What are mammals?

Mammals are any member of the group of vertebrate animals in which the young are nourished with milk from special mammary glands of the mother. In addition to milk glands, mammals are distinguished by several other unique features such as hair, the possession of a neocortex, and the presence of a chain of three tiny bones in the middle ear.

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Classification

Mammal classification has been through several iterations since the class was first defined. No classification system is universally accepted. In 2005, Wilson and Reader provided a collection of concise, detailed information about how mammals could be classified.

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Definitions

If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.

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McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level, is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings.

Types of mammals

Useful links

• Mammals - Australian Museum
• List of mammals of Australia
• Australian Mammals A-Z List
• Mammals of Australia