The Molluscs Flashcards
In terms of number of species in a living phylum where do the molluscs rank?
The 2nd largest living phylum.
What is the disparity of the mollusk phylum like?
(disparity=highly diverse array of body forms) It is very high
How many classes of molluscs are there?
At least 8 living and extinct
How well are molluscs represented in the fossil record?
They are represented extremely well.
The molluscan bauplan
Soft body, hard shell, slippery skin
Radula
The feeding organ of a mollusc. It has rows of sharp teeth that it uses like a chainsaw.
Bivalve morphology relative to “HAM”
highly modified.
No head or radular, large mantle cavity, large paired gills, large muscular food, body surrounded by 2 separate valves.
How were incurrent and excurrent siphons formed on bivalves?
The fusion of the mantle margins
Body plan of bivalves
Body surrounded by 2 separate valves joined by a dorsal toothed hinge.
What muscles do bivalves use to close?
adductor
If an organism is going to be moving through substrate would it be better for their shell to be smooth or rough.
relatively smooth.
Boring bivalves
Bivalves that occupy a cavity that they create
nestling bivalves
Occupy a pre-existing cavity
reclining bivalves
Partially buried in the soft substrate
bysally attached bivalves
Foot secretes stout threads (byssus) that attach them to the substrate. (ex-muscles)
cemeted bivalves
Secretes shell material that cements them to hard surfaces (ex-oysters)
bivalve beak
The spot where the 2 shells meet, marks where growth starts
bivalve umbo
The part where the shell bends over
pallial sinus
a mark left in a bivalve shell that tells you about the size of the siphons
cephalopods
includes octopuses, squid, cuttlefish, nautilus and ammonoids.
Includes the largest mollusks
All are carnivores
All modern forms are pelagic-rapid swimmers
Nearly all fossil forms have an external shell
Of all the living forms only the nautilus still has an outer shell.
General morphology of a cephalopod
+Close association of head and food
+foot has developed prehensile tentacles around the mouth
+funnel for “jet propulsion
+paired gills
+radula
+calcified beak in some forms
+well developed sense organs and nervous system
hyponome
a funnel used in cephalopods for “jet propulsion”
3 main groups of cepholopods
Cotecoidea, nautiloidea, ammonoidea
cotecoidea
squids, octopus, cuttlefish (late Triassic to recent)
Morphology of the nautilus
large chaimbered shell, planispiral and coiled above the body, septa, suture, animal lives in outermost chaimber, behind body is fluid filled space, large jaw
suture
junction between the septa and external wall shell
Goniatitic
Ammonoid suture. Saddles and lobes are both smooth
Ceriatitic
Ammonoid suture. Sadlles smooth, lobes wrinkled
Ammonitic
Ammonoid suture. Saddles and lobes both wrinkled.
siphuncle
Pumps out the water in nautiloids and ammonoids. Down the middle in the nautoloids, down the edge in ammonoids
Ammonoids
Mostly coiled shells, oddly curled heteromorphs, complex septa. Extinct
Monoplacophora
Well known fossil form ranging from early Cambrian to the Devonian. Cap like shells with rows of paired muscle scars, metameric, living representatives discovered in deep sea in 1952.
bellerophontids
paleozoic molluscan fossils characterized by a distinctive planispiral, symmetrically coiled shell.
It is controversial if they are torted or not, if they are they are gastropods, if they are not they are monoplacophora.
Defences in gastropod shells
More ornamentation, tighter coiling, thick lipped apetures could all strengthen the shell.
Why do we think there was such an increase in antipredatory features of gastropod shells during the mesozoic?
There was also an increase in shell breaking predators (ex-crabs) at the time which lead to an evolution arms race.
Does CaCO3 dissolve more readily in cold or warm water?
In cold water
Prosobranchs
One of the divisions of snails. Mostly marine, full torsion effect, well developed shell.
Opisthobranchs
A division of snails. Nearly all marine, gills highly modified or lost (they respire through their body surface), shells reduced or lost, flat and frilly shape to help with respiration, torsion reduced.
Pulmonates
Terrestrial land snails, no gills, mante cavity functions as a lung.
The first Gastropods
The cambrian, probably from a coiled ancestor, have more or less increased steadily since the cambrian.
Possible explinations for torision
Maybe it protects the head,
Maybe it made it easier to use sense organs and gills,
Maybe because the shell is easier to carry on your back,
There is no consensus
Consequences of torsion
Anus is above head.
Apeture
The place where the shell opens up
Suture
The spiral along the gastropod shell
Crepidula fornicata
The slipper limpit. A serial hermaphrodite, individuals pile on eachother- males will eventually turn into females as the pile grows.
Gastropods as predators
Some may have been parasitic, other may have been brachiopod predators.
The Paradigm Approach
“The structure that would be capable of fulfilling the function with the maximum efficiency attainable under the limitations imposed by the nature of the materials”
How the paradigm approach works
A function is suggested for a morphologic feature, based on knowledge of nature, materials, and engineering a theoretical model is made. This model is than compared with what you actually observe. This degree of correspondence acts as a test of the paradigm as a functional hypothesis.
Problems with the Paradigm model
- Assumes that natural selection produces an optional structure for a particula function.
- There may be multiple optimal structures
- Engineers can be wrong
- Functioning requirements may produce sub-optimal structures (ex-horseshoe crabs have to burrow, walk, and swim)
- The comparison is visual and qualitative and therefore subjective
biomechanics
Examins the interrelationships between biological structures and physical processes
Paleobiomechanics
The consequence of physical processes for organisms existed in the past the same way that they due today. (ex- the physics of fluid flow are the same today as they were in the cambrian.)
Holdfast
Holds crinoids and blastoids to the substrate
Constructional morphology
Observed morphology reflects three interacting factors:
- Phylogeny (history)
- Adaptation (function)
- Growth (morphogenic)
Earliest bivalves (age)
early cambrian, but they are rare
Bivalve siphon origin (geologic time scale)
Probably in Devonian
Rostroconchia
extinct, look like bivalves, but lack a hinge, their valves are fixed in place. They are interpreted to be semi-infaunal and sedimentary. They eventually gave rise to either bivalves or scaphopods.
The meaning of Gastropoda
“stomach foot”
About what % of named molluscs are snails?
About 80%
Gastropods
Found in nearly all marine, freshwater, and terrestrial environments, about 80% of named molluscs are snails, Nearly all are epifaunal, though some are pelagic. Nearly all are mobile (crawl on the bottom by using muscular contractions of the foot, though some are sessile)
Can be grazers, deposit feeders, carnivores, scavengers, parasities or suspension feeders
What makes a snail?
- large broad foot
- well developed head with eyes and tentacles
- well developed radula
- 90% of the time they have a shell (usually helically coiled and usually external)
- body can be withdrawn into shell, and the opening is closed by operculum
- Torsion
operculum
the “lid” on a snail shell
Evolutionary history of ammonoids
- first appear in the Devonian
- origin on goniatitic sutures in late Devonian
- Major expansion in late Permian
- All go extinct at the end of the cretaceous
- *Tremendous amount of turnover (repeated history of diversification and extinctions)
Coleoids
- Octopus, squids, cuddlefish
- No shell or reduced internal shell
- Relatively rare as fossils except for belemnites
What is usually found in the mantle cavity of molluscs
paired gills
