Molluscs Flashcards
Single, undivided, coiled shell or shell-less. Mantle cavity faces the front. Muscular foot is flattened and used for locomotion. Head is well developed with eyes and tentacles, e.g., snails and slugs. Late Cambrian-Recent
Class Gastropoda
Two-valved, hinged shell encloses the body. No head but foot well developed. Gills are modified for respiration and filter-feeding, e.g., mussels, oysters, and scallops. Lower Cambrian-Recent
Class Bivalvia
Internal or external shell or shell-less. Head and sensory organs well developed. Living representatives are intelligent and actively
carnivorous. Late Cambrian-Recent
Class Cephalopoda
External, buoyant, chambered shell. Chambers are connected and partition walls are flat, e.g., Nautilus. Upper Cambrian-Recent
Subclass Nautiloidea
External, buoyant, chambered shell. Chambers are connected and partition walls are folded, e.g., ammonites. Lower Devonian-Upper
Cretaceous
Subclass Ammonoidea
Shell internal and reduced or absent, e.g., belemnites, squids, and octopuses. Carboniferous-Recent
Subclass Coleoidea
have an elongate, unsegmented body with a distinct head. The internal organs are held between a muscular foot, a modified lower part of the body, and a calcareous shell secreted by an underlying tissue known as the mantle
Molluscs
are the largest and most diverse class of molluscs. They live in marine, freshwater, and terrestrial environments and have exploited the widest variety of habitats and developed a remarkable range of feeding strategies may have a calcareous shell or be entirely soft
bodied.
Gastropods
Gill in front, Cap shaped or conispiral, mostly marine, ex: limpets, winkles, whelks
Main subclass gastropods:
Prosobranchiata
Gill behind, due to distortion; shell lost or very reduced; marine; Ex:Sea-slugs and sea hares
Opisthobranchiata
Mantle cavity modified into lung; where present, conspiral or planispiral; terrestrial; Ex: Land snail and slugs
Pulmonata
Most fossil gastropods are prosobranchs and this subclass is divided into three orders:
- Archaeogastropoda:
Cambrian to
Recent - Mesogastropoda:
Carboniferous to
Recent - Neogastropoda:
Cretaceous to Recent
are laterally compressed molluscs enclosed within a pair of hinged shells or valves. Valves are closed by the adductor muscles. The shell is opened by relaxing these muscles and
water currents are drawn into the cavity
Bivalves
Types of bivalves:
Numerous teeth in radial or subparallel arrangement
Taxodont
Types of bivalves:
Small simple teeth at valve margin
Dysodont
Very large teeth positioned either side of ligament pit
Isodont
Types of bivalves:
Large grooved teeth
Schizodont
Types of bivalves:
Large teeth(termed cardinal teeth) flanked by smaller (lateral) teeth
Heterodont
Types of bivalves:
Teeth reduced to ridges or absent
Desmodont
The major life habits of bivalves are:
(i) burrowing in soft substrates;
(ii) boring and cavity dwelling;
(iii) attached (cemented or by byssus threads);
(iv) unattached recumbant; and
(v) intermittant swimming
Bivalves that burrow in soft substrates tend to be equivalved and have a distinct pallial sinus
Bivalves that bore into hard
substrates typically have
elongate thin shells that are
resistant to abrasion.
is acheved by the foot, which penetrates the sediment and swells. The muscles in the foot then contract, drawing the shell down through the sediment.
Burrowing
Epifaunal bivalves exploit three living
strategies:
1.) Attachment to the substrate by byssus threads
2.) Cementation to hard surfaces
3.) Recumbent, free lying on the sediment
surface stabilized by the shell morphology
Bivalves that are able to swim do so only
intermittently. Shells tend to be thin, to reduce weight. There is only one large adductor muscle scar which provides the strong contractions needed for swimming.
Swimming bivalves
are the most morphologically complex group of molluscs. They occupy the same ecological niche as fish and they are arguably the most sophisticated group of invertebrates. The class includes active, jet-propelled predators with highly developed sensory structures. All cephalopods are marine.
Cephalopods
Cephalopods are divided into three
subclasses:
Nautiloidea, Ammonoidea, and
Coleoidea.
have an external, chambered
shell with simple sutures between its chambers
Nautiloids
also have an external shell, always coiled, with variable and more complicated sutures.
