Description: Brachiopods are marine animals that secrete a shell consisting of two parts called valves. Their fossils are common in the Pennsylvanian and Permian limestones of eastern Kansas.
Brachiopods have an extensive fossil record, first appearing in rocks dating back to the early part of the Cambrian Period, about 525 million years ago. They were extremely abundant during the Paleozoic Era, reaching their highest diversity roughly 400 million years ago, during the Devonian Period. At the end of the Paleozoic, however, they were decimated in the mass extinction that marks the end of the Permian Period, about 250 million years ago. This event, known as the Permo-Triassic mass extinction, may have killed more than 90 percent of all living species. It was the largest of all extinction events (larger than the major extinction at the end of the Cretaceous that killed off the dinosaurs).
Although some brachiopods survived the end-Permian extinction, and their descendants live in today's oceans, they never achieved their former abundance and diversity. Only about 300 to 500 species of brachiopods are exist today, a small fraction of the perhaps 15,000 species (living and extinct) that make up the phylum Brachiopoda.
Shale slab containing numerous brachiopods belonging to the genus Hystriculina, from the Pennsylvanian Americus Limestone Member, Foraker Limestone, Chase County, Kansas.
The name brachiopod comes from the Latin words for arm (brachio) and foot (pod) and refers to a paired, internal structure, which specialists initially thought was used for locomotion. This structure, called the lophophore, is actually used for feeding and respiration and is one of the features common to all brachiopods.
Another distinctive feature of all brachiopods is that their valves are bilaterally symmetrical--in other words, the right half is a mirror image of the left half. Humans also are bilaterally symmetrical. This bilateral symmetry of each valve differentiates brachiopods from clams and other bivalved mollusks, with which they are sometimes confused. Clam valves are
typically bilaterally symmetrical along the line of commissure (where the
valves join), with the left and right valves being mirror images of each
Brachiopod shells come in a variety of shapes and sizes. Sometimes the bottom valve is convex like the top valve, but in many species the bottom valve is concave or occasionally conical. In some brachiopods (see photo below), the top valve is concave and the bottom is convex. The outer surface of the valves may be marked by concentric wrinkles or radial ribs. Some brachiopods have prominent spines, but usually these are broken off and are found as separate fossils.
Two specimens of the brachiopod genus Reticulatia, which has a concave top valve (left) and a concave bottom valve. These Pennsylvanian fossils are from the Americus Limestone Member of the Foraker Limestone in Greenwood County, Kansas.
The exceptional preservation of the brachiopod fossil in the center of the photo (note the delicate spines still attached to the valve) indicates that the specimen was buried quickly in undisturbed sediments. This limestone slab was collected near Topeka, from the Coal Creek Limestone Member of the Topeka Limestone.
The shells of living brachiopods typically range in size from less than 0.25 inches to just over 3 inches in length or width. Fossil brachiopods generally fall within this same range, though some adults have shells that are less than 0.04 inches in diameter, and an exceptional few have shells that are 15 inches across.
Most brachiopods live in relatively shallow marine water, up to about 650 feet (200 m), but some species have been found at depths of more than a mile. Because many fossils species are found in shales, which form from deposits of mud and silt, we know that some brachiopod species thrived in muddy environments. By studying living species, researchers can make inferences about how ancient brachiopods lived. Although they are mobile during their larval stage, many adult brachiopods are fixed to some object on the sea floor by a fleshy stalk that protrudes through an opening in the shell. Unable to move in pursuit of food, brachiopods use the lophophore to pump water through the interior cavity and recover food particles from it.
Brachiopods are one of most common fossils found in the Pennsylvanian rocks in eastern Kansas. They are also common in the younger Permian rocks. However, in spite of their abundance in many Cretaceous rocks worldwide, brachiopods are almost never found in the Cretaceous rocks of Kansas.
Brachiopods commonly found in Kansas rocks: Neospirifer, Farley Limestone Member, Wyandotte Limestone, Johnson County; Meekella, collected near Beaumont, Butler County; Derbyia, Speiser Shale, Council Grove Group, Cowley County; Crurythyris, Beil Limestone Member, Lecompton Limestone, Douglas County; Phricodothyris, Lecompton Limestone, Douglas County; Neochonetes, Garrison Shale, Council Grove Group, Riley County; Hustedia, Topeka Limestone, Shawnee Group, Jefferson County.
Because of their worldwide abundance, diversity, and rapid evolution in the Paleozoic, brachiopod fossils are useful indicators of the ages of different rock layers. By matching the brachiopod species contained within rocks deposited in different locations, paleontologists can determine that the rock units were deposited at the same time.
Stratigraphic Range: Lower Cambrian to Holocene.
Taxonomic Classification: Brachiopods belong to Kingdom Animalia, Phylum Brachiopoda. The phylum is divided into three subphyla, the Linguliformea, Craniiformea, and Rhynchonelliformea.
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Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.