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.
Etymology
The name "wombat" comes from the now nearly extinct Dharug language spoken by the aboriginal Dharug people, who originally inhabited the Sydney area.[3] It was first recorded in January 1798, when John Price and James Wilson, Europeans who had adopted aboriginal ways.
Taxonomy
Though genetic studies of the Vombatidae have been undertaken, evolution of the family is not well understood. Wombats are estimated to have diverged from other Australian marsupials relatively early, as long as 40 million years ago.
Characteristics
Wombats dig extensive burrow systems with their rodent-like front teeth and powerful claws. One distinctive adaptation of wombats is their backward pouch. The advantage of a backward-facing pouch is that when digging, the wombat does not gather soil in its pouch over its young.
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.
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.