Molluscs part 2 - cephalopods and gastropods Flashcards
What are the most common single shelled molluscs?
Cephalopods and gastropods
What is the shell growth of molluscs like?
Incrementally grown
What 4 parameters are used to define the shell shape of molluscs?
Shape of generating curve or axial ratio of the shell’s cross-sectional ellipse
Rate of whorl expansion per revolution (W)
Position of generating curve with respect to axis (X)
Whorl translation rate (T)
How can different shapes of shell be produced in bivalves?
whorl translation (T) =0 but whorl
expansion (W) is very high
How can different shapes of shell be produced in gastropods?
gastropods controlled by translation rate (T)
What is the underlying reason for why shells of molluscs do not occupy all morphospace?
Iterative evolution
What is iterative evolution?
The resurgence/ reoccurrence of morphologies throughout evolution
What is an example of iterative evolution of molluscs?
Ammonite in the Jurassic and the Nautiloid in the Silurian
How is the Cephalopoda class divided?
Into 3 subclasses:
1. Coleoidea
2. Nautiloidea
3. Ammonoidea
What is the age range of coleoidea?
Carboniferous to recent
What are some examples of coleoidea?
Octopuses
Squid
Cuttlefish
Paper nautilus
What is the fossil record of coleoidea like?
Typically poor due to being soft bodied except paper-nautilus
What is the paper nautilus?
Type of coleoidea which has a very thin mineralised exoskeleton for reproductive purposes
What will the mineralisation of coleoidea typically be?
Internal (besides Argonauta)
What is the scientific name of the paper-nautilus
Argonauta
What are the belemnites?
Geologically important group of molluscs with a bullet shape
What are the three components of belemnites?
Bullet shaped guard
Alveolus
Pro-ostracum
What is the bullet guard?
massive cylinder of calcite
formed through concentric growth
Why are belemnites so important?
Used as a standard for Jurassic ocean composition
What is the Alveolus of belemnites?
A hollow in the anterior end
What is the alveolus filled by?
anteriorly expanding phragmocone
formed from aragonite
What will aragonites and calcite look in fossil sample?
Aragonite is shiny
Calcite is dull
What does the pro-ostracum look like?
Flattened spatula (used as protection)
In what direction does the pro-ostracum project?
Anteriorly
What might some soft tissue fossils of belemnites be like?
Show tentacles and hooks
Ink sac at shell anterior
What is the shell (phramocene) like in Nautoloids?
external
How is a nautilus shell internally divided up?
Into chambers (camerae) separated by simple sutures
How are the chambers in a nautilus shell connected?
By a siphuncle
What is a siphuncle and what is its purpose?
Porous tube needed equalize pressure or
transfer gas between chambers
How will the siphuncle of nautiloids be positioned?
Central/ subcentral
What directions does the septal neck point in nautiloids?
Backwards
Where is the body of the nautilus housed?
In the body chamber
How can the age of nautilus be predicted?
The body chamber moves forward every 2 weeks
What are the internal parts of nautilus’s?
Head foot
Lateral eyes
Hyponome
Four gills
Tentacles
Parrot like jaw
What is the purpose of the hyponome?
A way to move rapidly/ escape which is done by the mantle cavity rapidly compressing sending out a jet of water for propulsion
What is the name of straight shelled forms of nautilus?
orthocones
What is the name of partially coiled shelled forms of nautilus?
cyrtocones
What forms can nautilus be found?
Cyrtocones
Planispirals
Orthocones
When were Nautiloids the dominant cephalopod?
Cambrian to Devonian
Where did orthocones and cyrtocones live and hunt?
Near the sea floor
What problem is faced by orthocones and cyrtocones?
As shell grows buoyancy is a problem due to a shift in gravity and centre of buoyancy
How was the buoyancy problem of cyrtocones and orthocones prevented?
thickening of cameral and siphuncular
What is the age range of ammonoids?
Lower Dev. - Upper Cret
What will ammonites usually be like?
Shell normally planispiral
Siphuncle is near the venter
Septal necks forwardly directed
Shells ribbed often with tubercles and spines (hydrodynamics)
What is the main basis for ammonite classification?
Sutures
What are goniatites?
Most ancient ammonite which can be identified by simple folds of suture
How are ceratites identified?
increase in number of saddles and lobes and lobes become frilled
How can ammonites be identified?
Due to very high level of suture complexity
How can the sexual dimorphism of ammonites be described?
strongly sexually dimorphic
How are adult ammonites divided sexually?
Microconch
Macroconch
What is associated with marcoconches?
Large numbers of embryonic ammonites
What are heteromorphic ammonites
Partially uncolied
Why is it thought heteromorphic ammonites evolved?
As there was a transition form living in the water column to living near the sea floor
What is the general info about habitat and feeding of gastropods?
Wide variety of ecological niches
Marine, freshwater and terrestrial
Creeping, floating and swimming
Grazing, suspension feeding, predatory and parasitic
What process have gastropods have experienced which makes them have their mouth and anus be so close?
Torsion
What is the problem with torsion?
Anus above mouth and dont want to eat poo
What does the mantle cavity being above the head allow for in gastropods?
mantle cavity and gills rotated to above the head allows head to be retracted into shell before the foot - which carries
the operculum
What is the operculum?
Protective cover like part
What characteristics will all gastropods share?
All have true head, usually with tentacles and eyes more or less continuous with the body, visceral elements lie within the shell
How did gastropods adapt to combat the effect of their anus being above their mouth?
Development of siphons
Slit band (scar allows siphon out of side)
What are gastropods good at surviving in?
High stress environments
What is the effect of the fossil record due to gastropods being able to survive high stress environments?
Leads to high abundance of one gastropods and thus low diversity
