300 – 200 million years ago

Year forty-four
The age of the dinosaurs

The age of the dinosaurs

As time went by, the position of archaeosaurs' legs continued to change. Now they were directly beneath the body, the knees were straight when the reptile stood and they were able to walk like cats, dogs and us. Just before Earth's 44th birthday (just over 200 million years ago) the archaeosaurs evolved into dinosaurs.

The earliest dinosaurs were one or two metres long and carnivores, but gradually they evolved into monsters 12 m long or more. Although some were still carnivorous (for example Tyrannosaurus), others were plant eaters or herbivorous (like the huge Apatosaurus). Some walked on two legs like the hadrosaurs, while others walked on four legs like Tricerotops. Dinosaurs lived in tropical and subtropical countries (even Antarctica, which was warm at that time). Their fossils have been found on every continent. Many different types lived in Britain. One of the first was called Saltopus, which lived in Scotland.

Dinosaurs were not all huge. Compsognathus, which ate small lizards, was only 30 cm tall, the size of a chicken! And the largest creature ever to live was not a dinosaur: the world record goes to a mammal — the blue whale.

The first mammals

The first mammals

Mammals are warm blooded, hairy creatures that suckle their young. The babies are almost always born live, but a few (the spiny anteater and the duck-billed platypus) still lay eggs like the reptiles from which they evolved.

Mammals evolved from 'mammal-like reptiles', such as the cynodont Thrinaxodon. They laid eggs, but may have suckled their young, although little is known about them. The first mammals evolved at about the same time as the dinosaurs. The oldest known evolved about 210 million years ago during late Triassic times. They were very small, about 10 cm long, shrew-like animals. Their young had a set of milk teeth, which means they must have suckled their babies. They lived on insects and were probably nocturnal. Bones that had once formed part of the jaw in their reptilian ancestors evolved so that they were now found in the ears of mammals. These tiny bones in the middle part of the mammals ears gave them very sharp hearing.

However, evolution of the huge number of different kinds of mammals that we see today had to wait another 150 million years. The 'Age of the Mammals' had to await the extinction of the dinosaurs about eight months ago in our imaginary person's life (65 million years ago in real time).

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