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The mid-Cretaceous period was one of the warmest intervals of the past 140 million years1–5, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume6. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.

The Paleocene flora from Seymour Island, Antarctica, first collected by Nordenskjöld on the Swedish South Polar Expedition (1901–1903), was described by Dusén (1908) as having 87 leaf taxa making it one of the most diverse floras of this age in the Southern Hemisphere. The original descriptions of these leaf impressions included 37 pteridophytes and one conifer. Many species identified by Dusén were based on single fragmentary specimens. Major new collections housed at the British Antarctic Survey (Cambridge, UK), together with the original collections held at The Swedish Museum of Natural History, form the basis for a taxonomic revision of the flora. This paper concentrates on the pteridophytes and conifers. The diversity is considerably reduced from Dusén's estimates with just three ferns (Cladophlebis aemulans, Cladophlebis seymourensis, Sphenopteris angustiloba) and two conifers (Araucaria imponens, Elatocladus seymourensis) recognized.

In the basal Purbeck Formation of Dorset a paradoxical association of evaporites and fossil forest vegetation is found, representing well-developed gymnosperm forests which grew on the borders of the shallow, hypersaline Purbeck lagoon which covered southern England during the late Jurassic. The dominant tree was a cheirolepidiaceous conifer which appears from its morphology to have been adapted to growing in a semi-arid environment.