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BRACHIOPODA
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MORPHOLOGY
AND ECOLOGY
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Figure
1
- Brachiopod Internal Morphology
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Modified from Clarkson (1986)
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Symmetry
and Orientation
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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).
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Figure 2
-
Brachiopod External Morphology
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From Clarkson (1986)
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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.
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Figure
3 - Brachiopod Symmetry
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From Boardman et al (1987)
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Morphological
Terms
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Beak: Initial point of
growth of a valve. It can be located by tracing radial
ornamentation to its origin.
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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. 
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Astrophic: Not having a well
defined hinge line. See these specimens for examples.
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Strophic: having an
elongated and well defined hinge line. See these specimens
for examples.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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Cardinal Process: A knob at
the mid-line of brachial valve interior to which the
diductor muscles attach.
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Figure 4 -
Brachiopod
Internal Shell Morphology
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From Clarkson (1986)
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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.
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Paleoecology
and Valve Shape
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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:
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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.
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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.
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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.
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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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Note that the fold is in the brachial valve and the sulcus
is in the pedicle valve. 
Note that one of the specimens also bears attachment points
for spines, which served as an additional adaptation for
reclining in soft sediments.