BRACHIOPODA

MORPHOLOGY AND ECOLOGY


Figure 1 - Brachiopod Internal Morphology

Modified from Clarkson (1986)


Symmetry and Orientation

The two valves of brachiopods are quite different. The pedicle valve bears all (or most) of the pedicle opening; it is the lower (therefore ventral) of the two valves. The brachial valve contains supports for the lophophores; it is the upper (therefore dorsal) of the two valves. The anterior of the brachiopod is the part opposite of the beak and hinge line; whereas the beak and hinge area are posterior. It should be noted that terms such as dorsal, ventral, anterior and posterior are in reference to soft-body parts and may not reflect the living position of the animal. The width of the brachiopod shell is measured as the maximum dimension parallel to the hinge line; whereas the length of the shell is measured normal to the width (from the beak to commissure).


Figure 2 - Brachiopod External Morphology

From Clarkson (1986)


All brachiopods exhibit bilateral symmetry. The plane of symmetry (a mirror plane) is parallel to length and passes through the middle of both valves; it is normal to the commissural plane.


Figure 3 - Brachiopod Symmetry

From Boardman et al (1987)


Morphological Terms

Beak: Initial point of growth of a valve. It can be located by tracing radial ornamentation to its origin.

Commissure and Commissural Plane: The commissure is the juncture of the growing margin of the two valves. The commissural plane is the plane between two valves. Notice in one specimen the commissure is rather straight whereas in the other specimen it has a zig-zag trace.

Astrophic: Not having a well defined hinge line. See these specimens for examples.

Strophic: having an elongated and well defined hinge line. See these specimens for examples.

Interarea: Plate of curved surface of a valve between the beak and hinge line; typically bears the triangular or in some cases a circular pedicle opening. See these specimens for examples.

Fold and Sulcus: In many brachiopod shells, an "anticline" (fold) along the mid-line of one valve is associated with a "syncline" (sulcus) along the midline of the other valve. On these specimens, Note that the fold is in the brachial valve and the sulcus is in the pedicle valve.

Costae and Plicae: Costae are radial ridges originating from the beak that are visible only on the valve exterior; whereas plicae are radial ridges visible on both valve exterior and interior. A shell is termed plicate if it exhibits plicae and costate if it has costae. See these specimens which illustrate plicae.

Pedicle Opening: Aperture of slit from which the pedicle emerges. Some brachiopods do not have a functional pedicle and thus do not have any opening. Several types of openings are common including:

1. Circular opening near beak of pedicle valve such as in this specimen.

2. Triangular opening usually in the pedicle valve interarea but may be expressed in both valves. Note that in this specimen the base of the triangle is along the hinge line.

3. Slit in either one valve (the pedicle) or in both valves, whose openings are produced by gapes. This is a common type in the inarticulate brachiopods such as Lingula.

Teeth and Sockets: Knob-like protrusions (teeth) on the hinge of the pedicle valve fits into the small depressions (sockets) on the hinge of the brachial valve (see Figure 4 below).

Adductor Scars: Mark the attachment sites of valve closing muscles. They occur as a small pair of impressions in the pedicle valve interior and as larger impressions on the brachial valve interior.

Diductor Scars: These mark the attachment sites of valve opening muscles. They occur as a large pair of impressions only on the pedicle valve interior where they are either outside or enclose the smaller adductor muscle scars.

Cardinal Process: A knob at the mid-line of brachial valve interior to which the diductor muscles attach.


Figure 4 - Brachiopod Internal Shell Morphology

From Clarkson (1986)


Lophophore Supports: Three common types include:

1. Brachiophores are a pair of short prongs pointing inward along the brachial valve hinge line.

2. Spiralia are a pair of spiral structures occupying much of the shell interior, but are attached to the brachial valve. Axis of coiling can be vertical or horizontal.

3. Loops are hoop shaped structures, sometimes bent double, that are attached to the brachial valve.


Paleoecology and Valve Shape

All brachiopods are filter feeding, sessile (non-mobile) bottom dwellers. They are exclusively marine, but inhabit a variety of bottom environments at various depths and latitudes. Brachiopods are either free-living or rooted by their pedicle to the substrate. During life, they can be oriented either vertically, inclined, or horizontally to the substrate. Typically brachiopods oriented vertically during life will have equally biconvex shells, whereas inclined and horizontally oriented ones will be unequal inflation being plano-convex, concavo-convex. Please become familiar with the accompanying figure indicating inflation types. The relationship between the animal and the substrate can be classed into the following:

Infaunal: Living totally buried within the sediment. Brachiopods living this way are oriented posterior downward, and are usually stabilized by their downward projecting pedicle. Lingulid inarticulates are among the only brachiopods to exploit this infaunal environment.

Semi-Infaunal: In this position, the animal is oriented vertically (posterior downward) and is only partially buried in the sediment; they may or may not be attached by their pedicle.

Reclining: In this position, the animal is in effect floating horizontally on (or partially within) the sediment with the pedicle valve as the lower valve. Generally, reclining brachiopods have a concavo-convex or plano-convex shape. Other modifications include large surface area and spines to help the critter float. The pedicle opening is usually not present. Note that one of the specimens also bears attachment points for spines, which served as an additional adaptation for reclining in soft sediments.

Epifaunal: In this position, the brachiopod is attached either to the sediment or other object (e.g., marine plants) by their pedicle.


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