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Fossil fern. Fossilised frond (leaf) of an unidentified fern from the early Cretaceous period. Ferns are flowerless plants that reproduce using spores. They dominated many habitats during the Cretaceous period. It was found on Alexander Island in the Antarctic. The polar climate was warmer during the Cretaceous than it is today. (KEYSTONE/SCIENCE PHOTO LIBRARY/BRITISH ANTARCTIC SURVEY)