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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– 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

S u bp h ylu m A m p h ine u ra(in c lu d e s C h ito n s)

C a m b ria n -H o lo ce ne

C la ss M o n op la cop h o raC a m b ria n -H o lo ce ne

C la ss G a s tro p o daC a m b ria n -H o lo ce ne

Subcla ss Nau tiloide a(Late Ca m brian-H olocene )

Subclass A m m onoide a(Devonian-Cretaceous )

Subcla ss Co leoide a(Devonian-H olocene; m o stly Jurassic )

C la ss C e ph a lop o daC a m b ria n -H o lo ce ne

S u b p hylu m C yrtoso m aC a m b ria n -H o lo ce ne

C la ss R o s tro co n ch iaC a m bria n -P e rm ian

C la ss S cap h op o daO rd o v ic ia n -H o lo ce ne

C la ss P e le cyp o daC a m b ria n -H o lo ce ne

S u b ph ylu m D iaso m aC a m b ria n -H o lo ce ne

P h ylu m M o llu sca

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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

• 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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Gastropodshells

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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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Siphuncleterminology

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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

lobessaddles

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Cephalopodsuturepatterns

orthoceratitic

gonitaitic

ceratitic

ammonitic

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Cephalopod classification(must know)

Subclass N autilo ideas tra igh t o r co ile d sh e llo rth oce ra tit ic su tu res

(L a te C a m b ria n-H o loce n e)

Subclass Am m onoideas tra igh t o r co ile d sh e ll

g o n ia tit ic , ce ra tit ic , o r am m on it ic su tu res(D e von ia n -C re ta ce o u s)

Subclass C oleoideain te rn a l sh e ll

in c lu de s be le m n ites(D e vo n ia n -H o loce n e ; m o s tly Ju ra ss ic)

Class Cepha lopoda

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Subclass Coleoidea: belemnites

Squid-like organism; typically, the only preserved partis 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” (Ordovician-Holocene)

sediment

water

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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 non-marine

• 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 morphologybeak

hingecommissure

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Pelecypod internal shell morphology

adductor muscle scar

hinge witharticulating teeth

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Bizarre pelecypods

Rudists(extinct; up to 2m)

Oyster(yum-yum;pearls, too!)