Biology / Unit 10 Bio 2305 short lesson, about 20 seconds
Three Living Mammal Clades
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The idea
Monotremes, marsupials, and placentals form three clades: monotremes lay eggs; marsupials finish development in a pouch; placentals nourish young internally until birth.
In practice
A platypus, a monotreme, lays eggs; a kangaroo, a marsupial, carries young in a pouch; most other mammals are placental.
Where people slip
Egg-laying does not exclude monotremes from Mammalia — they nurse young with milk, meeting the criterion of Bio 2303.
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