Mollusca

mollusks 

Mollusca 

blötdjur 

molluscs

What is a mollusc? 

Fundamental organization (hypothetical archimollusc): 

- shell secreted by a layer of tissue called the mantle 

- mouth and anus at opposite end (but in gastropods both anterior) 

- mantle cavity bears gills (but pulmonate gastropods have no gills) 

- above mantle cavity is the visceral mass 

with gut, nervous, circulatory and muscular system 

- shell is of calcium carbonite (calcite or aragonite) (but may be secondaryly lost) 

- shell typically external (but in some groups it became internal) 

- grow by accretion (calcium carbonate is added to the edge of the shell by the mantle) 

- generally marine (but also few freshwater terrestrial groups) 

Mollusca

systematics main groups 

Mollusca

Bivalvia 

Mollusca – systematics – Bivalvia

Bivalvia (=Pelecypoda, Lamellibranchia) 

pair of valves (right and left valve) 

bilobed mantle 

valves articulate along a dorsal hinge line 

no head 

typically bilaterally symmetric 

(plane of symmetry passing between the valves, = commissural plane) 

prominent ventral foot 

known since the Early Cambrian, but diversify not prior to Ordovician 

but still not a very common faunal element during the Paleozoic 

Mollusca – systematics – Bivalvia

Main features of the shell 

muscle scars ligament dentition lunule pallial line beak 

homomyar internal cardinalia (escutcheon) sinupalliate orthogyrate 

heteromyar external lateralia integripalliate prosogyrate ! 

monomyar amphidetic opisthogyrate 

prosodetic 

opisthodetic ! 

Mollusca – systematics - Bivalvia

Basic for systematics are the gill type and the hinge dentition 

Gills: Protobranchs (deposit feeders, most primitive) 

Filibranchs (suspension feeders) 

Mollusca – systematics – Bivalvia – gill types 

Eulammelibranchs (suspension feeders) 

Septibranchs (carnivores, most derived)

Dentition: Various types and subtypes 

taxodont – dysodont – isodont – schizodont – desmodont – pachydont – heterodont 

Mollusca – systematics – Bivalvia – dentition

Types of dentition 

Taxodont – many small similar teeth & sockets all along hinge plate (e.g., Glycimeris and Arca) 

Schizodont – two or three thick teeth with prominent grooves (e.g., Trigonia) 

Dysodont – small simple teeth near the edge of the valve (eg Mytilus) 

Heterodont – few teeth varying in size and shape, distinquished as cardinal teeth, beneath the umbo, and 

lateral teeth which lie obliquely along the hinge plate (e.g., most recent bivalves) 

Isodont – teeth very large and located on either side of a central ligament pit (e.g., Spondylus) 

Desmodont – teeth very reduced or absent (e.g., Mya) with a large internal process (the chondrophore) 

carrying the ligament 

Mollusca – systematics – Bivalvia – dentition

taxodont 

Taxodont – many 

small similar teeth & 

sockets all along hinge 

plate (e.g., Glycimeris 

and Arca)

Dysodont – 

small simple 

teeth near the 

edge of the 

valve (eg 

Mytilus) 

dysodont 

no teeth just crenulation

heterodont 

Heterodont – few teeth varying in size and shape, distinquished as cardinal teeth, 

beneath the umbo, and lateral teeth which lie obliquely along the hinge plate (e.g., 

most recent bivalves) 

cardinalia and lateralia

Isodont –teeth 

very large and 

located on either 

side of a central 

ligament pit (e.g., 

Spondylus) 

isodont 

two teeth correspond to two grooves

schizodont 

Schizodont – two or three thick teeth with prominent grooves (e.g., Trigonia) 

teeth have crenulations (”teeth with teeth”)

desmodont 

Desmodont – teeth very reduced or absent (e.g., Mya) with a large internal 

process (the chondrophore) carrying the ligament 

internal process (the chondrophore) carries the ligament

pachydont 

Pachydont – large, heavy and massive teeth (e.g., rudists)

