Fusulinids cover limestone, collected from Lopburi province of Thailand
Description: Fusulinids were small, marine organisms that were common inhabitants of the world's seas during the Pennsylvanian and Permian Periods, from about 315 to 250 million years ago. The earliest fusulinids occur in rocks deposited during the late Mississippian Period, more than 320 million years ago. Fusulinids became extinct during the mass extinction at the end of the Permian Period, about 250 million years ago.
Their shells resemble a grain of rice, with respect to both size and shape. 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.
These Pennsylvanian fusulinids belong to the genus Triticites, which gets its name from the Latin word for wheat. Triticites is a common fossil in Thailand rocks.
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.
This cutaway view of a fusulinid test shows the complex structure of these single-celled organisms. The prominent line on the outside of the text, the antetheca, was the growing surface, where new chambers were added (drawing by Roger B. Williams, KU Paleontological Institute).
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 over two inches long, an amazing size for a single-celled organism.
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.
 | Cross section of the common fusulinid Triticites, showing the distinctive internal structure of its chambers (drawing by Al Kamb, KU Natural History Museum, Invertebrate Paleontology). |
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.
Stratigraphic Range: Upper Mississippian to Upper Permian.
Taxonomic Classification:Fusulinids belong to the Kingdom Protoctista, Phylum Protozoa, Order Foraminiferida, Suborder Fusulinina, Family Fusulinidae.
Sources
Buzas, Martin A., Douglass, Raymond C., and Smith, Charles 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, Raymond C., Lalicker, Cecil G., and Fischer, Alfred G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.
Williams, Roger B., 1975, Ancient Life Found in Kansas Rocks--An Introduction to Common Kansas Fossils: Kansas Geological Survey, Educational Series 1, 42 p.
Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.
