Applied Biostratigraphy

Single-celled animals (“protista”), heterotrophic (which means they require food from an external source, just like we do) and some are heterotrophic/symbiotic combination. Most have a mineralised wall of some kind (there are some rare “naked” forms): a secreted wall will be calcareous (calcium carbonate - CaCO3) of various crystal arrangements; an agglutinated wall can be comprised of any externally-acquired material (usually sand/silt/mud particles) with a calcareous, non-calcareous or organic cement. Only certain benthic (bottom-dwelling) forms exhibit agglutinated shells.

Forams are single or (mainly) multi-chambered. All chambers possess one or more openings (Latin; “foramen” - which gives them their name) which connect to other chambers. They inhabit virtually every marine & marginal marine aquatic niche on earth. They are biostratigraphically and palaeoenvironmentally significant and the predominant subject of study for microfossil specialists

Subgroup: Planktonic Forams

They have a calcareous (secreted hyaline) shell structure. Their geologic range is Middle Jurassic – Recent (they are still living – “extant”). They inhabit open marine water depths between the surface and several thousand metres (over the course of the normal life-cycle) but live mostly in the top 0 – 50/100m of the water column. Many species exist with symbiotic algae and therefore mainly inhabit the photic zone in eutrophic waters (waters with little or no nutrients) to allow the algae to photosynthesise and provide nutrition.

Left: Various fossil planktonic forams from the Cenozoic. The aperture (terminal opening) can be seen on several specimens and forms an important part of determining the genus and species of a particular specimen. Right: A living planktonic foram – note the bubbly protoplasmic material which aids buoyancy, supported by spines which are lost on fossilisation. The shell is the dark mass in the centre.

The percentage of planktonic forams in individual fossil foraminiferal assemblages tends to increase (up to 90%+) with increasing water depth therefore statistical measurements of an entire assemblage in a sample can give useful paleo-water depth indications. They are rare at depths shallower than middle shelf (because there is insufficient water depth to carry out the breeding cycle), evolve rapidly, and provide excellent stratigraphic resolution

Subgroup: Benthic Forams

A further subgrouping separates Calcareous (secreted hyaline, aragonitic or porcellanous CaCO3) shelled forams or Agglutinated (the accretion of externally-derived particles to the cell’s surface) forms of wall structure.

Their geologic range is Cambrian – Recent (they are “extant”).

Some forms can achieve large sizes (perhaps even up to 10 cms in length - remarkable for a single-celled organism!) under warm, clear waters with the incorporation of symbiotic algae within the shell as an additional nutrition source – these are often grouped under the term “Larger Forams”.

Benthic forams inhabit virtually all marine sediment substrates either on top of (“epifaunal”) or burrowing within (“infaunal”) including hypo- and hyper-saline environments and can also live on the surfaces of plants (“epiphytic”). Naked, unfossilisable forms may occur in freshwater.

The agglutinated forms without any calcareous components in the shell predominate in ultra-deep waters (below the CCCD), more restricted environments and/or in conditions of low dissolved O2.

Benthic forams generally can be used to provide very useful paleoenvironmental information and, combined with planktonic forams, can be used to estimate paleo water depths.