Spores are organic, unicellular products of asexual reproduction. They comprise part of the life cycle of many plants, algae, fungi, and protozoa, and can be classified based on their origin, dispersal/mobility, function, and morphology. They preserve well in the fossil record because many of them are composed of sporopollenin  and represent some of the earliest evidence of plant life on land [2-4]. Both spore tetrads and trilete spores were present by the Ordovician ; thus, they are incredibly important markers of terrestrialization . In addition to their importance regarding the evolution of plants, they are also used in more modern settings as (paleo)environmental indicators, including salinity , and paleoecological  and archaeological  studies of deposits in marine, peat, soil and lacustrine settings.
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9. McAndrews, J.H., Turton, C.I. 2010. Fungal spores record Iroquoian and Canadian agriculture in 2nd millennium A.D. sediment of Cawford Lake, Ontario, Canada. Vegetation History and Achaeobotany, 19, 495-501.