August 11. Permian fusulinids

Permian fusulinid from Iowa about 12 mm long (Mark Wilson photo)

Fusulinids, one-celled animals that made shells, had proliferated in the Carboniferous and they continued to expand during the Permian. These critters were really large compared to most single-celled organisms, and some Permian types were more than a centimeter long. Their cross-sections reveal complex internal compartments where the animal lived, and some had coiled spiral chambers. Their shells were calcite, not silica like diatoms and radiolarians, and sometimes they were so abundant that they are the principal component of limestones. 

Fusulinids were an order within the phylum of foraminifera, amoeba-like single-celled organisms that made shells with tiny holes in them. Most of them probably lived within the mud on the sea floor, but some probably were floaters. Either way, they had thin filament-like pseudopods that they extended from the shell to catch food. 

Although there are many living kinds of foraminifera, fusulinids were among the many forms of life that became extinct at the end of the Permian in the great mass extinction event then. The had a run of about 190 million years though – pretty successful for a little animal.

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Today’s geological birthday is Robert Thomas Hill, born August 11, 1858, in Nashville, Tennessee. Hill was a pioneer in the study of Texas geology, and he taught some of the first geology courses at the University of Texas.

—Richard I. Gibson

Permian fusulinids 

Photo by Mark Wilson (public domain) Permian fusulinid from Iowa about 12 mm long

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).