Ammonoids
have an internal and reduced shell. In some coleoids the shell is absent
Coleoids
is the only living cephalopod that retains an
external, coiled shell.
Nautilus
are curved sections of line that “point” towards the body chamber.
Saddles
are the converse - curves that are directed away from the body chamber.
Lobes
0nly ammonoids with an ammonitic suture pattern should be termed
ammonites
adjustable buoyancy mechanism, its shell chambers contain gas and seawater. The siphuncle removes ions from solution in the seawater, drawing water from the chambers into the mantle cavity. Gas bubbles then diffuse into the space making the animal more buoyant and able to float higher in the water column.
Nautiloids
had a chambered, planispirally coiled shell with complex sutures. As with nautiloids, the chambers were connected by a tube called the siphuncle, although in ammonoids the siphuncle usually ran along the outer, ventral margin of the shell rather than through the center of the chambers.
Ammonoids
Ammonites had an external, chambered, usually planispiral shell. The shell can be divided into three parts:
a.) the body chamber, where the animal lived
b.) the phragmocone, the chambered part of the shell
c.) and the protoconch, the first chamber to form
in ammonites can be described as two classes of individuals with juvenile ontogeny identical up to a size (shell diameter) from which two different morphotypes develop: a macroconch (female) and a microconch (male).
Sexual dimorphism
Some ammonite groups developed bizarre or “_____________” shell forms, particularly in the late Cretaceous. These forms were considered nonfunctional, evolutionary dead ends. However, physical modeling has shown that they were stable and well adapted for floating within the water column.
heteromorph
- includes all cephalopods except Nautilus
- make up the majority of the fossil coleoids
- have an internal skeleton with a robust, bullet-shaped, calcite counterweight
- abundant in Jurassic and Cretaceous rocks
- shell: internal and reduced / absent
- modern species: diverse, highly modified, active predators w/ sophisticated sensory systems
Coleoids
- had an internal skeleton unlike any living coleoid
(i) the robust anterior counterweight -
the rostrum / guard;
(ii) the buoyancy mechanism - the
phragmocone, a chambered conical
section with a siphuncle; and
(iii) the pro-ostracum - the support for
an open body chamber - soft tissues are known from a few exceptionally preserved specimens
- tentacles had small hooks and some belemnites had ink sacs similar to those of modern squids
Belemnites
- elongate, smooth shell with a posterior gape
- shell: approximately 8 cm (from umbo to shell edge)
- no dentition ; a deep pallid sinus is present
- modern species: infaunal ; living in soft sediment in burrows 30 cm deep
- both siphons are enclosed within a protective sheath
Mya
- extremely elongated, thin shell w/ posterior and anterior gapes
- lives infaunally in muds and sands in the intertidal zone
- during feeding: anterior part of the shell is close to sediment-water interface During low tide the animals burrows actively down into deeper sediments using the muscular foot.
Ensis
- able to bore into wood
- cylindrical shell ; extremely reduced (approximately 1.2 cm from umbo to shell edge)
- Sharp sculpture on the external surface is used for tunneling into the substrate
- the animal is essentially worm-like and lives encased in the burrow, growing to fill the excavated space
Teredo
- a.k.a. rudist bivalves
- highly modified forms of cementing bivalves
- has two strikingly different valves:
(i) lower valve is conical (approximately 12cm in height) with thick walls
(ii) upper valve is reduced to a small flat lid - coral-like rudists tended to grow in groups, feeding in calm, clear waters above the sea bed
Radiolites
- subclass Prosobranchiata, order Mesogastropoda
- multiwhorled shell with a high pointed spire has a simple, circular aperture and spiral ribbing on the external surface
- no siphonal canal
- height is approximately 5 cm
- usually found buried with the spire facing downwards in soft sediment in marine, shallow water environments; food is collected fiom water drawn into the mantle cavity
Turritella
- subclass Prosobranchiata, order Mesogastropoda
- has an almost planispiral shell (diameter approximately 1 cm)
- most species have smooth shells (morphology varies)
- living in a range of freshwater environments, it feeds on algae and plants
-some species live entirely within water whilst others need to surface for air
Planorbis
- has a moderately high-spired shell with an oval aperture and short siphonal canal ; height is approximately 8 cm
- for protection the head and foot can be withdrawn into the shell and the operculum closes the aperture
- living in seawater up to a depth of 200 m, semi-infaunally with its siphon extended above the sediment drawing clean water into the mantle cavity and is carnivorous
Buccinum
- is a terrestrial gastropod, the bristly snail, belonging to the subclass Opisthobranchiata
- has a molfied, untwisted, mantle cavity that functions as an air-breathing lung
- shell is generally thin, smooth, and conispiral ; height is approx. 5 mm.