Orientation of a bivalve shell 

what is posterior – anterior – right – left ? 

ligament typically posterior 

posterior adductor muscle scar stronger developed 

pallial sinus posterior / shell gaps posterior 

posterior part of shell typically better developed 

umbo (beak) typically points anterior (prosogyre) 

byssal notch anterior 

Oysters: left valve bigger/cemented 

Mollusca – systematics – Bivalvia – orientation

ight 


ight 


ight 


ight 


left 


Ecology 

marine and fresh water 

typically benthic, infaunal or epifaunal 

include burrowing, browsing, cemented, free lying, swimming, boring forms 

filter feeders, deposit feeders, carnivores 

Mollusca – systematics – Bivalvia – ecology

Infaunal bivalves 

Both detrivorous and filtering strategies 

Most Palaeozoic groups are infaunal detrivores 

Probably the oldest of all bivalve life-modes 

Burry thorugh sediment with the muscular foot 

Extensions of the mantle (siphons) allow water 

transport 

Shell modified to specific substrate requirements

Infaunal bivalves 

Water 

Sediment 

Venus is a shallow burrowing 

form with short retractable 

siphons. 

Shell Features (shallow 

burrowers): 

• Equivalved 

• Thick(ish) valves 

• Adductor muscles 

roughly equal in size 

• Commonly with 

strong external 

ornament 

Internal view 

of right valve 

Note difference in 

size of pallial sinus 

between the two 

bivalves. (Generally 

the bigger the 

indentation the 

bigger the siphon 

and consequently 

the deeper the 

bivalve could 

burrow) 

Mya arenaria is a 

sluggish bivalve 

which burrows quite 

deeply in firm sand 

or mud. Its long 

siphons can be 

retracted, but not all 

the way back into the 

shell 

Internal 

view of 

left valve 

Foot 

Water 

Sediment 

Shell features 

(deeper 

burrowers): 

• Generally more 

elongate shells 

• Some have 

gapes in the shell 

commissure to 

allow siphons to 

remain outside 

when shell is 

closed 

• Dentition 

reduced

Sessile Epibenthic bivalves 

Attaches to hard subsrates and becomes immobile 

Many groups have evolved this lifemode independently 

Allows effective filterfeeding 

Mytilus (common blue mussel) and many others attach 

by chitinous threads (byssus) secreted by the foot 

Oysters attach by cementing one valve (left) to the 

substrate and adapt to the shape of the substrate

Motile epibenthic bivalves 

Lie exposed on the seabed 

Mostly filterfeeders 

Acute sensory system including photophores (eyes) and 

sensory tentacles along the mantle edge 

Escape strategy: Rapid closure of the valves creates 

jetstream and the mussel can thus swim short distances 

Some Jurassic bivalves may have been permanent 

swimmers

Soft sediment recliners and mudstickers 

Some byssally and cementing forms have 

evolved secondary soft sediment life-modes 

Larvae attaches to small objects and develops 

shapes that allows the bivalve to survive on the 

sediment surface 

Gryphaea (devils toenail) 

Pinnate bivalves

Reef-forming bivalves 

Rudists (Jurassic-Cretaceous) reef builders 

Differential valves 

Cone-shaped right valve 

Left valve acts as a lid 

Probably had symbiotic algae like modern 

Tridacna 

Evolved from oysters? 

Modern Tridacna clam

Reef-forming bivalves

Reef-forming bivalves

Rock boring bivalves 

Several groups of bivalves can produce 

livingchambers by boring through rock and wood 

Lithophaga 

calcareous substrates (corals, limestone 

etc.) 

Valves without gape 

Exclusively chemical excavation 

Pholadids 

All types of substrates 

Wood, corals, granite, lead cables, plastic, 

amber etc. 

