana, turtles, and many other reptiles. Some birds have evolutionary history starting from reptiles also. On the other hand Molecular genetics studies also show mammalian evolution from reptiles. The aquatic dinosaurs of that time also managed to evolved into the present day sharks and whale.