form the second largest phylum of invertebrates. Mollusca
comes from the Latin molluscus or "soft", referring
to their soft body. Clams, oysters, scallops, abalone, squid,
and snails are
familiar molluscan food items. The
of molluscan forms evolved from a simple limpet-like
ancestor with a
have a soft, unsegmented body with bilateral symmetry.
The basic molluscan body plan includes an internal or external
the mantle secretes a limy shell. The mantle surrounds
a soft body divided into a head, a ventral muscular
and a dorsal visceral mass that contains internal organs.
The head contains sensory organs and a mouth equipped with
a ribbon-like string of tiny teeth, known as the radula.
The foot may be modified for crawling (snails and chitons),
burrowing (clams and tusk shells), or as tentacles (cephalopods).
Many molluscan shells are composed of
calcite and aragonite (both calcium carbonate) (Rich,
1996, pp. 252-253). During the fossilization process aragonite
recrystallizes into the more stable form of calcium carbonate,
calcite (Pinna, 1985, p. 113). The basic
molluscan body plan has evolved into a great diversity
of marine, freshwater and even terrestrial species. Molluscan
vary from the microscopic to the largest invertebrates
known. Molluscs are the most common invertebrates in
the marine environment. They represent a significant proportion
benthic organisms (seafloor organisms), some are nektonic (free
swimming), while others are planktonic (floating passively).
Molluscs can be found in freshwater streams, lakes, springs,
Terrestrial forms are found from the tropics to arctic regions
(Ivanov, 2001, p. 95).
The Success of Molluscs
hard calcareous shell that many molluscs secrete increases the
chances for fossilization. Although not as numerous as arthropods,
molluscs have the best fossil record of any living marine
invertebrate phylum. All of the major molluscan classes (except
scaphopods) were established in the Cambrian (Prothero, 1998).
Overall, molluscs played a minor role in the Cambrian. In the Ordovician
nautiloids became major predators. Ammonoids evolved from nautiloids
and were also important predators. Their diversity and abundance
make them important tools for biostratigraphy from the Devonian
through the Permian. Gastropods and bivalves were present during
the Paleozoic, but played second fiddle to brachiopods, bryozoans,
crinoids and corals. The largest mass extinction, at the end of
the Permian, would allow molluscs to dominate the marine
realm. Two genera of Ammonoids survived into the Mesozoic
an adaptive radiation. Ammonites were rapidly evolving and abundant.
Mesozoic time is marked by ammonites. Gastropods and bivalves flourished
in the Mesozoic seas because of their ability to evade predators
by swimming and burrowing; traits lacked by the diminishing brachiopods.
At the end of the Cretaceous ammonites would go extinct, leaving
bivalves and gastropods the most successful molluscs in modern
seas (Protheros, 1998, p. 274).
living classes of shell-less, worm-like molluscs have no
fossil record (classes Caudofoveata and Aplacophora). Chitons
range from the Cambrian to the present. Chitons (Class
of many shells")
cling to surfaces with a broad muscular
foot. The teeth
radula are made of the mineral magnetite. The chiton's
shell is made of 8 valves, which fall apart after the organism
dies. Chitons have a poor fossil record.
(Class Monoplacophora-"single shell bearers")
were thought to range from Cambrian to Devonian. In 1952
a living species
waters near Costa Rica (Proteros, 1998, p. 275).
This primitive limpet-like molluscan shows pseudo-segmentation,
suggesting that molluscs may have evolved from segmented
worms instead of flat worms.
(Class Gastropoda-"stomach foot") are the largest,
most successful class of molluscs and include snails, slugs,
and nudibranchs. Gastropods
glide on their muscular foot. They have eyes on the end
of tentacles; a second pair of tentacles possesses olfactory
organs. Gastropods generally have coiled viscera that are
carried in a single shell, although some species have reduced
or lost the shell.
Gastropods range from the Cambrian to recent times. Terrestrial
gastropods first appear in the Carboniferous. The Miocene-aged
snail Ecphora gardnerae gardnerae is the state
fossil for Maryland. The Illinois state fossil, Tullimonstrum
gregarium has a segmented, sausage-shaped body with
a proboscis ending in a claw and teeth. This organism may
a type of shell-less gastropod predator (Sheldon and Nudds,
2004, p. 67).
(Class Cephalopoda-"head foot") include the cuttlefish,
squid, nautilus, octopus, and the extinct belemnites, and ammonoids.
Cephalopods are strictly marine organisms. Cephalopods
are active predators and the most intelligent invertebrate.
brain and eyes as sophisticated as vertebrates. Cephalopod
eyes have a structure very different from vertebrate eyes and
represent an independent line of evolution. Their foot is modified
into tentacles, the mouth into a powerful beak,
a jet propulsion system. Squid and cuttlefish also possess
fins that can be used for swimming. Some cephalopods have chambered
shells; others have solid shells reduced
or no shell at all. Chambered shells possess a fleshy stalk,
called a siphuncle, which connects the chambers to the body of
the organism. The siphuncle allows the organism to change the
gas or fluid composition of the unoccupied chambers, thus keeping
the buoyancy near neutral. In the living Nautilus the
process of changing the buoyancy is very slow, taking weeks.
