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Science Olympiad
Phylum Mollusca
Molluscs 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, octopus, and snails are familiar molluscan food items. The variety of molluscan forms evolved from a simple limpet-like ancestor with a flat cap-shaped shell.

Molluscan Characteristics

Molluscs have a soft, unsegmented body with bilateral symmetry. The basic molluscan body plan includes an internal or external shell lined with a sheet of tissue called the mantle. In most molluscs the mantle secretes a limy shell. The mantle surrounds a soft body divided into a head, a ventral muscular foot, 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 layers containing the minerals 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 species vary from the microscopic to the largest invertebrates known. Molluscs are the most common invertebrates in the marine environment. They represent a significant proportion of benthic organisms (seafloor organisms), some are nektonic (free swimming), while others are planktonic (floating passively). Molluscs can be found in freshwater streams, lakes, springs, and marshes. Terrestrial forms are found from the tropics to arctic regions (Ivanov, 2001, p. 95).

The Success of Molluscs

The 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 and underwent 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).

Subphylum Amphineura

Two 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 Polyplacophora-"bearers of many shells") cling to surfaces with a broad muscular foot. The teeth on their 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.

Subphylum Cyrtosoma

Monoplacophorans (Class Monoplacophora-"single shell bearers") were thought to range from Cambrian to Devonian. In 1952 a living species was found in deep 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.

Gastropods (Class Gastropoda-"stomach foot") are the largest, most successful class of molluscs and include snails, slugs, abalones, sea hares, 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 represent a type of shell-less gastropod predator (Sheldon and Nudds, 2004, p. 67).

Cephalopods (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. They have a relatively large 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, and the mantle and siphon make up 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 to support structures 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. The living Nautilus is negatively buoyant and must swim towards the surface to feed at night. Cephalopods first appear in the Cambrian. Straight-shelled nautiloids with shells as long as 10m appear 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.

Coiled 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 the shell. 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 lines become more complex 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 to 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 siphuncle of the nautiloid runs through the center of the septae. Nautiloid suture lines are simple. Nautiloid septae have a concave curvature towards the body chamber. Baculites was a straight-shelled ammonoid that lived during the Cretaceous. Baculites started out in a coiled form, but after the first centimeter or so 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.

Subphylum Diasoma

Rostroconchs are the only extinct class of molluscs (class Rostroconchia). They range from Cambrian to Permian. Superficially, they look like bivalves.

Bivalves (class Bivalvia-"two valves" or Pelecypoda-"hatchet foot") make up the second largest group of molluscs and include such organisms as clams, scallops, 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 symmetrical 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 cephalopod predators. Swimming scallops make their first appearance during the Triassic. Rudistid bivalves became dominant reef 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.

Scaphopods 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, and 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.



Nautilus showing Siphuncle
Nautilus with Siphuncle
Cretaceous; Albian Stage
Mahajanga, Madagascar

Nautilus showing siphuncle
Siphuncle and Sutures
Cretaceous; Albian Stage
Mahajanga, Madagascar
Ammonite Showing Suture Lines
Ammonite showing Sutures
Cretaceous; Albian Stage
Mahajanga, Madagascar
Ammonties Showing Siphuncle
Ventral Siphuncle
Cretaceous; Albian Stage
Mahajanga, Madagascar

Cylindroteuthis oweni
Kelovey Formation, Jurassic
Rizan, Russia

Fox Hills Formation, Cretaceous
South Dakota

9.5 cm long

Exogyra ponderosa
Left Valve
14 cm long


Gryphaea arcuata
"Devil's Toenail"

Left valve (right valve on bottom)
6 cm long

Worthenia tabulata
Wise County, Texas
2.4 cm tall

Santa Monica Mountain, California
3.5 to 4.5 cm

Platyceras sp.
Moscow Formation
Little Beards Creek
Leicester, New York
2.5 cm long

Venus clam
Fox Hills Formation, Cretaceous
South Dakota
4.5 cm

Treptoceras duseri
(Formerly Orthoceras duseri
6 cm

Treptoceras duseri
(Formerly Orthoceras duseri
showing siphuncle


Ivanov, M., Hrdlickova, S. & Gregorova, R. (2001). The Complete Encyclopedia of Fossils: A Comprehensive Guide to Fossils from Around the World. Netherlands: Rebo Publishers.

Pinna, G. (1990). The Illustrated Encyclopedia of Fossils. New York: Facts on File.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Rich P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The Fossil Book: A Record of Prehistoric Life. Mineola, NY: Dover Publications, Inc.

Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.

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