Over time, reptiles evolved into mammals. Mammalian ancestors such as pelycosaurs, therapsids, and cynodonts started developing mammalian traits. These gave rise to early mammals that still had some reptilian traits. After the extinction of dinosaurs, these early mammals flourished and evolved into modern mammals.
Pelycosaur: Reptile-like land vertebrate.
Therapsids: Evolved from pelycosaurs and have more mammalian traits.
Cynodonts: Evolved from therapsids and later evolved into mammals.
Monotreme: Earliest mammal that reproduces by laying eggs.
Marsupial: Early mammal in which the embryo is born at an early, immature stage and completes its development outside the mother’s body in a pouch on her belly.
Placental Mammal: Mammal with a placenta that sustains the fetus while it develops.
Pelycosaurs (275 million years ago): reptile-like land vertebrate
Some pelycosaurs evolved into therapsids
Monotremes evolved 150 million years ago–still had some non-mammalian traits such as laying eggs
Mammals were traditionally grouped into 17 orders. Recently, scientists have started to classify mammals based on their ancestors. The new classification method, phylogenetic classification, separates mammals into 4 superorders.
Traditional classification puts mammals into 17 orders:
Phylogenetic classification groups mammals by common ancestor.