July 17. Fusulinids

Foraminifera, or forams, are small single-celled animals that produce shells of calcite, calcium carbonate. The name foraminifera means “hole-bearing” because the shells have tiny perforations in them. The shells can be quite complex, ranging from snail-like spirals and assemblages to blob-like forms that reflect the amoeba-like shapes of the animals. Some float in water, but many live in the mud on the sea floor. 



Forams are known from the Cambrian and they exist today, totaling more than a quarter million species both living and extinct. Most forams were microscopic, but during the Pennsylvanian one group called fusulinids reached sizes greater than a quarter inch long. Fusulinid means “spindle like” and their diverse shells range in appearance from rice grains to tiny footballs. Fusulinids began during the Silurian and really thrived during the Pennsylvanian, but they disappeared in the end-Permian extinction. They are excellent index fossils, and because they are small they are often used in biostratigraphic studies of sedimentary rocks of the late Paleozoic Era. 

Fusulina cylindrica

Fusulinids were mostly marine, and they are common in the limestones that formed in the remaining shallow seas of Mississippian and Pennsylvanian time. In some places, they were so abundant that they make up most of the rock over short time spans.

—Richard I. Gibson

Upper photo by Wilson44691 under Creative Commons license.

Fusulina cylindrica from Naco Limestone, Arizona. Largest individual is ¼ inch long. Photo from USGS Prof. Paper 21, Geology and Ore Deposits of the Bisbee Quadrangle, Arizona, by F.L. Ransome, 1904 (public domain).