Fusulinids were small marine organisms that were common inhabitants of the world's seas during the Pennsylvanian and Permian periods, from about 323 to 252 million years ago. The earliest fusulinids occur in rocks deposited during the late Mississippian Period, more than 323 million years ago. Fusulinids became extinct during the mass extinction at the end of the Permian Period, about 252 million years ago.

Fusulinids cover limestone slab.
Fusulinids cover this limestone slab, collected from the Beil Limestone, Chautauqua County.

Fusulinids were single-celled organisms, about the size and shape of a grain of wheat. Unlike multicellular animals, which accomplish basic life functions (such as locomotion, feeding, digestion, and reproduction) through a wide range of specialized cells, fusulinids and other single-celled organisms have to carry on these same functions within the confines of a single cell. As a result, the cell is highly complex.

In fusulinids, this complexity is evident in the structure of the hard calcium carbonate shells, called tests. Internally, the tests, which are made up of calcium carbonate, are divided into a series of chambers. By studying living relatives of the fusulinids (a group called the foraminifera), scientists know that the tests were secreted by the protoplasm, the living material within the cell. As fusulinids grew, the test coiled around itself, adding chambers along its longitudinal axis.

The earliest fusulinids were minute, smaller than the head of a pin, and somewhat spherical in shape. During their 80 million years on earth, fusulinids evolved rapidly, typically becoming progressively longer and narrower. By the late Permian Period, some forms were more than 2 inches long, an amazing size for a single-celled organism.

Pennsylvanian fusulinids of the genus Triticites.
These Pennsylvanian fusulinids belong to the genus Triticites, which gets its name from the Latin word for wheat. Triticites is a common fossil in Kansas rocks.

As fusulinids evolved, the internal test walls also became increasingly complex, with more ornate subdivisions of their internal chambers. Fusulinids look fairly similar from the outside. In order to identify them, scientists usually examine a cross section of the fossil test under a microscope.

Because of their rapid evolution and their occurrence in the rocks from around the world, fusulinids are extremely useful in correlating the ages of sedimentary rocks from different parts of the earth. By matching the kinds of fusulinids contained within sedimentary rock formations, geologists can show that far-flung rock strata—as widely separated as Kansas and Russia—were deposited at approximately the same time.

By studying the rocks in which fusulinids are found, geologists can determine what kind of environment they lived in. Apparently, fusulinids preferred a clear-water, offshore environment and may have been reef dwellers. The mass extinction at the end of the Permian Period decimated the world's reefs and their occupants.

Fusulinid fossils are found on all continents except Antarctica and are common in the Permian and Pennsylvanian rocks of eastern Kansas. In fact, some Kansas limestones—for example, the Cottonwood Limestone Member of the Beattie Limestone, the Tarkio Limestone Member of the Zeandale Limestone, and the Americus Limestone Member of the Foraker Limestone—are made up almost exclusively of fusulinid fossils.

Stratigraphic Range: Upper Mississippian to Upper Permian.

Taxonomic Classification: Fusulinids belong to the Kingdom Protoctista, Phylum Protozoa, Order Foraminiferida, Suborder Fusulinina, Family Fusulinidae.


Text and photos from Windows to the Past: A Guidebook to Common Invertebrate Fossils of Kansas, Kansas Geological Survey Educational Series 16.


Buzas, M. A., Douglass, R. C., and Smith, C. C., 1987, Kingdom Protista; in, Fossil Invertebrates, R. S. Boardman, A. H. Cheetham, and A. J. Rowell, eds.: Boston, Blackwell Scientific Publications, p. 67-106.

Moore, R. C., Lalicker, C. G., and Fischer, A. G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.

Williams, R. B., 1975, Ancient Life Found in Kansas Rocks—An Introduction to Common Kansas Fossils: Kansas Geological Survey, Educational Series 1, 42 p.