Foraminifera (formally called Foraminiferida) are amoeba-like, single-celled protists (very simple micro-organisms). They have been called 'armoured amoebae' because they secrete a tiny shell (test) usually between about a half and one millimetre long. They get their name from the foramen, an opening or tube that interconnects all the chambers of the test. Fossilised tests are found in sediments as old as the earliest Cambrian (about 545 million years ago) and foraminifera can still be found in abundance today, living in marine and brackish waters.

One of the first fossilised foraminifera to have evolved is Platysolenites antiquissimus. It lived about 545 million years ago during the early Cambrian and has been found in rocks in Wales and in a borehole sunk below Oxfordshire. They are agglutinated tubes three or four centimetres long.

Platysolenites antiquissimus
The photograph (left) shows a cross-section of one of the tubes (which is about two millimetres across) and although it is slightly squashed, the agglutinated sand grains and the tubular structure are clearly visible.

The test, which is the part that is preserved as a fossil, can take many different shapes. (Click on the images to enlarge them.)

The simplest is a sphere or a tube with an aperture (an opening) at one end:

Tubular Rhizammina

Chambers may be added in a single row, like a string of beads (uniserial):

Uniserial Nodosaria

Two rows of chambers (biserial):

Biserial Loxostomum

Three rows (triserial):

Triserial Bulimina

In some types of foraminifera the chambers are added in a spiral and take a number of forms.

Planispiral, like a Catherine wheel:

Planispiral Cornuspira

Trochospiral, like a tiny snail:

Trochospiral Asterigerinata

Streptospiral, where each chamber is half a whorl:

Streptospiral Quinqueloculina

Still other foraminifera have very complex tests:

Globular Lagena

Foraminifera were first discovered about 2 000 years ago. The pyramids in Gizeh, Egypt, are in part built out of a Palaeogene limestone which contains huge numbers of Nummulites gizehensis, a large foraminifer that grew to several centimetres across.

Nummulites gizehensis

The Arctic rotaliid,  Elphidium