Two major groups called the articulates and inarticulates vary in terms of their hinges, muscle complexity, and composition matter.
The shells of brachiopods are hinged at the rear end (posterior), while the front (anterior) can be closed for protection or open for feeding. The articulate brachiopods have toothed hinges and simple opening and closing muscles. The other group called the inarticulate or 'lingulates' do not join at the hinge, but instead have a complex network of muscles to open and close the valves
​
The shells of inarticulate brachiopods are made up organic matter with calcium phosphate, and usually appear brown or black. After being fossilized, it turns either blue-black or pinkish-white, with shiny grey on the surface.
​
​Articulate brachiopods show a greater range of shapes and are much more diverse. Their shells are made of calcium carbonate (calcite, in this case), which in well-preserved layers shows two structural layers. Most shells have fine radiating ribs, and concentric growth lines on the outside. Muscle scars on the inside of the shell and variety of hinge structures can be used to distinguish many species.
Brachiopods are considered to be a type of marine 'shellfish' with their soft body enclosed in a pair of shells. Hence, they often resemble molluscs including clams and mussels, but they are not related to these at all!
All brachiopods have bilaterally symmetrical valves on the upper and lower surfaces, unlike the bivalve molluscs with left and right arrangement.
Each shell is symmetrical about the midline; however, the two shells are differed in size (unlike more bivalves). Their similarity in shape to Roman oil lamps has led to brachiopods being also called 'lamp-shells'.
Modern brachiopods range in size from 0.25 inches to just over 3 inches in length and width. Fossil brachiopods fall within the same range, though few exceptions including Gigantoproductus and Titanaria have shells that are 15 inches across.
​
Since brachiopods are exclusively marine, the morphological variation can be correlated with changing hydrodynamic conditions as well. Based on the strength of the currents, the shells can have variable shapes.
Size & symmetry
FUN FACT!
Lingula is often considered the ultimate living fossil - has retained its form for the last 500 million years!
​
The pedicle valve or the ventral (lower) valve bears the pedicle opening for the stalk-like structure called the pedicle. On the other hand, the brachial valve or the dorsal (upper) valve contains supports for the lophophore, used for feeding and respiration. The pedicle valve is typically larger than the brachial.
the two valves
articulates vs. inarticulates
Two major groups called the articulates and inarticulates vary in terms of their hinges, muscle complexity, and composition matter.
The shells of brachiopods are hinged at the rear end (posterior), while the front (anterior) can be closed for protection or open for feeding. The articulate brachiopods have toothed hinges and simple opening and closing muscles. The other group called the inarticulate or 'lingulates' do not join at the hinge, but instead have a complex network of muscles to open and close the valves
​
The shells of inarticulate brachiopods are made up organic matter with calcium phosphate, and usually appear brown or black. After being fossilized, it turns either blue-black or pinkish-white, with shiny grey on the surface.
​
Articulate brachiopods show a greater range of shapes and are much more diverse. Their shells are made of calcium carbonate (calcite, in this case), which in well-preserved layers shows two structural layers. Most shells have fine radiating ribs, and concentric growth lines on the outside. Muscle scars on the inside of the shell and variety of hinge structures can be used to distinguish many species.
Infaunal
Semi-infaunal
Reclining
Epifaunal
orientation
Brachiopods are either free-living or rooted by their pedicle to the substrate. They can be either be vertically, inclined, or horizontally oriented to the substrate. Brachiopods that are oriented vertically will have equally bi-convex shells. On the other hand, inclined and horizontally oriented ones will have unequal plano-convex, concavo-convex shells.
Following is the classification based on the relationship between the animal and the substrate:
Infaunal - brachiopods are completely buried within the sediment with posterior oriented downward and are stabilized by the pedicle projecting downward. Inarticulates or lingulates are the only brachiopods to exploit this infaunal environment.
Semi-infaunal - they are oriented vertically (posterior downward), but are only partially buried in the sediment. Therefore, they may or may not be attached by their pedicle
Reclining - horizontally oriented on or partially within the sediment with lower valve being the pedicle valve. Reclining brachiopods typically have large surface area and spines to help them float. The pedicle opening is not present in these animals.
Epifaunal - either attached to the sediment or surrounding things like marine plants by their pedicle
lophophore
Inside the shell is the filamentous feeding organ called the lophophore. This consists of pair of ciliated, twisted projections create water currents and filter out microscopic food particles. Fossil as well as modern brachiopods have a variety of accessory supports for the feeding apparatus
​
Some brachiopod lineages, particularly Rhynchonellida, developed a zig-zag commissure to increase the amount of water filtered and still protect its lophophore from large particles. This is done by bringing the sensitive mantle edges closer together allows the animal to have more control over the material that is coming in.
​
​
