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Mollusca

Mollusca Mollusca

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mollusks Mollusca blötdjur molluscs

mollusks

Mollusca

blötdjur

molluscs

Mollusca

One of the largest and most diverse animal groups today

~200 000 extant species (130 000 described)

Strong shell gives exceptionally good fossil record

~70 000 described fossil species

Well represented from the earliest Cambrian onwards

general things

Mollusca

Mollusca

What is a mollusc?

Fundamental organization :

- 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

- muscular foot facilitates movement

- shell is of calcium carbonate (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 and terrestrial groups)

Mollusca

Typical features: Shell

Strongly mineralised = hard

1-2 components (up to 8)

Simple, compact shapes

Usually coiled to some degree

= Facilitates excellent fossil record

No other animal group can compete!

Typical features: Shell

complex structure (often important for classification)

calcitic or aragonitic, or both; with organic matrix

often 3 shell units

- periostracum

- ostracum

- hypostracum

6 primary structure types

- simple prismatic (a)

- compound prismatic (b)

- sheet nacreous (c)

- foliated (d)

- crossed-lamellar (e)

- homogeneous (f)

periostracum (organic)

ostracum (calcitic, aragonitic)

hypostracum (nacreous)

Typical features: Radula

Most molluscs have a radula (rasping

tongue)

The radula is ribbon-like and bears

many small teeth arranged in rows

Found in at least some representatives

of all molluscan classes except

Bivalvia

Allowed early molluscs to graze on

bacterial films on soft sediment?

Now specialised to allow herbivory,

detrivory and carniverous diets

Mollusca

Typical features: Foot

Muscular structure below visceral

mass and mantle cavity

Creeping sole in Tryblidiida,

Polyplacophora, Gastropoa

-Presumably primitive feature

-Movement by ciliary movement or

rythmic muscular contraction

Modifications for burrowing in Bivalvia

-Movement by hydrostatic pressure

and muscular contraction

Modified into tentacles and hyponome

in cephalopods

Reduced or abscent in some groups

Tryblidiida

(”monoplacophorans”)

Scaphopoda

(tusk shells)

Polyplacophora

(chitons)

Bivalvia

Phylum Mollusca

- 8 extant Classes

Gastropoda

Cephalopoda

Solenogastres

(aplacophora)

Caudofoveata

(aplacophora)

Gastropoda

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

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

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

shell generally anisometrically coiled in some manner and external

radula typically present

torsion of the visceral mass is the single unique defining characteristic (synapomorphy)

known since Late Cambrian

Mollusca – systematics – Gastropoda

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)

Mollusca – systematics – Polyplacophora

Tryblidiida (”monoplacophorans” sensu stricto)

simple cap-shaped shells

never a diverse or abundant group

well-represented in the fossil record from the late Cambrian to the Devonian

first appeared in the Middle Cambrian

until the discovery of Neopilina in 1952, considered to be extinct

coiling exogastric

distinguished by the fact that virtually all organ systems are found in multiples

(gills, nephridia, muscles) pseudo-segmentation or serialization

common ancestry with annelids and arthropods have been suggested

serialization ≠ segmentation

Mollusca

Bivalvia (=Pelecypoda, Lamellibranchia)

pair of valves (right and left valve)

bilobed mantle

valves articulate along a dorsal hinge line

typically bilaterally symmetric (plane of symmetry passing between the valves,

= commissural plane)

no head

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

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, most living forms)

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

Mollusca – systematics – Cephalopoda

Scaphopoda (tusk shells)

small group of mostly small, infaunal or semi-infaunal, marine mollusks

shell typically a few centimeters in length

(largest form found 30 cm in length)

shell is a slender tapering and usually gently curved tube

shell is open at both ends, the large end of the shell being anterior

exterior has finely spaced growth lines and usually some

type of longitudinal ornamentation or ribs.

protrusible burrowing foot extends from the anterior end

no gills, respiration by direct exchange with mantle tissue

head is poorly-developed

numerous tentacles (called captacula) and a radula

animal feeds on microscopic organisms and organic detritus.

Scaphopoda first appear in the Ordovician or the Devonian or even the early Carboniferous

rare in fossil record

Scaphopods probably evolved from a tiny, semi-infaunal helcionellid ancestor

(via conocardioid rostroconches)

Mollusca

Pinnocaris (Ordovician)

not a scaphopod!

