They are endotherms. It's believed that mammals evolved form a small reptilian ancester before the age of the dinosaurs. But they showed a dramatic speciation after the dinosaurs became extinct. Restricted to the antipodes. These include the duck-billed platypus, and several species of spiny anteater.
Females have a ventral pouch containing the nipples.
The placential mammals. Females lack a marsupium, fetuses complete development within the uterus.