Classification of foraminifera based on structure and composition of the shell

Classification Based on Shell Structure

The shell structure of Foraminifera is a primary basis for classification. It describes the arrangement of chambers within the test.

• Unilocular: These have a single chamber. They are relatively rare and typically found in early Foraminifera. Example: Some primitive agglutinated forms.
• Multilocular: Possessing multiple chambers, these are the most common type. Further sub-classification is based on chamber arrangement:
  • Planispiral: Chambers are arranged in a flat, spiral plane. Example: Nummulites (Large benthic foraminifera common in Paleogene sediments).
  • Spiral: Chambers are arranged in a coiled, spiral fashion, but not in a single plane. Example: Globigerina (planktonic foraminifera).
  • Serpentine: Chambers are arranged in a curved, serpentine manner. Example: Some species of Textilaria.
  • Tubular: Chambers are arranged in a straight or slightly curved tube. Example: Operculina.
  • Globular: Chambers are arranged in a spherical or rounded shape. Example: Some species of Hantkenina.

Classification Based on Shell Composition

The composition of the foraminiferal test is another key classification criterion, reflecting the environmental conditions during their formation.

• Calcareous: Tests are composed of calcium carbonate (CaCO3), typically aragonite or calcite. These are common in warmer, oxygen-rich waters. Example: Most planktonic foraminifera like Globorotalia.
• Agglutinated: Tests are constructed by cementing together foreign particles (sand grains, shell fragments, etc.) with a cementing agent. These are often found in colder, lower-oxygen environments, or in brackish waters. Example: Astrononion, Ammodiscus.
• Chitinous/Organic: Tests are made of organic material, primarily chitin. These are less common and often found in deep-sea environments. Example: Species belonging to the genus Xenophyophore (giant foraminifera).
• Hyaline: A type of calcareous test that is transparent and composed of calcite. Example: Many species of Orbulina.

Combined Classification – A Tabular Representation

Shell Structure	Shell Composition	Example	Typical Environment
Planispiral	Calcareous	Nummulites	Warm, shallow marine (Paleogene)
Spiral	Calcareous	Globigerina	Open ocean, surface waters
Tubular	Agglutinated	Operculina	Brackish or low-oxygen marine
Multilocular (irregular)	Chitinous	Xenophyophore	Deep-sea abyssal plains

The interplay between shell structure and composition provides valuable insights into the paleoecology of foraminifera. For instance, the presence of agglutinated tests often indicates lower salinity or oxygen levels, while the abundance of calcareous tests suggests warmer, well-oxygenated conditions.