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Summary
The conquest of dry land by vertebrate animals began with the evolution of the first four-legged, amphibious animals ∼360 million years ago (1, 2). Amniotes originated ∼50 million years later (1) and have since become the most diverse clade of land-living vertebrates, including mammals, turtles, lizards, snakes, crocodiles, and birds. Evolutionary changes in reproduction were crucial for the move from the sea via swamps to dry land. However, the reproductive structures and early life stages of amniotes fossilize poorly. Exceptional insights into early amniote reproduction are offered by recent fossil discoveries (3–6). The fact that these fossils come from ancient seas and lakes and not from dry land helps to explain the paradox that there is an older fossil record for live-bearing amniotes than for egg laying in amniotes.
