Brachiopods are the most abundant fossils in Wisconsin. Most people are not familiar with living brachiopods because modern species inhabit extremely deep regions of the world’s oceans, and their shells are rarely found on modern seashores. But during the Paleozoic, thousands of different species of brachiopods teemed in the near-shore and deep-sea environments of Wisconsin. The number of brachiopod species has decreased since the extinction at the end of the Permian (about 245 million years ago). Now, only about 250 living species of brachiopods exist; more than 30,000 fossil species have been identified in the fossil record.
Brachiopods have two valves (shells) that are generally of unequal size and shape, but the right and left halves of each valve mirror each other. Near the tip of the bottom shell (the pedicle valve), a fleshy stalk (the pedicle) emerges through a hole (the pedicle opening) and attaches the animal to the sea bottom. The other shell is known as the brachial valve, which contains the brachidium. The brachidium supports the gill-like lophophore, which has many filaments and cilia (hairs) that create currents to bring microscopic food particles to the mouth. The large surface area of the filaments is used to obtain oxygen from water and eliminate carbon dioxide. The two valves join at the hingeline, which is on the shell near the pedicle. Muscles hold the two valves together so that the soft parts are protected. The two valves meet at the commissure.
Other shell features are useful for identifying brachiopods. A sulcus (a groove-like depression) is present on many brachiopod shells, and a fold (a raised ridge) can be found on the opposite valve. Costae are elevated ribs on the shell. Growth lines are concentric rings representing successive periods of growth.
Brachiopods are divided into two main groups: the articulates and the inarticulates. Articulates have hinge structures on their shells; inarticulates do not. Because articulates greatly outnumber inarticulates in Wisconsin, we focus on the five orders of articulate brachiopods found in Wisconsin: Orthida (orthids), Strophomenida (strophomenids), Pentamerida (pentamerids), Rhynchonellida (rhynchonellids), and Spiriferida (spiriferids).
Wisconsin’s Ordovician rock holds many orthids. Several characteristics of orthids distinguish them from other brachiopods. Both valves of orthid shells are convex, and one valve is larger and deeper than the other. Most orthids have costae. The hingeline of the orthid shells is normally straight and somewhat long and is bordered by a large interarea, a flat, shelf-like region. Orthids lived from the Cambrian to late Permian, and most Ordovician rock contains diverse assemblages of these brachiopods.
Strophomenids were especially abundant during the Ordovician. Strophomenids usually have large, semicircular shells, with one concave and one convex valve. A short, flat interarea borders the hinge region, and the hingeline is straight, like that of the orthids, but may contain a row of small teeth. Muscle scars are conspicuous features of the internal shell. The strophomenids in Wisconsin, particularly the genus Strophomena, commonly cover entire horizons (layers) of rock. Strophomenids were devastated along with other groups during the Permian extinction, but managed to survive a little longer, into the Triassic, before becoming extinct.
Pentamerid shells are extremely abundant in the Silurian rock of eastern Wisconsin. These brachiopods are large and egg-shaped, with curved hingelines and pronounced shell beaks. They possess a unique internal structure found near the hinge; it is called the spondylium, a raised, spoon-shaped platform used for muscle attachment. Pentamerids arose in the Cambrian, were most common in the Silurian, and became extinct in the Devonian. Some quarries in Dodge County contain Silurian rock with millions of large pentamerid molds and casts.
Most rhynchonellids in Wisconsin are small and marble-like in shape. The commissure is zigzag in outline. These small shells have prominent ribs, and a fold and sulcus are usually present. Rhynchonellids arose in the Ordovician. This group is still present in some of the Earth’s oceans.
Spiriferids are among the most beautiful brachiopods. They are somewhat triangular; many possess shell extensions that give them the appearance of having wings. The spiriferids have a unique spiraling support for the lophophore as well as a fold and sulcus. This group arose in the late Ordovician, but can be found mostly in Devonian rock in the Milwaukee metropolitan area. Two main groups of spiriferids are found in Wisconsin: spiriferidinids and atrypids.
Spiriferidinids have straight hingelines and lateral wing-like shell extensions. Some members of this group, such as Eospirifer, are superficially similar to pentamerids. Platyrachella, because of its interarea, looks suspiciously like an orthid. For the most part, however, triangular-shaped spiriferids are found in Wisconsin.
Atrypids are triangular, and they do not have the wing-like shape of some other spiriferids. These spiriferids have curved hingelines and two convex valves. Most atrypids have ribs and a small beak. Atrypa is a common Devonian brachiopod.