GEOL 331 Principles of Paleontology
Fall Semester 2008
Mollusca
Mollusca
- Second only to Arthropoda in diversity
- Protostomians
- Trochozoans (have a trochophore larva, also in Annelida, Pogonophora, Echiura,
Siphuncula, and Nemertini)
- United by following synapomorphies:
- A calcareous shell with three layers: thin outer organic periostracum; thick calcareous
prsimatic layer; thin innermost nacreous layer (aka "mother of pearl") made of thin
sheets of aragonite
- Secreted by a mantle
- Anus directed into a mantle cavity which houses the ctenidia (or "gills") and
osphradium (a water chemical sensory organ)
- A mouth with a radula
- Nephridia (kidney-analogs)
- Ventral foot
- Coelomic cavity small, and restricted mostly to area surrounding heart and gonads
- Blood sinuses form a hemocoel, which serves as the hydrostatic skeleton in some mollusks
- Veliger larva stage (after trochophore) in
Gastropoda, Bivalvia, and Scaphopoda.
Possible shell-less mollusks: Kimberlla
of the Vendian and trace-fossil Climactichnites
of the Cambrian
Possible Mollusca Phylogeny (combines some molecular and some morphological studies):
Mollusca
- Aplacophora (possibly paraphyletic)
- Testaria: shelled, although a fossil aplacophoran is also shelled
- Polyplacophora
- Conchifera: single shell, ancestrally
- Monoplacophora (possibly paraphyletic)
- Unnamed A
- Cyrtosoma: single shell, head is better developed and distinct
- Diasoma: shells are dorsoventrally elongated; infaunal filterfeeders
For a long time a hypothetical archimollusc (or HAM,
basically a univalve unsegmented monoplacophoran with a single pair of gills, or an
untorted gastropod) was suggested as the ancestral morphotype. However, because
the outgroups are semi-segmented animals, it is more likely that aplacophorans and polyplacophorans
represent the ancestral condition. HAM actually represents a good model for the most recent common
ancestor of "Unnamed A" above.
Aplacophora
- As name implies, are mostly unshelled
- Consequently, have almost no fossil record
- However, recent discovery of a shelled aplacophoran
from the Silurian of England: reconstruction here and
here.
Polyplacophora (chitons) (also called Amphineura)
- 8 valves
- Mouth anterior, anus and mantle cavity posterior
- Paired muscles, (bipectinate) gills, nephridia, hearts
- Fossil record from Upper Cambrian to today
- Have the likely-ancestral life habit of algae scraping on hard substrate
- A few fossil forms with more elongate and narrow valves: possibly crevice dwellers
Monoplacophora (long a gastropod subclass Tryblidiacea)
- A single conical valve
- Mouth anterior, anus and mantle cavity posterior
- Paired muscles, gills, nephridia, hearts
- Fossil record in the lower Paleozoic, but found in 1950s living off Costa Rica
- As with polyplacophorans, have the ancestral molluscan life habit
HAM is arguably the common ancestor of Diasoma + Cyrtosoma
Gastropoda (snails)
- One of the most diverse clades of mollusks, with 80% of molluscan species
- Univalve, conical, often coiled shells
- Cephalization: head contains sense organs such as eyes and tentacles
- Torsion: during development, body rotates counter-clockwise
- Veliger (“winged”) larvae begin untorted
- As it grows, body (with digestive, nervous, & other systems) swings around
- Gastropod systematics plagued by paraphyly (probable paraphyletic groups indicated by "*")
- Prosobranchiata* (most marine gastropods)
- Bellerophontacea (Cambrian to Triassic)
- Paired muscle scars, so may not be true gastropods
- Planispiral shells
- Flared apertures, suggesting that mantle (& foot) enwrapped ventral part of shell
Archaeogastropoda*
- Shells are normally planispiral or helical
- One or (normally) two aspidobranch (bipectinate) gills, two nephridia
- Herbivorous
- Aperture not notched: no siphon
- Slit-shelled forms among early archaeogastropods (allows excretion away from inhalent
current, but does weaken shell)
- Cambrian to Holocene
- Includes Haliotis (abalone)
- Mesogastropoda*
- Helical shells
