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Foraminifera and their Classification

<i>Rugoglobigerina rugosa</i> (Plummer). B. Huber photo.


What they are and their importance to global stratigraphy

Foraminifera are single-celled, typically shell-forming amoeboid protists that are abundant in ocean habitats ranging from shallow marshes to deep abyssal depths. Foraminifera shells are preserved in sediments around the world. Shells are commonly subdivided by chambers added during growth though some forms occur as simple tubes or hollow spheres. The size of the foraminiferal shell generally ranges from 0.05 mm to 0.7 mm, although some may be as large as 20 cm in diameter. Benthic (bottom dwelling) foraminifera evolved during the earliest Cambrian time (~540 million years ago) and are highly diverse, with over 4000 species existing today. Benthic species ranges are generally long (averaging about 20 million years) and some species are extremely useful for determining sediment ages. The diversity and type of benthic foraminifera allow determinations of ancient environmental conditions such as depth, oxygenation, and salinity. Planktonic (floating) foraminifera evolved during the middle Jurassic (~170 million years ago) and are quite low in diversity, with 40 species living in the upper water column of the ocean today. Because of their relatively short species duration (averaging ~5 million years) planktonic foraminifera are very useful for determining the age of ancient marine sediments.

Classification of forams

Cushman wrote the textbook on it and why his method was so important

At the turn of the twentieth century foraminifera were studied by relatively few researchers who were interested more out of curiosity than for understanding their taxonomy and evolution. Beginning with his 1906 commission to study foraminifera obtained from dredges taken during expeditions of the U.S. Fish Commission steamer, the Albatross, Joseph Cushman realized that the existing foraminifera taxonomy was grossly oversimplified and a new classification based on careful observations from the stratigraphic record would provide the basis for understanding their taxonomy, diversity and phylogeny (history of relationships) through time. Financial compensation for his study of the Albatross collections and his employment with the U.S. Geological Survey and the Marland Oil Company enabled Cushman to travel abroad to study foraminiferal collections housed at museums in London, Vienna, and Berlin. His observations and illustrations of important museum type specimens, accumulation of a vast library of foraminiferal publications and study of foraminifera samples from a wide range of ages and environments led Cushman to develop a much more complex classification scheme. Though his new scheme was initially considered unorthodox and unusable by his colleagues, Cushman's belief in the value of foraminifera in stratigraphic correlation and his new classification scheme gradually became broadly accepted by his colleagues, particularly after publication in 1928 of his book Foraminifera, Their Classification and Economic Use. The following three editions of his book included a number of improvements to Cushman's taxonomy and classification scheme, and much the basic taxonomic and phylogenetic framework is still regarded as valid today. While the taxonomy and phylogenetic history of foraminifera has vastly increased in detail and complexity since Cushman's death in 1949, his contributions to their study led to many new discoveries and a significant increase in understanding the geologic history of the earth.

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