Classes

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5 Classes Of the Phylum Mollusca
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All mollusks have A mantle which Secretes its
CaCO3 Shell (if it has one).
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Chitons Are Common In present Day but Dont
Preserve As Fossils well
One of 9 plates
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Scaphopods Are common But have changed Little
since The Silurian
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Some Common Pelecypods

a.k.a bivalves
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Class Pelecypoda hatchet foot(a.k.a.
Bivalvia)Main divisions of groups are based on
shell and hingeline types
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Rough classification of Pelecypods based on
mode of life1) active ones that dig or burrow
on or in the sediment2) burrowing or boring
into wood or rock3) Sedentary groups that live
attached to rocks, wood, ships,etcThis is an
informal grouping because it is complicated by
the fact that some forms have more than one mode
of life
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Surface dwellers
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Burrowers
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Subclass Prionodesmacea Cam. Rec.
Prismatic shell structure, hinge teeth either
lacking or poorly developed mostly sedentary,
few burrowers
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Like many Primitive Pelecypods This order is
still Common today
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Order Taxodonta Ord.-Rec. Small
numerous hinge teeth, 2 suborders, one
with protobranch gills and one with
filibranch gills
Note long Hinge line With many small teeth
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Order Schizodonta Ord. Rec. Filibranch gills,
few hinge teeth diverging from beak
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Order Isodonta Triassic-Rec. Filibranch gills,
pair of symmetrically placed teeth on each
valve
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Order Dysodonta Ord.-Rec. Essentially toothless,
best known forms are Pectens (scallops) 6
suborders with 2 that have filibranch gills
(including Pectens) and 4 with eulamellibranch
gills (including oysters)
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Subclass Teleodesmacea Ord.-Rec. All
but one order eulamellibranch gills,
porcellaneous shell structure, teeth specialized
and distinct cardinals and laterals accompanied
by external ligaments.Some have primitive
internal ligamentsCan be burrowing, sedentary,
or boring
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Order Heterodonta Sil. Rec. Eulamellibranch
gills with prominent hinge and Lateral teeth
(hence the name hetero) these are forms that
are most common as clams
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Lateral teeth
Hinge teeth
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Order Pachyodonta Jur. Rec. Mostly thick
shelled, sedentary, with Eulamellibranch gills,
highly inequal valves And thick teeth. Some of
the most distinctive Of all pelecypods with
one group (the Rudistids) That only lived in
the Cretaceous
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Order Desmodonts Ord.-Rec.This group has all
Eulamellibranch gills except The Poromyacea
which have septibranch gills This group is
mostly boring and has internalLigaments with
heterodont-like teeth
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Ordovician/ Silurian Representative forms
Paleoconchs(7) taxodonts (4,8,11,13)
schizodonts( 10,18) Dysodonts(1-3,5,6,12,14,17)
Heterodonts(15) desmodonts(9)
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Representative Devonian forms
Paleoconchs(7,10,12,17,19)
taxodonts(6.9.13.16) schizodont (23)
dysodonts(1,3-5,11,20-22)
heterodonts(14,15,18) desmodonts(2,8)
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Representative Forms of the Carboniferous Dysodo
nts dominant(2,4, 7,8,11,13,14,17,19-24) Paleocon
chs(9,12,15) Taxodonts(1,6,10)
Schizodonts(18) and Primitive
Heterodonts (3,5,16)
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Representative Permian forms Taxodonts(3,4)
schizodont (5) dysodonts(2,6-10,12) Heterodont(1
1) Desmondont(1)
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Representative Triassic/Jurassic forms
Triassic are 1-5, Jurassic Are 6-13
Schizodonts(3,4,7,8a,b) dysodonts(1,2,5,6,11-13)
heterodont (9) desmodont (10) Not shown
here are the first isodonts or pachyodonts

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Representative Cretaceous forms Isodonts(10,13)
dysodonts (1-7, 12,14,15) desmodonts (8.9.11)
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Other Cretaceous forms
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Representative Upper Cretaceous Forms Taxodonts(2
,5,8,9) schizodont(3) Dysodonts(7,12)
heterodonts (1,6,10,11) desmodonts(4,13)
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An upper Cretaceous Ostrea bank
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Representative Paleogene forms
Heterodonts(5,7,9,11, 15-17) taxodonts(
1-4, 13,14,18) also
dysodonts(6,12) and desmodonts(8,10)
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Other Paleogene forms
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Representative Neogene forms
Heterodonts(3-5,12,13) Taxodonts(1,9)
dysodonts (7,10,11) desmodonts(6,8) Isodont(2)
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More Neogene Forms
Desmodonts(1-3, 5, 10,12) taxodonts(4,6,7) Dysod
ont(8) heterodont (9)
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Other Neogene forms
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Class Pelecypoda hatchet foot

(a.k.a. Bivalvia)
Main divisions of groups are
based on shell and hingeline types
Subclass Prionodesmacea Cam.
Rec. Prismatic
shell structure, hinge teeth either lacking or
poorly developed
mostly sedentary, few burrowers
Order Palaeoconcha Ord.-Rec.
Protobranch gills, no hinge
teeth Order Taxodonta
Ord.-Rec. Small
numerous hinge teeth, 2 suborders, one
with protobranch gills and
one with filibranch gills
Order Schizodonta Ord. Rec.
Filibranch gills, few hinge teeth
diverging from beak
Order Isodonta Triassic-Rec.
Filibranch gills, pair of symmetrically
placed Teeth on each valve
Order Dysodonta Ord.-Rec.
Essentially toothless, best known forms
are Pectens (scallops)
6 suborders with 2 that have filibranch
gills (including Pectens)
and 4 with eulamellibranch gills
(including oysters)
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Subclass Teleodesmacea Ord.-Rec. All
but one order eulamellibranch gills,
porcellaneous shell structure, teeth
specialized and distinct cardinals and laterals
accompanied by external ligaments.Some
have primitive internal ligaments Can
be burrowing, sedentary, or boring Order
Heterodonta Sil. Rec.
Eulamellibranch gills with prominent hinge and
Lateral teeth (hence the name
hetero) these are forms that are most
common as clams Order Pachyodonta
Jur. Rec. Mostly thick shelled,
sedentary, with Eulamellibranch gills,
highly inequal valves and thick teeth. Some of
the most distinctiveof all
pelecypods with one group (the Rudistids)
that only lived in the Cretaceous Order
Desmodonts Ord.-Rec. This group
has all Eulamellibranch gills except the
Poromyacea which have septibranch
gill This group is mostly boring and has
internal Ligaments with heterodont-like
teeth
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With few exceptions Pelecypods have Very long
lived Orders and Generally are not Considered
good Index fossils However they are One of the
most Common fossils in The geologic record And
provide good Paleoenvironmental control