Valves with wide anterior gape 

Excavation by movement (abrasion) 

Shell ornament of teeth and rockfragments 

wedged between them act as ”sandpaper”

Cephalopoda 

Mollusca – systematics – Cephalopoda

Cephalopods 

most highly evolved molluscs (especially eyes and brain) 

a high level of cephalization (concentration of sensory and neural centers in the head) 

group includes the modern Nautilus, argonauts, squids, octopuses, cuttlefishes 

as well as the fossil ammonites and belemnites 

2 main groups: Palcephalopoda (nautilids and endoceratids) 

Neocephalopoda (orthoceratids, ammonites, belemnites) 

typically bilaterally symmetrical 

shell, if developed, subdivided in chambers by septae 

chambers are connected by a tube (siphuncle) 

hyponome and tentacles are homologue to foot of bivalves and gastropods 

mouth with powerful horny beaklike jaws and a radula 

radula less developed than in gastropods 

since Late Cambrian 


Neocephalopods 

Spirula 

Loligo (Squid) 

Octupus 

Sepia (Cuttlefish)

Shell remains

Palcephalopoda (Nautilus + fossils)

Shell terminology 

shell wall 

peristome 

aperture 

growth line 

septum 

camera / 

chamber 

septal neck 

Mollusca – systematics – Cephalopoda – shell morphology 

phragmocone living chamber 

protoconch

Mollusca – systematics – Cephalopoda – morphology – shell

The suture = junction between septa and shell wall 

- most important for taxonomy and phylogeny of Ammonitoidea 

- particular types characterize distinct families and orders 

saddles: point in apertural direction 

lobes: point backward 

prosuture – primary suture 


Shape of shell 


The cephalopod jaw 

Modern Cephalopods have a horny beak, either two simple plates or more 

complex structures 

There is also a radula with rel. simple, undifferentiated teeth


Classification 

Old: Nautiloidea – Ammonoidea – Coleoidea 

Palcephalopoda (~Nautiloidea) – Neocephalopoda (Orthoceratoidea, Ammonoidea, Coleoidea) 

Palcephalopoda 

shell well developed and large, originally slightly curved 

siphuncle was situated between the center and the ventral surface. 

siphuncle generally large with internal deposits (important tax. feature) 

Neocephalopoda 

siphuncle thin and empty 

phragmocone originally straight with the siphuncle situated at or near the center 

later the position of the siphuncle shifted to the ventral surface (Bactritida), 

the shell became coiled (Ammonoidea) 

the shell became internal, reduced or absent (Coleoidea) 


Palcephalopoda (= Nautiloidea, + several Paleozoic groups, excl. orthoceratids) 


Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea) 

Mollusca – systematics – Cephalopoda 

Ammonoidea - Goniatitida



Ammonoidea - Ammonitida


Choristocerataceae 

Late Jurassic 

Spirocerataceae 

Middle Jurassic 


Ammonoidea – heteromorphic ammonites 

Ancyloceratina 

latest Jurassic to end Cretaceous


Coleoidea 

•Coleoids have little skeletal material 

•Consequently are rare as fossils 

•Fossisl date back to the Carboniferous 

•Probably derived from orthocone 

Neocephaolopds in the Devonian



Coleoidea – Belemnitida 

•Belemnites were squid-like with internal shell (Phragmocone) 

•The posterior of the phragmocone had mineralised deposits 

(rostrum or guard) 

•The rostrum is a massive, calcareous structure and hence 

fossilise extremely well (contrary the phragmocone) 

•Probably worked as counterbalance (compare darts) 

•Belemnites were common in the Jurassic and Cretaceous 

•No modern cephalopods produce a rostrum

Evolution 

Plectronoceras 

Mollusca – systematics – Cephalopoda – evolution 

evolutionary explosion 

high diversity 

increase in size

Ecology 

entirely marine 

active predators (all are carnivorous) 

active swimmers 

swimming is by rapidly expelling water from the mantle cavity 

the water is forced out through the hyponome (“jet propulsion“) 

