The
evolution of mammals has passed through many stages since the first appearance of their
synapsid ancestors in the late
Carboniferous period. The most ancestral forms in the class Mammalia are the egg-laying mammals in the subclass
Prototheria. By the mid-
Triassic, there were many synapsid species that looked like
mammals. The lineage leading to today's mammals split up in the
Jurassic; synapsids from this period include
Dryolestes, more closely related to extant
placentals and
marsupials than to
monotremes, as well as
Ambondro, more closely related to monotremes. Later on, the
eutherian and
metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since
Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.