Mesozoic era → Mammals (225 MA)

Mammals evolved during the Triassic period, about 225 million years ago. Some scientists believe that mammals evolved from a group of extinct mammal-like reptiles, Theriodontia, which were Therapsids. These primitive mammals were tiny and are thought to have been nocturnal. The first placental mammals appeared at the beginning of the Cretaceous period.

There are more than 4.000 different species of mammals. The smallest is the hog-nosed bat, which weighs 0.05 ounces. The largest is the blue whale, which can be 100 feet long and weigh 150 tons. But whether they live on land or water, all mammals share some common characteristics.

All mammals:

• Are vertebrates (which means they have a backbone or spine).

• Are endothermic. Also known as “warm-blooded,” endothermic animals regulate their own body temperate which allows them to live in almost every climate on Earth.

• Have hair on their bodies.

• Produce milk to feed their babies. This allows them to spend more time with their young and teach them important skills they need to survive on their own.

For more information, click on the following link:
http://www.enchantedlearning.com/subjects/mammals/Evolution.shtml