- One pectinibranch gill (the left), one heart, one nephridium
- Ordovician to Holocene
- Two sexes
- Neogastropoda
- Variety of shells
- Siphonate, one pectinibranch gill (the left), one heart, one nephridium
- Includes most of marine snail diversity
- Includes most of the carnivores, including borers
- Cretaceous to Holocene
- Sometimes Mesogastropoda + Neogastropoda called Caenogastropoda
- Opisthobranchiata
- Most likely derived from mesogastropods
- Single gill, auricle, nephridium, but often detorted
- Shells reduce or lost: consequently, little fossil record
- Two main groups:
- Pteropoda: planktonic forms
- Nudibranchia: sea hares, nektonic forms
- Mississippian to Holocene
- Pulmonata
- Terrestrial or freshwater
- No gills: oxygen absorbed by vascularized mantle cavity
- Most have shell, but some (slugs) have lost it
- Pennsylvanian to Holocene
Scaphopoda (tusk shells)
- A single tall conical valve, open at both ends
- Gills very reduced: respiration over mantle surface
- Mostly sediment-sitting detritivores and foram-eaters
- Ordovician to Holocene record, but very simple shell design shows little variation throughout Phanerozoic
Cephalopoda
- Univalve conical shell
- Exclusively marine
- Predatory (possible planktonivores in ammonoids)
- Most sophisticated brains among invertebrates
- Camerate (chambered) shell: may be coiled or straight
- Beak
- Phylogeny is better understood than gastropods and bivalves:
- Basalmost form: Plectronoceras of Upper Cam
- Starting in Ordovician: succession of “nautiloid” forms
- Intially orthocones (straight) or cyrtocones (cuved)
- Various forms among “nautiloid" grade:
- Endoceratida, Ordovician-Silurian, with endocones (cone-in-cones)
- At least some forms with ectocochlear (external-to-shell) tissues
- Many with cameral deposits, to help stabilize shell?
- Others developed various other strategies for stabilizing shell: coiling, losing distal sections, etc.
- Nautilida (coiled, only living clade, 4 gills)
- Ammonoidea
- Devonian-Cretaceous: VERY important index fossils in Mesozoic
- Complex sutures
- Marginal (ventral) siphuncle
- Most planispiral, but some orthocones and some heteromorphs
- Did have radulae, but beak has become aptychi and anaptychi
- Probably planktonivores
- Coleoidea
- Poor fossil record, except for lagerstätten and Belemnoidea:
- Internalized shell, surrounded by a guard
- Hooks on arms
- Two main clades:
- Aulacocerida (possibly Devonian; definitely Mississippian through Jurassic)
- Belemnitda (Jurassic and Cretaceous: earlier reports of Tertiary belemonoids seem to be in error)
Rostroconchia (rostroconchs)
- Cambrian-Permian, but peak is Ordovician
- Univalved, resemble a non-hinged bivalve
- Begin as limpet-like protoconchs, grow a pair of valves down (forms the adult dissoconch)
- May have had a protrusible foot
Bivalvia aka Lamellibranchia aka Pelecyopoda (bivalves)
- Cambrian-Holocene, but much less abundant in Paleozoic than post-Paleozoic
- Bivalved, with right & left valve
- Ligament to open, muscles to close
- Dentition of hinge used in classification schemes
- Foot (large, protrusible muscle)
- Enlarged gills
- Used for feeding as well as breathing: lophophore-analogs
- Gill morphology, broken down into:
- Protobranch: small, leaf-like (primitive condition, found in polyplachophorans & cephalopods)
- Filibranch: lamellar sheets of individual filaments in a W-shape
- Eulamellibranch: as above, but with cross-partitions
- Septibranch: (only in rock-borerers) small gills transverse to inner chamber
- Phylogeny is still poorly understood
- Life habits:
- Paleozoic forms:
- Shallow burrowers: probably the basal condition
- Labial palp feeders: extrude tentacles to collect food particles
- Epifaunal byssate forms
- Some rare deep burrowers
- During Mesozoic, added:
- Lots of deep burrowers
- Intertidal forms
- Large epifaunal forms:
- Inoceramids
- Rudists
- Ostreoida: oysters, including coiled soft sediment dwellers
- Rock- and wood-boring bivalves
- Swimmers (scallops)
To Syllabus.
Last modified: 22 August 2008
