Planktonic Foraminifera floating forms of the order Foraminifera, class Sercodina, phylum Protozoa; the single celled most primitive animal. They are most numerous in the surface and near surface parts of the open warm ocean water and mostly live about to a depth less than 2,000m. They lack locomotary organs and are distributed by ocean currents. In some cases they are carried thousands of kms in the ocean and commonly die far away from their birth places. There are only a few species of planktonic foraminifera but they are represented in the ocean in numerous quantities. The tests of Globigerina makes up a large part of the sediments, covering about 35% of the ocean bottom and making up 80% of the fauna in the bottom sediments at many localities. Their presence has been even reported from great depths ranging from 3,000 m to 5,000 m. The common five genera of the Planktonic foraminifera belong to the families of Globorotaliidae. They are Globigerina, Globigerinoides, Globigerinella, Orbulina and Globorotalia.
Planktonic foraminifera are considered good guide fossils for the correlation of regional and inter-regional sedimentary strata due to their vast distribution, short stratigraphic ranges with simple identifying characters. A number of authors have proposed various biozones based on the planktonic foraminifera from Cretaceous to Holocene.
Planktonic foraminifera may lead to an erroneous interpretation as their tests are carried by ocean currents to a long distance to almost all depositional environments of the open ocean though their distribution is mainly restricted to the tropical and subtropical environments.
In Bangladesh planktonic foraminifera (particularly nummulite) are noted in the Eocene Limestone Formation encountered in the subcrops of the north-western Bogra Shelf region and in the outcrops of the Sunamganj and Sylhet districts. Nummulitic Sylhet Limestone Formation is not traced in the south and south-eastern geosynclinal portion of the country. Planktonic foraminifera may be isolated from the bhuban formation of Bangladesh if detail work is carried out. In Bangladesh two studies have recently been conducted on the planktonic foraminifera from two gravity cores of the northeast bay of bengal and planktonic foraminifera from core samples of the continental slope of the Bay of Bengal. [Mujibur Rahman Khan]