Mississippian Period—358.9 to 323.2 MYA

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fossil crinoid
Mississippian age fossil crinoid, Mammoth Cave National Park, Kentucky.

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Geologists in North America use the terms “Mississippian” and “Pennsylvanian” to describe the time period between 358.9 and 298.9 million years ago. In other parts of the world, geologists use a single term and combine these two periods into the Carboniferous. Only in North America is this section of rocks easily divisible into a younger (Pennsylvanian) and older (Mississippian) subperiod.

In 1870 Alexander Winchell introduced the term “Mississippian” into American stratigraphic terminology for the well-exposed strata of the Mississippi Valley. In 1891 Henry Shaler Williams coined the name “Pennsylvanian” (from the state of Pennsylvania) for the strata as a counterpart to Winchell’s Mississippian strata. T. C. Chamberlain and R. D. Salisbury elevated both terms to system status in their influential geology textbook of 1906, and they justified this division largely on the basis of the widespread unconformity that separated the two. The U.S. Geological Survey has recognized these systems officially since the mid-1950s. However, neither has found use outside North America (Eicher 1976).

The Carboniferous

In parts of the world other than North America, the Mississippian and Pennsylvanian subperiods are combined into a single period called the Carboniferous. Mississippian represents earlier Carboniferous rocks, and Pennsylvanian represents later Carboniferous rocks. Two British geologists, William Conybeare and William Phillips, proposed the name “Carboniferous” in 1822 for the strata in north central England that contained coal beds. The term “Carboniferous” (“coal bearing”) is descriptive, but Conybeare and Phillips expected that the Carboniferous System would be widely recognized by its distinctive fossils rather than its lithology (Eicher 1976).

Significant Mississippian events

During the Mississippian Period, shallow seas covered much of North America. Mississippian fossils are abundant in portions of the Midwest and South and include vast beds of limestone and marble. For example, the domed ceiling of the Jefferson Memorial in Washington, D.C., is made of Indiana limestone that was deposited during the Mississippian Period.

This period is sometimes called the “Age of Crinoids” because the fossils of these invertebrates are major components of much Mississippian-age limestone. Also noteworthy in this period is the first appearance of amphibians.

Learn more about events in the Mississippian Period

The Mississippian Period represents the last time limestone was deposited by widespread seas on the North American continent. Limestone is composed of calcium carbonate from marine organisms such as crinoids, which dominated the seas during the Mississippian Period. Crinoids, commonly called “sea lilies,” are delicate animals that typically anchor themselves to the seafloor. Although they may resemble plants, they are actually related to sea stars and sea urchins. They feed on algae and other tiny marine organisms. From the abundance of crinoids in Mississippian rocks, scientists infer a time of warm, clear seas. Crinoids are filter feeders requiring high calcium carbonate concentrations to build their skeletons, which is only possible in warm waters. When the vast quantities of crinoids died during the Mississippian Period, their remains became part of the extensive limestone of this age. Today only a few hundred known forms remain.

Though early tetrapods, which appeared in the Devonian Period, are often referred to as “amphibious,” the first true amphibians (of the order Temnospondyli) appeared during the Early Carboniferous (Mississippian) Period. During the Late Carboniferous (Pennsylvanian) Period and into the Permian and Triassic periods, amphibians were extremely diverse, including many large and small forms. Some resembled newts and salamanders, while others bore a resemblance to snakes or eels. Some large-snouted forms (e.g., Archegosaurus) resembled small (about 5 feet [1.5 m]) crocodiles, though true crocodiles are reptiles and did not appear until the Triassic. The drying out of the coal swamps during the Pennsylvanian and Early Permian diminished many of the environments of these Paleozoic amphibians, with the result that many types died out. They were supplanted by reptiles in the Triassic Period—the "Age of Reptiles". However, both large and small amphibians still continued to flourish in rivers and lakes of the Late Permian, alongside mammal-like reptiles. During the following Triassic Period many genera had large, sometimes very flattened heads and very weak limbs; some of these, such as Paracyclotosaurus, Cyclotosaurus, and Mastodonsaurus were up to 10 feet (3 m) in length. Apart from a few stragglers, all these large amphibians died out during the Triassic extinction event, and most Jurassic amphibians belonged to modern groups, which are familiar-looking by today’s standards.

Visit—Mississippian Parks

Every park contains some slice of geologic time. Below, we highlight selected parks associated with Mississippian Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Paleozoic

Last updated: September 11, 2020