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INTRODUCTION
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This laboratory discusses two
classes of molluscs: the bivalves and rostroconchs; of which
the bivalves have the most robust fossil record. Although
these two groups have species that are far too long-ranging
for precise stratigraphic correlation and zonation, their
morphologic attributes are very diagnostic of their diverse
life-habits and paleoecology. As such, this laboratory deals
with a minimum of classification (only to class level) and
concentrates more on paleoecology.
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Like other molluscs, these two
classes have a fleshy mantle encasing the visceral mass and
a muscular foot. Unlike gastropods and cephalopods, bivalves
secrete two shells rather than one. Additionally, bivalves
have lost the radula, which is presumed to be a primitive
feature among all mollusks. The Rostroconchs are similar in
many regards to bivalves, especially in their hypothesized
soft parts, a significant difference is that the shell is
pseudobivalved.
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CLASSIFICATION
& GEOLOGIC RANGES
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Phylum
Mollusca
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Class
Rostroconchia (Cambrian-Permian)
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Class
Bivalvia (Cambrian-Recent)
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Subclass
Paleotaxodonta
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Subclass
Isofilibranchia
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Subclass
Pteriomorphia
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Subclass
Herteroconchia
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Subclass
Anomaldesmata
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Class
ROSTROCONCHIA
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Rostrochonchs are a relatively
minor Paleozoic group which may have an important
phylogenetic position within the phylum Mollusca. Some
authors suggest that rostroconchs evolved from an early
monoplacophoran ancestor and gave rise to both bivalves and
scaphopods, leaving the cephalopods and gastropods as
descendants from a separate monoplacophoran
stock.
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Although they may have an internal
anatomy similar to bivalves, rostroconchs are characterized
by a single, pseudobivalved shell which encloses the mantle
and muscular foot. The anterior part of shell has a gape
from which the foot could probably emerge and an elongated
tube on the posterior end called the rostrum which may have
aided in water filtration. Although only one example is
provided from the laboratory, 
please examine the illustration below.
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Figure 1 -
General
Rostrochonch Morphology
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From McRoberts (1998)
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Class
BIVALVIA
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Bivalves, sometimes called
Pelecypods (meaning axe foot) or, in older literature,
referred to as Lamellibranchs, are a very diverse and
abundant group of molluscs which inhabit a variety of marine
and non-marine environments. Their long geologic history and
variety of forms have made them the popular subjects of many
evolutionary and functional morphological studies.
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For those who work on modern
bivalves, often the characters such as the gill structure
and even color patterns have taken prominence in
classification. However, for those working on only the
preserved hard parts of fossils, usually features such as
differences in teeth provide the classification scheme.
Unfortunately, because bivalves have many morphologic
features with adaptive value, many of these features have
arisen more than once. As a result, it is often difficult to
erect a classification that reflects an evolutionary
history. The classification scheme given above (which you
are not responsible for) is derived in part from your text
and is an attempt to incorporate both hard and soft part
morphology.
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The shell of bivalve molluscs is
characterized by two calcareous halves, called valves, which
can be composed of either calcite and/or aragonite. Some
groups, such as the oysters, 
are exclusively calcitic, while others, such as the
pterioids,
have an aragonitic inner layer. The outermost layer of the
bivalve shell, called the periostracum, is composed of a
horny organic substance which in the example is the darker
material only on the external part of the shell.
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The beak is located in the dorsal
region of bivalves. The byssus or foot protrudes from the
anterior of the shell, while the posterior the shell is the
region of siphon protrusion (at least in some bivalves). The
plane between the two valves (or commissural plane) is the
plane of symmetry which separates the left from right valve.
Note there is not an upper and lower valve (as in
brachiopods) because most bivalves are oriented with their
commissure plane vertical. Although this symmetry is
retained in most bivalves,
the symmetry is secondarily lost in others. Note that in one
of the specimens, there is close to a plane of symmetry
within the valves.
This asymmetry corresponds to the living habit of the
beasts, and those who have lost their original bilateral
symmetry between the valves commonly live with their plane
of commisure not perpendicular to the sediment
surface.
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Left or right valves can be
determined by viewing the posterior end (as shown in the
figure below).
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Figure
2
- Determination of Left and Right Valve
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The two halves of the bivalve shell
are usually joined at the dorsal margin by a ligament which
acts as a spring. The ligament may be internal, as in
oysters, mussels, or scallops. Some internal ones consist of
horizontal or vertical bands which sit in grooves.
Other ligaments may be external.
The hinge margin may also be occupied by a series of teeth
and sockets collectively referred to as dentition (see
below). The opening and closing of the shell are controlled
by adductor muscles (which oppose the force of the ligament)
that often leave physical scars on the valve interior (see
below). Other muscles that leave scars include the pallial
muscles which attach the mantle to the shell, in addition to
smaller ones that control the siphons, foot, and/or byssus.
General terms that you will need to know are given in the
next section.
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please examine the illustration below.
are exclusively calcitic, while others, such as the
pterioids, 
have an aragonitic inner layer. The outermost layer of the
bivalve shell, called the periostracum, is composed of a
horny organic substance which in the example is the darker
material only on the external part of the shell.
the symmetry is secondarily lost in others. Note that in one
of the specimens, there is close to a plane of symmetry
within the valves. 
This asymmetry corresponds to the living habit of the
beasts, and those who have lost their original bilateral
symmetry between the valves commonly live with their plane
of commisure not perpendicular to the sediment
surface.
Other ligaments may be external. 
The hinge margin may also be occupied by a series of teeth
and sockets collectively referred to as dentition (see
below). The opening and closing of the shell are controlled
by adductor muscles (which oppose the force of the ligament)
that often leave physical scars on the valve interior (see
below). Other muscles that leave scars include the pallial
muscles which attach the mantle to the shell, in addition to
smaller ones that control the siphons, foot, and/or byssus.
General terms that you will need to know are given in the
next section.