- is found in a range of terrestrial habitats and
- is most common in humid environments with calcium-rich soils
Hygromia
- has an uncoiled, conical, cap-like shell & pronounced ribs that radiate from the apex strengthen the shell ; height of the cone is approx. 3 cm
- lives in the intertidal zone and clings to rocks using its foot
- during low tide the animal “clamps down” to avoid desiccation; at high tide the gastropod grazes the rock surface for encrusting algae
Patella
- has a goniatitic suture ; shell is inflated (cadicone) and the external surface is finely ribbed & the shell diameter is approx. 5 cm
- small tubercles line the margin of the moderately deep umbilicus
- is found in marine shales and is a useful zone fossil for the Upper Carboniferous
Gastrioceras
- this ammonite is typically serpenticone and the external surface is ribbed (the shell diameter is approx 6.5 cm)
- the body cavity is tubular and elongated
- numerical modeling has shown that the center of gravity and the center of buoyancy are very close together (see Fig. 9.16)
Dactylioceras
- has a compressed, oxycone shell sculpted with curved, sinuous ribs
- a “pie crust” keel is developed along the center ; the suture pattern is ammonitic & the shell diameter is approx 8 cm
- body chamber: large and positions of the centers of gravity and buoyancy show that the animal floated in a position similar to that adopted by modern Nautilus
Amaltheus
- this ammonite has an evolute shell with a wide umbilicus
- the whorl cross-section is broadly quadrilateral
- the external surface has a distinctive sickle-shaped ribbing pattern
Hildoceras
- this ammonite has a compressed shell with pronounced ribs that bifurcate towards the venter
- the suture pattern is ammonitic ; sexual dimorphism is known
- in the microconch, the smaller dimorph, the aperture is compressed and the lappets are developed
- the shell diameter of the macroconch is approximately 5 cm
Kosmoceras
- this ammonite has an evolute shell with a wide umbilicus
- the whorl cross-section is broadly quadrilateral
- the external surface has a distinctive sickle-shaped ribbing pattern
Scaphites
- has a small, spindle-shaped guard (approx 4 cm in length) with a long ventral groove in the area around the alveolus
- the soft-body morphology of belemnites is known from exceptionally preserved individuals associated with fossil Lagarstätten. Such specimens have long hooked tentacles and ink sacs.
Neohibolites
Located on the hinge plate These structures fit into their inverse on the other valve to insure that the valves fit tightly and securely
Tooth and Sockets
The adductor muscles actively hold the valves together. When the muscles relax the hinge ligament expands the shell opens
Anterior Adductor muscle scar
line that marks the attachment of the mantle to the shell
Pallial Line
A deflection in the pallial line. This feature indicates the presence of a retractable siphon. Depth of burrowing correlates with depth of indentation
Pallial Sinus
Surrounds the mantle cavity.
The outer part of the mantle secretes the shell and also has a sensory factor
Matntle
Modified for filter feeding. Cilia produce currents and trap food particles that are transported to the mouth
Gill
In burrowers the foot swells and the muscles contact, moving the animal through the sediments ``
Foot
Strengthens shell. In burrowing bivalves, growth lines may help move sediments as burrowing proceeds
Rib
Earliest formed part of the shell
Umbo
Each line marks the earlier position of the shell edge
Growth line
Macerates food
Beak
Usually a cartilaginous rod used to strengthen the mantle. Shell loss increased mobility
Internal remnant shell
directs the expelled jet of water
Hyponome
Expands when water floods in
Mantle Cavity
Conical cavity in Guard
Alveolus
bouyant chamber cone with small protoconch. A ventrally positioned siphuncle connects the chambers
Phragmocone
An expanded support for the belemnite soft body parts. This part was very fragile and is rarely preserved `
Pro-ostracum
Acted as counterweight to balance the animal, Enabled animal to swim in horizontal position, offsetting the weight of the soft part concentrated at the anterior. Most frequent preserved skeletal element, made of calcite
Guard or Rostrum
