Several independent lines of evidence suggest that this traditional account is false. There appear to be no theoretical reasons to assume that the evolution of terrestrial egg-laying was difficult, or required a structure as elaborate as the amniote egg. Oviparous vertebrates that reproduce on land typically, and probably primitively, lay eggs in underground nests. The physical conditions that eggs encounter in these nests are generally quite mild-not only milder than conditions above ground, but also milder than conditions in standing water. When mapped on any plausible phylogeny of the vertebrates, these character states show so much homoplasy that they cannot be polarized.
Amniotes are as likely as frogs or salamanders to retain primitive reproductive character states.
We do not know how the first terrestrial vertebrates or their ancestors reproduced, or even among which group terrestrial reproduction began. Evolutionary scenarios that assume such knowledge cannot be tested.