The shelled egg
Vertebrates cut the last tie to water
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The cliffs at Joggins in Nova Scotia preserve some of the oldest amniotes, found inside fossilised tree stumps. The amniotic egg has several membranes: the amnion surrounding the embryo in fluid, the allantois handling waste, and the chorion managing gas exchange. That construction is shared by reptiles, birds and mammals — and in mammals it is rebuilt into the placenta. It is one of the clearest cases where a single anatomical innovation opens an entire continent: together with seed plants, the egg makes dry interiors habitable.
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Amphibians must lay their eggs in water or they dry out. Then some animals evolved an egg with a shell and membranes around it — a small private pond to develop in, which could be laid anywhere. Vertebrates could now move into deserts and mountains, far from any water.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org