Mollusca

mollusks

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

Mollusca

systematics main groups

Mollusca – systematics – Bivalvia

Bivalvia

Mollusca – systematics – Bivalvia

Bivalvia (=Pelecypoda, Lamellibranchia)

pair of valves (right and left valve)bilobed mantlevalves articulate along a dorsal hinge lineno headtypically bilaterally symmetric(plane of symmetry passing between the valves, = commissural plane)

prominent ventral footknown since the Early Cambrian, but diversify not prior to Ordovicianbut 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 orthogyrateheteromyar external lateralia integripalliate prosogyrate !monomyar amphidetic opisthogyrate

prosodeticopisthodetic !

Gills: Protobranchs (deposit feeders, most primitive)Filibranchs (suspension feeders)Eulammelibranchs (suspension feeders)Septibranchs (carnivores, most derived)

Basic for systematics are the gill type and the hinge dentition

Mollusca – systematics – Bivalvia – gill types

Mollusca – systematics – Bivalvia – dentition

Dentition: Various types and subtypestaxodont – dysodont – isodont – schizodont – desmodont – pachydont – heterodont

Types of dentitionTaxodont – 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., Glycimerisand Arca)

dysodont

no teeth just crenulation

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

heterodont

cardinalia and lateralia

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

two teeth correspond to two grooves

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

schizodont

teeth have crenulations (”teeth with teeth”)

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

desmodont

internal process (the chondrophore) carries the ligament

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

pachydont

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

Mollusca – systematics – Bivalvia – orientation

Orientation of a bivalve shellwhat is posterior – anterior – right – left ?

ligament typically posteriorposterior adductor muscle scar stronger developedpallial sinus posterior / shell gaps posteriorposterior part of shell typically better developedumbo (beak) typically points anterior (prosogyre)byssal notch anterior

Oysters: left valve bigger/cemented

Mollusca – systematics – Bivalvia – orientation

right

Mollusca – systematics – Bivalvia – orientation

right

Mollusca – systematics – Bivalvia – orientation

right

Mollusca – systematics – Bivalvia – orientation

right

Mollusca – systematics – Bivalvia – orientation

left

Ecologymarine and fresh watertypically benthic, infaunal or epifaunalinclude burrowing, browsing, cemented, free lying, swimming, boring formsfilter feeders, deposit feeders, carnivores

Mollusca – systematics – Bivalvia – ecology

Infaunal bivalves

Both detrivorous and filtering strategies Most Palaeozoic groups are infaunal detrivoresProbably the oldest of all bivalve life-modesBurry thorugh sediment with the muscular footExtensions of the mantle (siphons) allow water

transport Shell modified to specific substrate requirements

Water

Sediment

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

Foot

Internal view of

left valve

Water

Sediment

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)

Venus is a shallow burrowingform with short retractable siphons.

Shell Features (shallow burrowers):• Equivalved• Thick(ish) valves• Adductor muscles roughly equal in size• Commonly with strong external ornament

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

Infaunal bivalves

Sessile Epibenthic bivalves

Attaches to hard subsrates and becomes immobileMany groups have evolved this lifemode independentlyAllows effective filterfeedingMytilus (common blue mussel) and many others attach

by chitinous threads (byssus) secreted by the footOysters attach by cementing one valve (left) to the

substrate and adapt to the shape of the substrate

Motile epibenthic bivalves

Lie exposed on the seabedMostly filterfeedersAcute sensory system including photophores (eyes) and

sensory tentacles along the mantle edgeEscape strategy: Rapid closure of the valves creates

jetstream and the mussel can thus swim short distancesSome 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

Pinnate bivalvesGryphaea (devils toenail)

Reef-forming bivalves

Modern Tridacna clam

Rudists (Jurassic-Cretaceous) reef buildersDifferential valvesCone-shaped right valveLeft valve acts as a lidProbably had symbiotic algae like modern

Tridacna Evolved from oysters?

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 gapeExclusively chemical excavation

PholadidsAll types of substratesWood, corals, granite, lead cables, plastic,

amber etc.Valves with wide anterior gapeExcavation by movement (abrasion)Shell ornament of teeth and rockfragments

wedged between them act as ”sandpaper”

Mollusca – systematics – Cephalopoda

Cephalopoda

Mollusca – systematics – Cephalopoda

Cephalopodsmost 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 symmetricalshell, if developed, subdivided in chambers by septaechambers are connected by a tube (siphuncle)hyponome and tentacles are homologue to foot of bivalves and gastropodsmouth with powerful horny beaklike jaws and a radularadula less developed than in gastropodssince Late Cambrian

Neocephalopods

Spirula

Loligo (Squid)Sepia (Cuttlefish)

Octupus

Shell remains

Palcephalopoda (Nautilus + fossils)

Mollusca – systematics – Cephalopoda – shell morphology

protoconch

septal neck

peristomeshell wall

growth line

camera / chamber

septum

aperture

phragmocone

livin

g ch

ambe

r

Shell terminology

Mollusca – systematics – Cephalopoda – morphology – shell

Mollusca – systematics – Cephalopoda – morphology – shell

The suture = junction between septa and shell wall

saddles: point in apertural directionlobes: point backward

- most important for taxonomy and phylogeny of Ammonitoidea- particular types characterize distinct families and orders

prosuture – primary suture

Mollusca – systematics – Cephalopoda

Shape of shell

The cephalopod jaw

Modern Cephalopods have a horny beak, either two simple plates or more complex structuresThere is also a radula with rel. simple, undifferentiated teeth

Mollusca – systematics – Cephalopoda – morphology – shell

Classification

Old: Nautiloidea – Ammonoidea – Coleoidea

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

Palcephalopodashell 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)

Neocephalopodasiphuncle thin and emptyphragmocone 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)

