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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