Brachiopods are marine bottom dwelling, suspension feeding, multicelled animals. They have a soft body enclosed in two shells that can be opened to feed. They are generally immobile but some can slowly relocate themselves on the ocean floor.
Brachiopods form their own phylum with two main branches, the Articulates and the Inarticulates. The Articulates have a hard, calcareous, shell. The shell has two valves that are hinged at one end and thus "articulate" (rotate) around the hinge to open. The Articulates are generally larger than the Inarticulates. The Inarticulates also have two valves. The Inarticulates valves are generally chitinos or chitno-phosphatic that is a softer material than the calcareous shells of the Articluates. The Inarticluates have only a rudimentary hinge or none at all. Those without hinges separate the two value margins to open them. The Articulates genera and species are much more numerous than the Inarticulates. While one species of Articulate brachiopods was about one foot in length, most are much smaller, usually no bigger than a golf ball and often less than that size. Inarticulates are usually less than 2 cm in size and often smaller
Clams, oysters and other bivalves, although somewhat similar to brachiopods, in fact belong in a different phylum, the Mollusca. Each brachiopod valve is bilaterally symmetrical from the center of the hinge to the point of the shell opening opposite to the hinge. That is, each brachiopod valve can be divided in two from hinge to front, and the halves are mirror images of each other. However the two brachiopod valves are not mirror images of each other. In contrast, bivalves are symmetrical along the hinge line so that each value tends to be a mirror image of the other. The soft body anatomy of a brachiopod is different from a mollusk. In addition, the brachiopod phylum has been very conservative. The earliest brachiopods are essentially the same as the modern ones. The phylum Mollusca has been much more adventuresome. Mollusks evolved forms able to move, with eyes and intelligence (snails, squid and octopus). Mollusks were able to invade fresh water and the land. Brachiopods have not done this.
Brachiopods appear in the earliest Cambrian and continue into the modern era. They greatly increased in genera and species in the Ordovician and continued to dominate the marine environment through the Permian. They declined in importance with the Permian extension event, being replaced by mollusks. Brachiopods are exclusively marine but they can tolerate brackish, estuary environments. They are generally found in near shore environments but some species are found at great depth.
A Brachiopod is a fairly simple organism. It has a feeding apparatus, the lothophore. The lothophore strains food particles from the water and channels the food to a mouth and digestive tract. There is a weak heart-like organ that pumps some fluid through the body cavity. However the brachiopod takes up most of its oxygen directly from the surrounding sea, particularly through the lothophore. The soft tissue of the lothophore may be supported by a hard apparatus that is preserved within the fossil shell. These are not found in the Ordovician fossils around Cincinnati.
The brachiopod is distinguished by an organ that is used to anchor it to the ocean floor. It is called the "pedicle" and is a fleshy, muscular stock or "foot". The pedicle could be inserted into the bottom mud and secured with the aid of a mucus like secretion. In some species, the pedicle is cemented to hard objects. Some brachiopods lose their pedicle at maturity.
Brachiopods have muscles that they use to open and shut their shells or values. Articulate brachiopods have one set of muscles to pulled the shell open (diductors) while they have another set of muscles to pull it shut (adductors). In inarticulate brachiopods, the muscles squeezed the body cavity, causing it to expand around the margins to open the shell.
Brachiopods have a simple nervous system and are able to open and close their shells to feed or to escape predators. They have no eyes or brains as we would think of them.
Soft body tissue is rarely preserved in fossils and in the Cincinnati Ordovician it is not found.
Brachial or Dorsal Valve
Pedicle or Ventral Valve
The following are definitions of terms used to identify brachiopod shell fossils.
For identification purposes, the proper orientation of an articulate brachiopod shell is to have the shell opening to the front and the hinge to the back. The pedicle or ventral valve would be on the bottom and the brachial or dorsal valve on top. This is the life position.
Pedicle or ventral valve: The shell half through which the pedicle soft tissue extended. This valve will generally show evidence of a hole or slot through which the pedicle extended. This is generally the larger valve.
Brachial or dorsal valve: The shell half that is not the pedicle valve. This was the top shell in life position.
Hinge: For articulate brachiopods, the contact point on which the two shells rotated to open and close.
Commissura or Margin: The contact point between the two shell valves that separates when the two valves open.
Anterior: The front of a properly oriented brachiopod.
Posterior: The back or hinge side of a properly oriented brachiopod.
Length of shell: Distance from the middle of the posterior side to a point opposite on the anterior side.
Width of shell: Distance from the widest point on a side of the commissura to the opposite point on the other side of a properly oriented brachiopod.
Fold: A raised area of the shell, as if the shell had been pinched.
Sulcus: A depression in the shell, as if the shell had been pushed in. The opposite of a fold.
Interarea: The exterior hinge area of both the pedicle and brachial valve of some articulate brachiopods. The interarea contains the opening for the pedicle, called the notothyrium, in the brachial valve. Opposite to the notothyrium is a notch like, trapezoid shaped feature in the pedicle valve called the delthyrium.
Ray or rib: Raised, thin lines on the valve exteriors, usually going from the hinge area to the margin of the shell. These lines would be from posterior to the anterior side on a properly oriented brachiopod.
Costa: A very strong ray, with significant elevation above the valve surface.
Growth line: Lines or steps in the exterior of a valve that run from margin to margin across a properly oriented brachiopod. These are thought to be growth lines indicating where the shell stopped growing at some point before resuming growth.
Identifying the rock strata in which a brachiopod is found will greatly aid in identification of the brachiopod. The more specific the information on the rock layer, the greater the aid in identification. This is because each strata of rock will have its own suite of brachiopod species. The converse is also true. Knowing the species of brachiopod found in a rock layer will greatly aid in identifying that rock layer.
The interior pattern of the muscle scars is generally unique to a species and greatly aids in identification. Professions will often abrade or dissolve the shell in order to record thin sections and thus be able to reconstruct the shape of the shell interior. Amateurs will generally have to be satisfied with shell interiors that have been naturally exposed. A shell valve with the interior exposed and which has been identified as to species can be compared to a specimen with only the exterior exposed to aid in identification.
It is not uncommon for brachiopod shells to be broken, swashed or mashed down. This can distort some of the characteristics used for identification. Because brachiopods are generally numerous at most collecting sites, look for complete or nearly complete specimens. This will also aid in identification.
As with anything, practice makes perfect. Do not be afraid to identify your brachiopods, You will make mistakes but you will also learn from those mistakes. You can always change the name to the correct one. Please note that the professionals change the names, usually at the genera level. They also sometimes disagree on the proper name, depending often on whether they think it is a different species or not. Ask someone with more experience if you are using the correct name. Ask them what they looked for to make the identification. Go to a natural history museum and compare their identifications. Look in fossil identification books. All of these will increase your skill at identifying the species in your brachiopod collection.
Good luck and good hunting.
Pictures of the Brachiopods Found on Our Field Trips
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Photos courtesy of John Tate, Jack Kallmeyer
and Bill Heimbrock
Descriptions courtesy of John Tate and Ron Fine
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