Mollusca – systematics – Cephalopoda

Mollusca – systematics – Cephalopoda

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

Mollusca – systematics – Cephalopoda

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

Ammonoidea - Goniatitida

Mollusca – systematics – Cephalopoda

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

Ammonoidea - Ammonitida

Mollusca – systematics – Cephalopoda

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

Ammonoidea – heteromorphic ammonites

SpirocerataceaeMiddle Jurassic

Ancyloceratinalatest Jurassic to end Cretaceous

ChoristocerataceaeLate Jurassic

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

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

Mollusca – systematics – Cephalopoda

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

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

Mollusca – systematics – Cephalopoda – evolution

Evolution

Plectronoceras

evolutionary explosionhigh diversityincrease in size

Mollusca – systematics – Cephalopoda – ecology

Ecologyentirely marineactive predators (all are carnivorous)active swimmers

swimming is by rapidly expelling water from the mantle cavitythe water is forced out through the hyponome (“jet propulsion“)

Swimmingswimming is by rapidly expelling water from the mantle cavity

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

Homo sapiens Octopus

Cephalopod eyesCamera eye fully comparable to oursFamous case of convergent evolutionForms from skin in the embryo, ours from extension of the brainNautilus has very primitive, pin-hole camera type eye

Mollusca – systematics – Cephalopoda – biostratigraphy

Biostratigraphy

especially Ammonoideaand in the Mesozoic

Mollusca – systematics – Polyplacophora

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 bodycreeping foot, a primitive feature in molluscsradula, mineralized with magnetitehead is poorly developed the girdle (perinotum), a band of muscular tissue, runs along the dorsal peripheryembedded in the girdle are small calcareous spines, scales or spiculesknown since the Late Cambrian (isolated plates)

Polysacos

Multiplacophorans

Stem group polyplacophorans?

Different numbers of sclerites

Best know is Polysacos from the Carboniferous

17 plates

marine, commonly occurring on rocks and seaweed in the intertidal zonefew species have also been found at depths down to 5000 metersphotoreceptor cells in the mantle and girdle.

the animal is thus able to detect light, which it responds negatively toactive at night, when they creep over rocks scraping algae and other microscopic organisms off the surface with their radula

Polyplacophoran Ecology

Mollusca – systematics – Polyplacophora

Mollusca – systematics – Gastropoda

Gastropoda

Mollusca – systematics – Gastropoda

Gastropoda

mollusks with a head and foot (the head-foot), and a mantle covering visceral masshead-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 externalradula typically presentTorsion is the single unique defining characteristic (synapomorphy) of the gastropodsknown since Late Cambrian

Mollusca – systematics – Gastropoda – torsion

Torsiontwisting of the body [it is entirely different from the spiraling of the shellfossil evidence suggests that early, non-twisted molluscs already had coiled shellssome 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

TorsionAdvantages:

allowed the gills better access to water flowallowing the animal to withdraw more deeply into the shellthe 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 – radula

The radulaimportant taxonomic feature in modern gastropodsno fossil radula confirmed, although there are descriptionscomposed of chitinous material and arranged as a long, coiled bandconsists of central, lateral, and marginal teeth

Shell terminology

Mollusca – systematics – Gastropoda

coiling:- dextral- sinistral

Gastropod opercula

Mollusca – systematics – Gastropoda – systematics

Traditional classification

Mollusca – systematics – Gastropoda – classification

Prosobranchia (shelled gastropods in which torsion is complete)

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)

classification based on gill and radula types -- unfortunately!

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 OpisthobranchiaOrder Pulmonata

Superorder Caenogastropoda(Prosobranchia in part)Order Architaenoglossa Order Neotaenioglossa Order Neogastropoda

Mollusca – systematics – Gastropoda – systematics

Modern classification

Patellogastropoda

Cellana radians

Neritimorpha

Haliotis (Haliotis) midae

Vetigastropoda

Turbo (Dinassovica) imperialis

Amblychilepas scutella

Oliva (Oliva) sericea textilina

Conus (Asprella) alabaster

Pusionella vulpina Morum (Oniscidia) exquisitum

Murex (Murex) aduncospinosus Malea ringens

Turritella ungulina

Caenogastropoda

Heterobranchia

Pulmonata

Philine angasi

Ophistobranchia

Glaucilla marginata

Mollusca – systematics – Gastropoda – ecology

Ecologymost are aquatic, marine, brackish and fresh waterseveral groups lives on land (most are Pulmonates)marine forms typically live in shallow watershighest diversity in tropical watersbut also known from arctic waters and hydrothermal vents in the deep seaone of the most adaptable forms with respect to:

salinity – preassure (water and air) – temperature (water and air) – humiditymost are herbivores, but also carnivore (Muricidae, Naticidae, Conidae) and omnivoremarine forms typically benthic, but also free swimming and floating formsfreshwater and terrestrial forms at least since Carboniferous

Palaeozoic gastropods

Relatively rareShell usually structurally weak:

With selenizone or anal slitLacking columella (central

strengthening rod connecting whorls)

PlatyceratidsLarge, loosely coiled shellUneven marginsLife attached to crinoids

Palaeozoic gastropods

BellerophontidsPlanispiral coilingSelenizone and deep sinusSelenizone often raised ExtinctName derived from ancient Greek

hero Bellerophon in recognition of the similarity to a greek helmet

Bucanella nana

Sinuites

Bellerophon

Mollusca – systematics – Gastropoda

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 thingsFoot modified to function as a suckerWhy?

ProtectionConserve moisture

Feeding by scraping algae

Secondarily untorted

Obs! Convergent evolution

Predation by Gastropods

Naticid

Muricid

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

Mesozoic marine revolution

Predator-prey arms raceJurassic 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