Aplacophora (shell less, vermiform molluscs)

No shell = not well known from fossils

Two groups, possibly not closely related

Solenogastres

Caudofoveata

Helcionellida

(Cambrian-Ordovician)

Rostroconchia

(Cambrian-Permian)

Mollusca

Phylum Mollusca

At least 4 extinct Classes

Stenothecoida

(Cambrian)

Hyolitha

Cambrian-Permian)

Helcionellida

laterally compressed coiled shell

typically 1mm to 2 cm long

typically bilateral symmetrical

coiling endogastric

some develop a snorkle

resemble gastropods, but are considered to be untorted

first molluscs to develop a mineralized shell

earliest Cambrian to Early Ordovician

Mollusca

Rostrochonchia

single, pseudobivalved shell which enclosed the mantle and muscular foot

anterior, probably downwards-orientated, part of shell has a gape

from which the foot could probably emerge

probably sedentary semi-infaunal lifestyle

no functional hinge, shell layers are continuous across the dorsal margin

and some or all of the shell layers are continuous across the ventral margin (pegma)

posterior of the shell is usually elongated into a flattened tube called the rostrum

endo- or exogastric, or both

Early Cambrian to Late Permian

Mollusca

gape

anterior

rostrum

posterior

Stenothecoida

Bivalved mollusks with two non-equal,

assymmetrical shells

No articulation

Only found in the Cambrian

Reef environments

Epibenthic filter feeders

Probably not closely related to proper Bivalves

Hyolitha

Probable molluscs

Early Cambrian to Permian

Most common in the Cambrian

One deep cone-shaped shell (conch)

One flat lid (operculum)

Sometimes two curved rods (helens)

Circular, triangular or rectangular

in cross-section

Two Orders:

Hyolithida

Triangular cross-section

Have helens

Lived on the sediment surface

Probably filter feeders

Orthothecida

Circular or rectangular cross-section

No helens

Burried in the sediment

Found with sediment filled gut

Possibly related to siphunculans, not molluscs

origins

Mollusca

Mollusca

Mollusca

Protostomes

Part of the Lophotrochozoan

branch among protostomes

(sometimes called Spiralia)

What is a mollusc?

Fundamental organization :

- shell secreted by a layer of tissue called the mantle

- mouth and anus at opposite end

- mantle cavity bears gills

- above mantle cavity is the visceral mass

with gut, nervous, circulatory and muscular system

- muscular foot facilitates movement

- shell is of calcium carbonate (calcite or aragonite)

- Shell is external

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

- marine

Mollusca

Hypothetical ancestral

mollusc

What’s the problem?

What’s the problem?

D C B A

What’s the problem?

D C B A

The HAM:

-A conceptual animal

-Has ALL the important

characters of modern

molluscs

-All molluscs derived from

HAM independently

Last common ancestor:

-A real, living animal

-Would NOT have all

charatcers of modern

molluscs

-Characters and groups

were acquired by steps

Mollusca

Precambrian

Mollusca

Mollusca

Ediacaran Fauna

First macroscopic ’animal’ fossils

Most are difficult to place in modern

groups

Sponge- or Cnidarian-grade animals?

Something completely different?

Ediacaran Substrate

Very stable soft sediment

No, or almost no burrowing activity

Bacterial films covering the surface

Led to enhanced preservation

And a fantastic food-sourse for the

first vagrant animals

Kimberella

Impressions in fine sandstone

Possible foot, mantle and ’soft’ shell

White Sea region of Russia

Associated with traces of mobility and/or

feeding with a scraping radula

One of the oldest convincing Metazoans

But is it a mollusc?

Typical features: Radula

Most molluscs have a radula (rasping

tongue)

The radula is ribbon-like and bears many

small teeth arranged in rows

Found in at least some representatives of

all molluscan classes except Bivalvia

Allowed early molluscs to graze on bacterial

films on soft sediment?

Now specialised to allow herbivory,

detrivory and carniverous diets

Mollusca

Cambrian

Mollusca

Mollusca

Helcionellida

laterally compressed coiled shell

typically 1mm to 2 cm long

typically bilateral symmetrical

coiling endogastric

some develop a snorkle

resemble gastropods, but are considered to be untorted

first molluscs to develop a mineralized shell

earliest Cambrian to Early Ordovician

Mollusca

Mollusca

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