What is a primary,common evolutionary feature of all reptiles,mammals,and birds?

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The amniotic egg, which contains a fluid-filled sac that protects the developing embryo, is a primary, common evolutionary feature of reptiles, mammals, and birds. This adaptation allows these animals to reproduce on land, away from aquatic environments. Teeth are only present in some members of these groups, and endothermic behavior (warm-bloodedness) is only present in mammals and birds, not reptiles. The gastrovascular cavity is a feature of some invertebrates and simple animals, but not reptiles, mammals, or birds.

What Is a Primary,common Evolutionary Feature of All Reptiles,mammals,and Birds