315 Mya
First Reptiles

Everyone has heard of the Dinosaurs, the huge terrifying reptiles that came to rule land, sea and air, but what creatures did they evolve from and how could reptiles become so dominant.

The main factor that enabled Tetrapods to move away from a dependence on water was the development of the Amniote egg.

This enabled all the Amniota (everything more advanced than amphibians) to become independent of the processes involved with egg laying by amphibians. Thus instead of laying many soft eggs in water, hard, or, in the case of turtles, leathery shelled eggs could be laid on land. This is a more energy expensive process, so fewer eggs could be laid as there was a better chance of hatching.

The most fundamental innovation is the evolution of another fluid-filled sac, the amnion, in which the embryo floats. Amniotic fluid has roughly the same composition as seawater, so that in a very real sense, the amnion is the continuation of the original fish or amphibian eggs together with its microenvironment, just as a space suit contains an astronaut and a fluid that mimics the earth’s atmosphere. All of the rest of the amniote egg is add-on technology that is also required for life in an alien environment, and in that sense, it corresponds to the rest of the space station with its food storage, fuel supply, gas exchangers, and sanitary disposal systems. —Richard Cowen, The History of Life

Thus the animals were free to take over the world.

Reptiles, Birds and Mammals are all descended from the first Amniotes.

Reptiles as we think of them developed other adaptations to help them survive on land. Their skins are, in general, scaly and tougher. They do not require to be hydrated, thus can live in most environments. This does not apply to very cold environments as reptiles are still exothermic.

Their body temperature is dependent upon the external environment. If their temperature drops too low, they become lethargic and vulnerable.

This diagram shows the relationships and development of the Amniotes. It is from a book called ‘Evolution’ by D. Prothero and C Buell.

Please note that Mammals and Reptilia have a common ancestor and mammals did not evolve from reptiles.

Birds, however, did.

It shows that present day reptiles such as Turtles, Snakes, Lizards and Crocodiles have a long history.

It can also be seen that Reptilia adapted not only to land, but returned to the sea – Turtles, Plesiosaurs & Ichthyosaurs.

They also conquered the air – Pterosaurs.

Reptiles were the dominant animals for around 250 million years. The immense Dinosaurs with which we are familiar developed around 220mya and were dominant for at least 150 million years.

Not all Reptilia laid eggs, and there is a famous fossil of an Ichthyosaur giving birth to a live baby. Common Lizards do the same today. The eggs are fertilized, develop and ‘hatch’ inside the mother.

315 Mya First Reptiles

Further reading:

315 Mya
First Reptiles