Mammals

Mammals are a class of tetrapods that include camels, chimpanzees, cats and rats and elephants, goats and sheep, horses and cows. The class of mammals can be divided into two main infraclasses: eutheria and the marsupials.

EUTHERIA = the crown placental mammals

Eutheria (Greek: "true beasts") is a group of mammals consisting of the first placental mammal and all of her decendants. The earliest known fossil eutherian, Eomaia, was found in Asia, and is dated to the Early Cretaceous period, about 125 million years ago. Most placental mammals belong to one of four main superorders:  Afrotheria, Euarchontoglires, Laurasiatheria, and Xenarthra.

Euarchontoglires + Laurasiatheria = Boreoeutheria

and

Xenarthra + Afrotheria = Atlantogenata.

Xenarthra diverged from Afrotheria relatively rapidly after Atlantogenata diverged from Boreoeutheria. There was considerable gene flow among the nascent clades prior to their divergence. This made it difficult to resolve the above phylogenic tree until relatively recently.

MARSUPIALS

The seven living orders of marsupials are divided into two clades: Ameridelphia and Australidelphia.

MONOTREMATA

Order monotremata = Subclass prototheria

Eutheria + Marsupials = Subclass theria