DK Science: Prehistoric Life
Since life began, more than 3.5 billion years ago, evolution has produced an enormous variety of living things. The earliest living things were simple, microscopic forms, such as bacteria. They evolved into increasingly complex creatures, eventually developing into the animals we know today. However, some animals, such as DINOSAURS, have become extinct.
Like many insects, dragonflies have barely changed since they first evolved. Insects are a remarkably successful group of animals that first appeared around 400 million years ago. About 320 million years ago, some insects evolved wings, making them the earliest animals to fly.
Between about 206 and 65 million years ago, long necked reptiles called plesiosaurs lived in the oceans. Elasmosaurus was one of the largest. As plesiosaurs have no living relatives, scientists can only guess how they lived and moved. The huge flippers may have worked like the oars of a boat, pulling the animal along, or they might have provided propulsion by moving up and down.
Beginning 540 mya, this era lasted for 290 million years. Fish appeared in the seas and rivers around 500 mya. Over time, some of these developed legs and lungs, giving rise to amphibians. Later, reptiles, such as Dimetrodon, evolved from these air-breathing amphibians.
This era lasted from 250 to 65 mya. It was dominated by dinosaurs, pterosaurs, and giant sea reptiles. Birds evolved about 150 mya. Placental mammals evolved from more primitive mammals at the end of the era.
An extinct group of land-living reptiles, dinosaurs had erect legs set under their bodies rather than out to the side. All dinosaurs fall into one of two types, depending on the shape of their hip bones. Their closest living relatives are birds, which evolved from meat-eating dinosaurs.
The hips of all dinosaurs had three bones, the ilium, ischium, and pubis. The so-called bird-hipped, or ornithischian, dinosaurs were all plant-eaters.