The first reptiles emerged approximately 320 million years ago during the Late Carboniferous period. One of the earliest known possible amniotes is Hylonomus, a small, lizard-like creature whose fossils were discovered in the hollow tree stumps of the Joggins Fossil Cliffs in Nova Scotia. The key evolutionary breakthrough of reptiles was the amniotic egg, which featured protective membranes and a semi-permeable shell that prevented desiccation. This freed land vertebrates from the need to return to water bodies for reproduction, unlike amphibians. Together with waterproof scaly skin, the amniotic egg allowed early reptiles to colonize dry inland environments, paving the way for their eventual dominance during the Mesozoic era.