Mollusks are a group of animals without back bones (invertebrates) that have an unsegmented body, often covered by a shell (exceptions include octopuses and squid). Mollusk shells are usually coiled, either strongly (such as in snails and some cephalopods) or weakly (as in bivalves). Though living mollusks have extremely diverse shell shapes, some bivalves, snails, and cephalopods—and the ancient group monoplacophorans—differ little from their Paleozoic relatives.
The bivalve mollusk (“clam”) genus Neotrigonia was discovered in 1802. Prior to this, naturalists had only known its family, the Trigoniidae, from Devonian through Cretaceous fossils. Many fossil trigoniid species (belonging to 20 genera) have been described from tropical environments around the world. Today, only six living species of Neotrigonia are known, all from ocean waters near Australia.
Left: fossil Trigonia thoracica from the Cretaceous of Tennessee. Right: Modern Neotrigonia margaritacea from Tasmania.
Neotrigonia is distinctive for having a very large muscular foot which it can stick far outside its shell, and which it uses to move across the seafloor by “jumping.” Like most clams, Neotrigonia is a filter-feeder, straining tiny plankton out of the water.
Why do only a few species of this formerly diverse and widespread group survive in only one corner of the globe? Neotrigonia does not seem better adapted than the common trigoniids of the Mesozoic. These clams were probably struck especially hard by the mass extinction that ended the Cretaceous (when non-avian dinosaurs became extinct). This extinction event was especially severe for marine species in the tropics. The few trigoniid species that survived the Cretaceous were located in an isolated geographic region far to the south of the family’s Mesozoic center of diversity. Following the extinction, there has been little opportunity for the Trigoniidae to reoccupy other areas.
Living slit shells (pleurotomarians) were first discovered alive in 1856. Before this, the group was known only from fossils of Cambrian to Cretaceous age. Living slit shells—of which 15 species are known—are rare and live only in deep water.
Modern shell of the slit snail Bayerotrochus teramachii from Taiwan (PRI 70103). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Maximum diameter of specimen is approximately 10.5 cm.
Fossil specimen of the slit shell snail Pleurotomaria sp. from the Jurassic Inferior Oolite of England (PRI 78776). Preservation is an interior mold. Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Maximum diameter of specimen is approximately 8.5 cm.
The slit is a characteristic that was once much more common in marine snails. It serves as an opening for a plumbing device, allowing water that enters the aperture of the shell to flow out over paired bi-fold gills. Most marine snails have only a single simple gill, which is less easily clogged by sediment, and a more elongated gill chamber which allows for better water flow.
Most living cephalopods—including octopuses, squids, and cuttlefish—lack external shells. The modern chambered nautilus is the last surviving representative of a group of ancient cephalopods (frequently called nautiloids) that had external shells. The seven species of modern nautilus live only in the southwestern Pacific Ocean, whereas their similarly shelled ancestors and relatives were once far more diverse, abundant, and widespread in the world’s oceans.
Living chambered nautilus on display at the Melbourne Aquarium. Photograph by Jonathan R. Hendricks.
Modern specimen of the chambered nautilus cephalopod Nautilus pompilius from the Philippines (PRI 70573). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Maximum dimension of specimen is approximately 17.5 cm.
Fossil shell of the extinct nautiloid Tainoceras monilifer from the Pennsylvanian Graford Formation of Palo Pinto County, Texas. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York (PRI 42134). Image by Jonathan R. Hendricks.
The modern nautilus uses the chambers inside its shell to regulate its buoyancy by pumping gas and fluid in and out of its shell. Like all cephalopods, a nautilus moves by jet propulsion, taking water into its muscular body and then forcing it out to thrust itself backwards.
Internal parts of a chambered nautilus shell. Learn more on the Digital Encyclopedia of Ancient Life.
Nautiluses spend daylight hours in deep water, but migrate into shallow water at night to feed. They are hunters and pick up food scents in the water column with chemical receptors on their tentacles. They consume animals such as fish, crabs, and lobsters with their sharp, beak-like mouth.
Unlike other cephalopods, nautiluses are relatively long-lived, reaching ages of 15 to 20 years or more. They grow slowly, maturing around 10 to 15 years of age, and produce a small number of relatively large eggs that require at least a year-long incubation period.
The first description of nautilus soft parts was published in 1705 by Dutch naturalist Georg Eberhard Rumphius, which confirmed that it was a cephalopod. The first detailed description of a living chambered nautilus was published in 1832 by English naturalist Richard Owen, based on a specimen collected in 1829 in the New Hebrides in the southwest Pacific (now the nation of Vanuatu). Knowledge of living nautilus allowed previously puzzling fossils, such as ammonoids (which are more closely related to squids and octopuses than to nautilus), to be interpreted as extinct cephalopods.
The first illustrations of a chambered nautilus, published by Rumphius in 1705. Left: external shell; center: soft animal; right: internal shell features.
Nautiluses occur across a very large geographic range in the Indo-Pacific region, persisting as numerous small, isolated populations. They are fished in many locations within this range, primarily for their shells. Nautiluses are highly vulnerable to overfishing because of their low reproductive rates, slow growth, and late maturity. In 2018, the U.S. Government listed the chambered nautilus as a “threatened” species.