Mollusca – systematics – Cephalopoda – ecology

Swimming 

swimming is by rapidly expelling water from the mantle cavity 

the water is forced out through the hyponome (“jet propulsion“)

Cephalopod eyes 

Camera eye fully comparable to ours 

Famous case of convergent evolution 

Forms from skin in the embryo, ours from extension of the brain 

Nautilus has very primitive, pin-hole camera type eye 

Homo sapiens Octopus

Biostratigraphy 

especially Ammonoidea 

and in the Mesozoic 

Mollusca – systematics – Cephalopoda – biostratigraphy

Polyplacophora 

Mollusca – systematics – Polyplacophora

Polyplacophora (chitons) 

primitive molluscs with eight, articulating (overlapping) aragonitic plates 

(except one Palaeozoic lineage had seven) 

generally oval in outline with a flattened body 

creeping foot, a primitive feature in molluscs 

radula, mineralized with magnetite 

head is poorly developed 

the girdle (perinotum), a band of muscular tissue, runs along the dorsal periphery 

embedded in the girdle are small calcareous spines, scales or spicules 

known since the Late Cambrian (isolated plates) 


Multiplacophorans 

Stem group 

polyplacophorans? 

Different numbers of 

sclerites 

Best know is Polysacos 

from the Carboniferous 

17 plates 

Polysacos

Polyplacophoran Ecology 

marine, commonly occurring on rocks and seaweed in the intertidal zone 

few species have also been found at depths down to 5000 meters 

photoreceptor cells in the mantle and girdle. 

the animal is thus able to detect light, which it responds negatively to 

active at night, when they creep over rocks scraping algae 

and other microscopic organisms off the surface with their radula 


Gastropoda 

Mollusca – systematics – Gastropoda

Gastropoda 

mollusks with a head and foot (the head-foot), and a mantle covering visceral mass 

head-foot can be withdrawn into the shell (sealed by operculum) 

typically with a univalve calcareous shell (maybe reduced, or pseudo-bivalved) 

shell generally coiled in some manner and external 

radula typically present 

Torsion is the single unique defining characteristic (synapomorphy) of the gastropods 

known since Late Cambrian 


Torsion 

twisting of the body [it is entirely different from the spiraling of the shell 

fossil evidence suggests that early, non-twisted molluscs already had coiled shells 

some modern gastropods have uncoiled shells, or even no shell at all] 

all gastropods undergo torsion during some stage of their development 

- displacement of many interior organs 

- digestive tract became U-shaped (anus and nephridia moved anterior) 

- nervous system acquires a twisted appearance (streptoneury) 

Mollusca – systematics – Gastropoda – torsion

Torsion 

Advantages: 

allowed the gills better access to water flow 

allowing the animal to withdraw more deeply into the shell 

the head was able to retract first (foot last, still able to swim) 

Disadvantages: 

anus and nephridia anterior 

the animal would be dumping its waste on its head 

Mollusca – systematics – Gastropoda – torsion

The radula 

important taxonomic feature in modern gastropods 

no fossil radula confirmed, although there are descriptions 

composed of chitinous material and arranged as a long, coiled band 

consists of central, lateral, and marginal teeth 

Mollusca – systematics – Gastropoda – radula

Shell terminology 

coiling: 

- dextral 

- sinistral 


Gastropod opercula

Traditional classification 

Mollusca – systematics – Gastropoda – systematics

Prosobranchia (shelled gastropods in which torsion is complete) 

classification based on gill and radula types -- unfortunately! 

