The modes

• Eggs, external — fish, amphibians, most invertebrates. Large numbers, little or no care, heavy losses.
• Eggs, shelled — reptiles and birds. The amniotic egg is what let vertebrates leave water for good: a private pond carried inside a shell.
• Live birth — almost all mammals, plus some sharks, snakes and lizards. Arrived at independently more than a hundred times across vertebrates.
• Both — the monotremes. The platypus and the echidna are mammals that lay eggs and then lactate.
• Seeds — a plant embryo with packed lunch and a coat, able to wait years for conditions.
• Spores — ferns, mosses, fungi. Single cells, produced in vast numbers, carried on air.

The borrowed part

The placenta depends on syncytin, the protein that fuses cells into the barrier separating maternal and foetal blood. Syncytin is the envelope gene of an ancient retrovirus, inserted into an ancestral genome and repurposed — and different mammal groups captured different retroviruses to do the same job.

So the defining mammalian feature is running on borrowed viral machinery, acquired more than once. That is set out in full under Nature’s own gene splicing, and it is the cleanest single link between these two sections.

Related

• Nature’s own gene splicing Open page: Nature’s own gene splicing
• Designed Life Open page: Designed Life