Transcript Slide 1

MOLLUSCA
MOLLUSCS
• Molluscs have a fleshy mantle encasing
the visceral mass and a muscular foot.
• Unlike gastropods and cephalopods,
bivalves secrete two shells rather than
one. Bivalves have lost the radula,
which is presumed to be a primitive
feature among all mollusks.
Class BIVALVIA
• Bivalves, sometimes called Pelecypods
(meaning axe foot) are a very diverse
and abundant group of molluscs which
inhabit a variety of marine and nonmarine environments.
• Their long geologic history and variety
of forms have made them the popular
subjects of many evolutionary and
functional morphological studies.
BIVALVES
• 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.
Feeding
• Most bivalves are filter feeders, trapping
suspended food particles as water passes
through their gills.
• Only two groups, the nuculoids and
cryptodonts, actively feed on organic
material within the sediment and are thus
true deposit feeders.
Relations to Substrate
• Infaunal
– Burrowing: Shells are usually equivalved and isomyrian (or
anisomyrian) with a distinct pallial line. They include: the nuculoid
burrowing deposit feeders, the shallow burrowing non-siphonate forms
lacking a pallial sinus, and deep burrowing siphonate forms identified by
a distinct pallial sinus.
– Boring: Shells are usually thick, equivalved, and cylindrical in cross
section. Some forms are moderately ornamented with ridges and stout
spines whereas others such as the "ship worms" are tubular in form.
• Semi Infaunal
– Byssally attached (endobyssate). Similar to many epifaunal byssate
forms (see below), yet maximum shell width (inflation) is at mid-line of
shell cross-section. Some forms can be elongated and fan-like with a
reduced anterior area. Examples include pen shells, and the mussellike modiolids, and some ark shells. The depth to which the bivalves
are partially buried can often be deduced by looking for encrusting
organisms that may have attached themselves above the sedimentwater interface.
Epifaunal
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Byssally attached (epibyssate).
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Reclining.
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Shells are commonly inequivalved with a larger lower (usually the left) valve which is more
inflated or convex while the upper valve may be planar. Some also exhibit spines, especially
on the lower valve, to aid in stabilization in soft substrates in a manner similar to some
brachiopods. Many have a small attachment area at beak where earliest growth stages were
cemented. The giant clam Tridacna, who has photosymbionts similar to hermatypic
scleractinian corals, is a recliner even though it had a functional byssus during its earliest
juvenile stages.
Swimming.
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Shells can be either equivalved or inequivalved depending on their orientation to substrate
during life. Usually, all epibyssate forms have a reduced anterior region. Some groups, such
as the blue mussels, are similar to endobyssate forms except the maximum inflation is below
the mid-line of the valves cross-section. Other forms may have a byssal notch and/or a well
defined auricle, or, as in the case of some arks, have a gape along the ventral margin.
Shells are usually equilateral but not equivalved. The lower (usually the left) valve is usually
slightly larger. Swimming forms are typified by having a greater umbonal angle (greater than
105°) than similar-looking epibyssate forms. Furthermore, swimming forms typically have a
single (monomyrian), large, centrally located adductor muscle.
Cementing.
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Shells are commonly inequivalved with the lower (usually left) valve assuming the form of the
object to which it is cementing, a condition called xenomorphism. In such cases, both valves
are usually highly variable in shape, as in the common oysters and other forms as well.
Some groups such as the Cretaceous rudists could reach very large sizes and were able to
form reefs mimicking corals in both morphology and ecology.
Class GASTROPODA
• Gastropods, including such common forms
such as snails, slugs, and whelks, occupy
both marine and non-marine
environments.
• Although many gastropods are
herbivorous grazers, several groups are
active carnivores able to drill through the
skeleton of the luckless victim.
GASTROPODA
• Gastropods are radulate organisms with a
torted body (e.g., the body is rotated 180o
so that the anus is above the head).
• Gastropods typically have a helical coiled
univalved shell whose opening (aperture)
may be closed by an operculum.
Class CEPHALOPODA
• Cephalopods are a class of mobile mollusks, most of
which are nektic or nekto-benthic.
• Cephalopods have a bilaterally symmetrical body, a
prominent head, and a modified foot in the form of
tentacles.
• Although during the Paleozoic and Mesozoic,
cephalopods achieved great diversity and abundance in
marine habitats, only two genera possessing skeletons
are known today.
• Superficially the shell or conch of cephalopods resemble
gastropods; however, most cephalopods coil in a plane,
whereas gastropods are helicoiled.
• Cephalopods with an external conch, the coiled shell is
chambered.
Shape of External Shells
• Several different shapes are common
among fossil and extinct cephalopods.
• These include orthoconic or straight,
brevicone, evolute planispiral, involute
planispiral, or heteromorphic.
brevicone
straight
evolute planispiral
involute planispiral
Subclass NAUTILOIDEA
• Nautiliods are an important group of
cephalopods with an external shell.
• However, unlike the ammonoids, the nautiloids
have living representatives in the genus
Nautilus.
• Nautiloid shells are external and are
characterized by either straight or slightly wavy
sutures.
• Nautiloid shells are either orthoconic, or they are
coiled, such as Nautilus;
• The siphuncle may be small or large, but is
typically centrally located.
Subclass AMMONOIDEA
• Very important extinct group of cephalopods
which includes all forms with an external shell
with fluted septa.
• Most are planispiral, but some may be
heteromorphic
• The siphuncles are generally small and ventral
in position.
• Division within the ammonoids is based upon
the grades of suture fluting.
• There are three grades
SUTURES
• Goniatite suture. Saddles and lobes are present. The
goniatite suture is characterized by undivided rounded
saddles and undivided angular lobes. Ammonoids with
this type of suture are called goniatites.
• Ceratite suture. Saddles are undivided whereas the
lobes are divided. Ammonoids with this type of suture
are called ceratites.
• Ammonite suture. Both the saddles and lobes are
divided. Ammonoids with this type of suture are called
ammonites. Although many of the ammonites are coiled,
there are many genera such as Baculites, which is
heteromorphic and encompasses a variety of coiling
shapes.
Goniatite suture
Ceratite suture
Ammonite suture