The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon.
At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds.
Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, though there are efforts to combat this.
Mammal classification has been through several iterations since Carl Linnaeus initially defined the class.
With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young.
Most mammals, including the six most species-rich orders, belong to the placental group.
The largest orders are the rodents, bats and Soricomorpha (shrews and allies).
Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30-meter (98 ft) blue whale.Living mammals are divided into the Yinotheria (platypus and echidnas) and Theriiformes (all other mammals).There are around 5450 species of mammal, depending on which authority is cited.Most mammals are intelligent, with some possessing large brains, self-awareness and tool use.Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation.