Figure 1. Emergence of traits along the mammalian lineage.

Amniotes split into the sauropsids (leading to birds and reptiles) and synapsids (leading to mammal-like reptiles). These small early mammals developed hair, homeothermy and lactation (red lines). Monotremes diverged from the therian mammal lineage ~166 Myr ago² and developed a unique suite of characters (dark-red text). Therian mammals with common characters split into marsupials and eutherians around 148 Myr ago² (dark-red text). Geological eras and periods with relative times (Myr ago) are indicated on the left. Mammal lineages are in red; diapsid reptiles, shown as archosaurs (birds, crocodilians and dinosaurs), are in blue; and lepidosaurs (snakes, lizards and relatives) are in green.