The first mammals
Small, nocturnal and in the shadows for 160 million years
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Morganucodon, known from fissure fills in Wales and south-west England, has the decisive trait: a jaw joint between the dentary and squamosal. The former jaw bones were freed and migrated into the middle ear as hammer and anvil — one of the best-documented transitions in the entire fossil record, and still visible in embryonic development. The long nocturnal phase left permanent marks: mammalian colour vision is poorer than that of reptiles and birds because two of four cone types were lost, while smell and hearing became unusually acute.
For children
The first mammals were the size of a shrew. They had fur, warm blood and sharp hearing, and lived at night while the dinosaurs slept. They stayed that way for 160 million years — almost the entire age of dinosaurs — never growing bigger than a badger.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org