400 – 300 million years ago

Year forty-three
Lobe-finned fish evolved into amphibians

Animals colonise the land

Shortly after the plants invaded the land, the first of the animals followed. Earth was about 43 years old (remember, that is only 3 years ago in our pretend person's life, but that is 300 million years ago in real time).

Water snails and worms managed to survive out of the water, although they could only live in damp soil. Amphibians also evolved at about this time: their living descendants include frogs, toads, newts and salamanders. The first amphibians were tetrapods. They did not go very far away from their rivers or lakes because they had to keep their skin wet and their eggs were soft and could not survive being dried out.

Lizzie and friends

The first reptiles evolved from the amphibians shortly before Earth's 43rd birthday (around 300 million years ago) and lizards still live today in warm countries. The oldest known fossil reptile, nicknamed Lizzie, was found near Edinburgh. This animal lived about 330 million years ago. Reptiles owe their success to the fact that the early ones had developed an egg with a hard shell that did not have to be kept in water like amphibians' eggs do. They did not need to keep their skins wet either, so they could move away from rivers and lakes. These were important changes, which separate the amphibians from the reptiles.

Reptiles spread all over the world: some learned how to fly (the pterosaurs, for example) and others became sea creatures and evolved fins (like ichthyosaurs and plesiosaurs). But most lived on the land, like Elginia, Ornithosuchus and Dimetrodon. One group of reptiles, the archosaurs, looked a bit like crocodiles. The legs of the early archosaurs were like those of the amphibians: they stuck out sideways their knees were bent like a newt's, and when they walked their bodies wiggled from side to side. In order to walk, they had to use a lot of energy just to pull their bodies off the ground. Slowly the archosaurs evolved and the position of their legs was more underneath their body so that they could walk more easily. They were evolving into dinosaurs.

Dimetrodon was a reptile, but not a dinosaur. Its fin was used to control body temperature. It was about 3.3 m long and lived in an area that is now in North America.

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