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Originally posted by nixie_nox
I think we have to see how far back egg layers go from bird to dinosaur and maybe even further to find the real answer.
Females of all vertebrates produce eggs, but the reptiles "invented" the eggshell -- a device that could keep the egg from drying out and allow reproduction away from water (or, at least, from extremely moist environments). With the exception of the platypus and echidna, mammals provide the developing embryo with a suitable environment within the mother's womb. The other major group of reptile descendants, the birds, not only have continued the reptilian tradition, but have evolved eggs of an improved design in a wide variety of sizes, shapes, colors, and textures.