Archaeogastropoda: holostome aperture = no siphonal canal (since Cambrian) 

Mesogastropoda: aperture typically with siphonal canal (since Ordovician) 

Neogastropoda: aperture siphonostome, often very long siphonal canal (since Cretaceous) 

Mollusca – systematics – Gastropoda – classification

Modern classification 

Incertae Sedis (primitive forms - Archaeogastropoda in part) 

Order "Tropidodiscida" ("Bellerophontina" in part) † 

Order Bellerophontida ("Bellerophontina" in part) † 

Subclass Eogastropoda (primitive forms - Prosobranchia / Archaeogastropoda in part) 

Order "Platycerida" † 

Order Patellogastropoda (Docoglossa) 

Order Cocculinida (polyphyletic?) 

Order Vetigastropoda 

Subclass Orthogastropoda (all other gastropods) 

Infraclass Neritimorpha (Archaeogastropoda in part) 

Infraclass Apogastropoda 

Superorder Heterobranchia 

Order Opisthobranchia 

Order Pulmonata 

Superorder Caenogastropoda(Prosobranchia in part) 

Order Architaenoglossa 

Order Neotaenioglossa 

Order Neogastropoda 

Mollusca – systematics – Gastropoda – systematics

Patellogastropoda 

Cellana radians 

Haliotis (Haliotis) midae 

Vetigastropoda 

Amblychilepas scutella 

Neritimorpha 

Turbo (Dinassovica) imperialis

Oliva (Oliva) sericea textilina 

Conus (Asprella) alabaster 

Caenogastropoda 

Pusionella vulpina Morum (Oniscidia) exquisitum 

Turritella ungulina 

Murex (Murex) aduncospinosus Malea ringens

Pulmonata 

Heterobranchia 

Ophistobranchia 

Glaucilla marginata 

Philine angasi

Ecology 

most are aquatic, marine, brackish and fresh water 

several groups lives on land (most are Pulmonates) 

marine forms typically live in shallow waters 

highest diversity in tropical waters 

but also known from arctic waters and hydrothermal vents in the deep sea 

one of the most adaptable forms with respect to: 

salinity – preassure (water and air) – temperature (water and air) – humidity 

most are herbivores, but also carnivore (Muricidae, Naticidae, Conidae) and omnivore 

marine forms typically benthic, but also free swimming and floating forms 

freshwater and terrestrial forms at least since Carboniferous 

Mollusca – systematics – Gastropoda – ecology

Palaeozoic gastropods 

Relatively rare 

Shell usually structurally weak: 

With selenizone or anal slit 

Lacking columella (central 

strengthening rod connecting whorls) 

Platyceratids 

Large, loosely coiled shell 

Uneven margins 

Life attached to crinoids

Palaeozoic gastropods 

Bellerophontids 

Planispiral coiling 

Selenizone and deep sinus 

Selenizone often raised 

Extinct 

Name derived from ancient Greek 

hero Bellerophon in recognition of the 

similarity to a greek helmet 

Bucanella nana 

Sinuites 

Bellerophon

Modern gastropods 


Cone shells 

Hunt with poisonous harpoons 

Poison sometimes extremely 

potent (deadly to humans) 

Prey is ingested whole or 

scraped with radula

Patellids 

Cap-shaped shell 

Sticking to rocks and other hard things 

Foot modified to function as a sucker 

Why? 

Protection 

Conserve moisture 

Feeding by scraping algae 

Secondarily untorted 

Obs! Convergent evolution

Predation by Gastropods 

Several groups of gastropods feed by drilling holes 

in mollusc shells 

Muricids are epibenthic with often highly ornate 

shells. Drill holes with straight sides 

Naticids are infaunal with very smooth, rounded 

shells. Drill countersunk holes by combining acid with 

radular activity 

Naticid 

Muricid

Mesozoic marine revolution 

Predator-prey arms race 

Jurassic to present 

Evolution of new predators (e.g. tools) 

- Crab and lobster claws 

Today: More shells are damaged than in 

Palaeozoic 

Led to new mollusc adaptions 

- glossy shells 

- varices on aperture 

- narrow aperture 

New ‘inventions’ forced the opponent to 

develop new counter methods 

Affected all benthic marine animals

Mollusca

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