Nautilus is negatively buoyant and must swim towards
the surface to feed at night. Cephalopods first appear in the
Cambrian. Straight-shelled nautiloids with
in the Ordovician. Many believe these predators were responsible
for defense adaptations seen in many Ordovician fauna, especially
trilobites (Protheros, 1998, pp. 298-299). Ammonoids first appear
in the Devonian and later become major predators during the Mesozoic.
Ammonoids become extinct at the end of the Cretaceous along with
belemnites. Six subclasses of Cephalopods are
recognized in the fossil record. Two of these subclasses have
living representatives. The Nautilus has six living
species and belongs to the subclass Nautiloidea. The living Nautilus is
often used to help paleontologist better understand fossil forms
bearing similar structures. All other living cephalopods, squid,
cuttlefish, and the octopus belong to the subclass Coleoidea.
Belemnites are an extinct coleoid related to the squid. Belemnites
had an internal chambered shell supported by a cylindrical calcium
carbonate structure called the guard. The guards of belemnites
are bullet-shaped. Belemnites are a common Mesozoic fossil. Belemnitella
americana is the state fossil for Delaware.
fossil nautiloids can look very similar to coiled ammonoids. Ammonoids
(subclass Ammonoidea) were the most successful and diverse
subclass of cephalopods. In general, the suture lines,
position of the siphuncle, and curvature of the septae
may be used to distinguish
between nautiloids and ammonoids. The siphuncle in most
ammonoids is ventral, running along the outer edge of
Septae have a convex curvature towards the body chamber.
Sutures, the line of contact between the inner shell
wall and the septum, are the most important taxonomic feature
on ammonoid shells. Suture lines are visible if the outer
shell is polished or eroded. The first ammonoids in the Devonian
have simple suture lines. Suture
through time and are a primary feature for identification.
It was once believed that the complex suture lines provided
increased shell strength allowing the organism
dive deeper. It is now believed that the complex
suture lines increase the membrane surface area thereby
allowing for quicker adjustments in the composition of
gas or fluid in chambers. This modification in structure
would increase the speed of buoyancy changes (Protheros, 1998,
p. 305). The
the nautiloid runs
the center of the
septae have a concave
chamber. Baculites was a straight-shelled ammonoid
that lived during the Cretaceous. Baculites started
out in a coiled form, but after the first centimeter
of growth produced a straight shell. It is suggested,
based upon buoyancy and counterweight studies that Baculites
oriented itself in the vertical position with its
head pointed downward.
are the only extinct class of molluscs (class Rostroconchia).
They range from Cambrian to Permian. Superficially, they
look like bivalves.
(class Bivalvia-"two valves" or Pelecypoda-"hatchet
make up the second largest group of molluscs and include
such organisms as clams,
and oysters. Bivalves have lost their head. Their body
is enclosed in a hinged calcareous shell. Their foot is
wedge-shaped and modified for burrowing. Their gills are
used for respiration as well as filter feeding. Bivalves
look superficially like brachiopods. Most bivalves are
symmetrical between the shells where as brachiopods are
through the shells. Brachiopods filter feed with a lophopohore,
while bivalves use their gills for this purpose. Many bivalves
burrow into the sediment, brachiopods could not burrow.
Bivalves range from the Cambrian to recent times. The first
freshwater bivalves appear in the Devonian. Bivalves with
siphons also make their first appearance during the Devonian.
During the Mesozoic burrowing bivalves with siphons underwent
a great adaptive radiation, maybe because of pressure from
predators. Swimming scallops make their first appearance
during the Triassic.
building organisms in the tropics during the Cretaceous
displacing coral. Rudistids grew in massive, thick accumulations.
These bivalves went extinct at the end of the Cretaceous.
Bivalve forms are restricted by the environment in which
they live, thus giving paleontologist a great tool for
studies in paleoecology. Pterotrigonia thoracica is
a Cretaceous-aged bivalve that was chosen as the state
fossil for Tennessee. The Pliocene-aged scallop Chesapecten
jeffersonius is the state fossil for Virginia.
or tusk shells (class Scaphopoda-"shovel footed") live
in long, tapering, tubular shells. The larger end contains
the animal's head and foot, which is oriented downward
into the sediment. They use their foot to dig in the sand
and catch prey with their tentacles. Scaphopods are the
most recent molluscan class to evolve and range from the
Ordovician to recent times.
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List inludes bivalves,
gastropods, and cephalopods. The bivalves are represented
by the following genera: Pecten, Gryphaea, Exogyra,
Pholadomya. The gastropods are represented by
the following genera: Conus, Turritella, Worthenia, Platyceras,
and Cypraea. Cephalopods are represented by the
following: subclass Ammonoidea (Ammonites) genus Baculites and Dactylioceras, subclass Coleoidea Order Belemnitida (Belemnites), subclass Nautiloidea genus Orthoceras and Nautilus.