Classes of the Phylum Brachiopoda (currently in Atlas)
What is a brachiopod?
While many types of animals were common on the Ordovician sea floor, perhaps none were more important than the brachiopods. Brachiopods are marine invertebrates belonging to the Phylum Brachiopoda, characterized by two bilaterally symmetrical valves.
During the Ordovician, brachiopods were the dominant shellfish and occurred abundantly on the seafloor globally. In fact, if you went to the beach anytime from 550 to 250 million years ago, most of the shells you would have collected would have been brachiopods. Only after the Permian mass extinction did brachiopods become less important than clams in the ocean ecosystem. Although you won’t find brachiopods at the beaches in North America today, they are still alive and most commonly living in colder ocean waters off the coast of the Pacific Northwest, Alaska, New Zealand, Antarctica, and other cooler oceans of the world.
Brachiopods are animals that lived either attached to or simply resting on the sea floor. An opening, known as the pedicle foramen, along the hinge line connecting their valves, allowed a foot-like muscle, the pedicle, to extend and attach the animal to the sea floor. At the other end of the animal’s body, the two valves would open to allow water to enter and waste to exit the body cavity. Brachiopods are filter feeders, so they required a continuous stream of water into their body chamber from which to extract plankton and oxygen. Brachiopods produced a water current by beating the cilia of their lophophore. When in danger, brachiopods would close their shells together for protection.
Brachiopods versus bivalves
Brachiopods superficially resemble clams but are not closely related to our modern sea shells. Clams, or bivalves, belong to the Class Bivalvia in the Phylum Mollusca, while brachiopods belong to their own phylum, Brachiopoda. The internal organs and muscular systems of clams and brachiopods are very different; clams feed using gills, while brachiopods have a feature called a lophophore but no gills. In fact, brachiopods are more closely related to another group of animals with lophophores that are also common in Cincinnatian rocks, the Phylum Bryozoa.
The easiest way to distinguish a fossil brachiopod from a fossil clam (which also occur in Cincinnatian rocks) is to determine where the line of symmetry falls between the two valves. In brachiopods, the plane of symmetry runs through each valve so that the right and left half of a single shell look identical but the two shells are different in shape and size. In a bivalve, the plane of symmetry runs between the shells so that the two shells look identical, but the right and left sides of the valves are different.
Brachiopods and the Richmondian Invasion
The types of brachiopods present in the Cincinnati region changed after the Richmondian Invasion.
Native Species: Prior to the invasion, the dominant species of brachiopods were members of the orders Orthida and Strophomenida.
- Orthid species included: Cincinnetina multisecta, Hebertella occidentalis, and Vinlandostrophia sp.
- Strophomenid species included: Rafinesquina alternata, Sowerbyella rugosa, and Strophomena sp.
Invasive Species: New species, with no ancestors within the Cincinnati region moved into the area from other geographic regions such as modern day Oklahoma, eastern Canada, and Montana/Wyoming. These new taxa included an additional order of brachiopods, the Rhynchonellida.
- Order Rhynchonellida: Hiscobeccus capax, Lepidocyclus perlamellosum, and Rhynchotrema dentata
- Order Strophomenida: Leptaena richmondensis, Eochonetes clarksvillensis, Holtedahlina sulcata, and Tetraphalerella neglecta
- Order Orthida: Glyptorthis insculpta, Plaesiomys subquadrata, and Retrorsirostra carleyi