Fossil flora of the Chatham Islands, eastern ‘Zealandia’: a window into the forest biome of the mid-Cretaceous south pole

The Chatham Islands, New Zealand, provide a unique perspective of the polar forest biome during the mid-Cretaceous global greenhouse (palaeolatitude ~ 70–80° S), when eastern Zealandia was attached to the West Antarctica sector of Gondwana. The palynological assemblage supports a Cenomanian-Turonian (~ 98–90 Ma) age for the ~ 400 m thick succession. The examined lithostratigraphic unit, the Tupuangi Formation, was deposited in a fluviodeltaic system; lithological and palaeopedological evidences suggest that the local depositional environments associated with the macrofloral remains were deltaic floodplains. Diverse macrofloral fossil assemblages were found on numerous hydromorphic palaeosol horizons, often associated with well-established root systems and in situ trunks, or entrained in overlying fine sandstone facies. This macroflora consisted of prevalent conifers, locally abundant angiosperms and ginkgos (Ginkgoites), and uncommon free sporing plants, including non-vascular plants (marchantiophytes and bryophytes), herbaceous lycopsids and ferns (Adiantites, Cladophlebis and Sphenopteris). The fern leaf and spore assemblage comprised a lower diversity and abundance than coeval localities of the Southern Hemisphere, including eastern Australia, the Antarctic Peninsula and mainland New Zealand. Quantitative microfloral data reveal intermittent overabundances of monospecific fern spores, but these were likely caused by the localised re-establishment of riparian fern taxa after disturbance of the floodplain environment. In contrast to the relatively depauperate fern component, the high conifer pollen diversity and abundance is unparalleled for mid-Cretaceous assemblages of the Southern Hemisphere, and consists almost exclusively of Araucariaceae, Cupressaceae and Podocarpaceae. The conifer-dominated assemblage is likely due to the relatively cooler, drier climate associated with the extremely high palaeolatitude. Furthermore, the palynology hints at a previously unreported microfloral subprovince, characterised by a very high abundance of cupressaceous pollen. A combination of macro-, meso- and palynofloral fossil remains indicate a well-established coniferous forest with a ginkgo-angiosperm understory, and subsidiary ferns, seed-ferns, lycophytes and non-vascular plants. This assemblage represents the highest southern latitude flora of the mid-Cretaceous studied to date, and thus provides a crucial biogeographic and ecological end-member during one of the most extreme global greenhouse regimes in geological history.