Phylum Mollusca:
stuff to know
• Important morphologic features (hard parts only)
• Classification:
– Subphyla; classes; subclasses within Class
Cephalopoda
• Molluscan phylogeny
• Ammonoid suture types
• Pelecypod genera:
– Pecten, Inoceramus, Gryphaea, Exogyra
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Mollusca—Phylum overview
• Representatives include: snails, slugs,
mussels, oysters, clams, squids, octopuses
• Size ranges from microscopic (snails) up to
18m (giant squids)
• Inhabit marine, freshwater and terrestrial
environments
• Aquatic types may be benthonic,
planktonic, nektonic, even flying (squids)
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Phylum overview (cont.)
• Mollusks are extremely diverse, so there are few
features common to all representatives
–
–
–
–
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Free-living metazoans
Dorsal calcareous exoskeleton
Muscular foot for locomotion
Visceral mass with major organ systems
Mantle cavity with gills (digestive and reproductive
systems open into mantle cavity)
– Radula (rasping structure in mouth)
– Head with mouth (maybe also tentacles and eyes)
– Mantle (tissue layer) that surrounds soft parts and
secretes shell
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“typical” mollusk
radula
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Phylum overview (cont.)
• Phylum originated in Early Cambrian
(earlier?) from a flatworm ancestor
– All major classes and subclasses originated by
Middle Ordovician
– Only one major class has become extinct
(Rostroconchia)
• Shells:
– mostly univalved or bivalved, aragonitic,
multilayered, with growth lines and muscle scars
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Mollusk shell and musculature
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Classification
Phylum Mollusca
Subphylum Amphineura
(includes Chitons)
Cambrian-Holocene
Subphylum Cyrtosoma
Cambrian-Holocene
Subphylum Diasoma
Cambrian-Holocene
Class Monoplacophora
Cambrian-Holocene
Class Rostroconchia
Cambrian-Permian
Class Gastropoda
Cambrian-Holocene
Class Scaphopoda
Ordovician-Holocene
Class Cephalopoda
Cambrian-Holocene
Class Pelecypoda
Cambrian-Holocene
Subclass Nautiloidea
(Late Cambrian-Holocene)
Subclass Ammonoidea
(Devonian-Cretaceous)
Subclass Coleoidea
(Devonian-Holocene; mostly Jurassic)
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Phylogeny of molluscan classes
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Monoplacophorans
• Cap-shaped to helical shell; bilateral symmetry;
soft parts not twisted; paired muscles; apex of
shell points anteriorly and overhangs head
• Important because ancestral to most other
mollusks
• Only group of organisms to be described
hypothetically before being discovered, AND to
be known as fossils before live specimens were
found
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Monoplacophorans
Bellerophon
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Monoplacophorans
• Most important group is bellerophontids
– Cambrian-Early Triassic
– Resemble gastropods
– Very common in Late Permian of Tethyan
region (e.g., “Bellerophon Limestone”)
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Gastropods
• Characterized by torsion of soft anatomy
• Head and foot regions combined or closely
associated
• External shell usually coiled in a corkscrew
helix
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Gastropod anatomy
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Gastropod shell terminology
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•
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Apex (earliest part)
Aperture (opening for head-foot)
Operculum (cap)
Whorl (coil of 360°)
Suture (contact between adjoining whorls)
Siphonal canal (opening for inhalent
siphon)
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Gastropod
shells
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Cephalopods
• Class includes Nautilus, squids, octopuses, extinct
ammonoids
• Highly evolved nervous system (cephalization;
eyes)
• Carnivorous and capable of swimming
(nektonic) (up to 70 km/hour)
• Foot and head closely associated
(indistinguishable in some)—hence the name:
kephalus + poda
• Possess hyponome (funnel for jet propulsion) and
arms or tentacles
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Cephalopods
• Shelled forms possess gas-filled chambers
• Buoyancy is controlled by (1) poise
adaptation of the shell (shell form) and (2)
adding or subtracting fluid from chambers
by the siphuncle
• Most living forms possess an ink sac
• Exclusively marine
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Nautilus soft anatomy
(shell not shown)
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Cephalopod shell morphology
• Chambered shell is divided into living chamber
and phragmocone
• Chambers separated by septa
• Suture is junction of septum with the outer shell
wall
• Siphuncle = tube with blood vessels, nerves and
mantle that extends from animal back through
phragmocone (usually ventral)
– Septal foramen = hole through which siphuncle passes
– Septal neck = projection of septum surrounding
siphuncle
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Cephalopod shell morphology
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Siphuncle
terminology
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Cephalopod sutures
• If suture is fluted, saddles point toward aperture
and lobes point toward apex
– Orthoceratitic = unfluted or with broadly undulating
lobes and saddles (Cambrian-Holocene)
– Goniatitic = distinct lobes and saddles that are
undivided (mostly Devonian-Triassic)
– Ceratitic = smooth saddles; serrated (“saw-tooth”)
lobes (mostly Triassic)
– Ammonitic = serrated saddles and lobes (mostly
Jurassic-Cretaceous)
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Cephalopod sutures
saddles
lobes
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Cephalopod
suture
patterns
orthoceratitic
gonitaitic
ceratitic
ammonitic
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Cephalopod classification
(must know)
Class Cephalopoda
Subclass Nautiloidea
Subclass Ammonoidea
Subclass Coleoidea
straight or coiled shell
orthoceratitic sutures
(Late Cambrian-Holocene)
straight or coiled shell
goniatitic, ceratitic, or ammonitic sutures
(Devonian-Cretaceous)
internal shell
includes belemnites
(Devonian-Holocene; mostly Jurassic)
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Subclass Coleoidea: belemnites
Squid-like organism; typically, the only preserved part
is the guard (= “fossil cigars”)
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Rostroconchs and Scaphopods
• Relatively uncommon
• Rostroconchs = strange, bivalved
mollusks (superficially resemble
pelecypods) (Cambrian-Permian)
• Scaphopods = “tooth shells” (OrdovicianHolocene)
water
sediment
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Pelecypods
• Clams, scallops, mussels, oysters, rudists
• Soft anatomy lacks a head region; no significant
sensory organs or radula
• Mostly infaunal or attached epifaunal
suspension feeders; some infaunal deposit feeders
• Typical shell is bilaterally symmetrical, with
right and left valves closed by adductor muscles
• Shells held together along hinge; line of junction
of two valves is commissure
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Pelecypods
• Exclusively aquatic; both marine and nonmarine
• Marine forms range from intertidal zone to
abyssal depths
• Mostly aragonitic; but oysters are calcitic
• Bizarre variants lack bilateral symmetry
(oysters, rudists)
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Pelecypod shell morphology
beak
commissure
hinge
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Pelecypod internal shell
morphology
hinge with
articulating teeth
adductor muscle scar
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Bizarre pelecypods
Oyster
(yum-yum;
pearls, too!)
Rudists
(extinct; up to 2m)
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