Mammal Footprints And Fossils Reveal Their Existence From The Ancient Era
Scientists claim that mammals were evolved from a group of reptiles called therapsids. They appeared millions of year ago during the Jurassic era. About 5000 species of mammals exist today which are distributed over different parts of the world. The mammal footprints reveal the existence of earlier mammal species among which many have disappeared due climate change and habitat loss. The exact number is unknown as the researches are still around on the fossils and footprints collected from different regions.
Cenozoic era (present age) is referred as the ‘Age of Mammals’ where the group was diversified. They are the dominant vertebrates in the Animal Kingdom. Monotremes are the oldest mammal group that is still alive today. The most diverse mammal group is rodents with more than 1700 species. Next to rodents are bats having 977 species. Primates, insectivores and marsupials are other mammal groups know for their diverse forms. The groups Aardvark, Dugongs & manatees and Flying lemurs have 1, 4 and 2 species respectively.
The largest living mammal is Blue Whale which is an aquatic mammal weighing about 110 to 160 tons. They measure around 20 to 30 meters in length. The smallest mammal alive today is Bumblebee bat which is also called as hog nosed bat. It measure just one inch and weighs 2 gram. They survive in the limestone caves of Burma and Thailand.
The defining feature of mammals is they are born alive and are nursed by mothers with the milk secreted from mammary glands. They are endothermic animals meaning they are capable of regulating their body temperature regardless of environment conditions. All mammals grow hair in some stages of their lives. Their lower jaw is made up of a single bone on the either sides and their middle ear consists of three bones. Mammals also feature an efficient circulatory system with a four chambered heart.