Virtual Museum: Molluscs

Bivalves, as their name indicates, have 2 valves or shells. Most are filter-feeders. This specimen is probably of the genus Cypricardella.

Lansing, NY

Hamilton Group, Middle Devonian, 397-385 Ma

personal collection

Gastropods are the most diverse group of modern day mollusks. They are commonly preserved as internal molds, such as these.

Taylor Mill, KY

Ordovician Period

personal collection

The majority of modern cephalopds (mollusks with at least 8 arms or tentacles surround a mouth with a beak) are soft bodied with an internal shell or no shell at all The living nautiluses have an external shell, as did most extinct groups.

The mostly soft-bodied forms such as this Trachyteuthis sp. squid are rarely preserved in the fossil record except in exceptional circumstances.

Lebanon

Late Cretaceous Period

Black Hills Institute Museum

Octopus fossils, such as this Keuppia levanter, are even rarer than squid fossils.

Sannine Limestone, Hajula, Lebanon

Late Cretaceous, Cenomanian, 95 Ma

Black Hills Institute Museum

Belemnites are an extinct group closely related to squid and octopuses. The differed in having a hard cone-shaped internal shell.

This Phragmoteuthis concaudia belemnite had hardened hooks on its arms to capture prey.

Posiden Shale, Holzmaden, Germany

Early Jurassic Period, Lias Stage

Black Hills Institute Museum

A model of an unidentified belemnite

The earliest cephalopods in the fossil record were the shelled Nautiloids. A few species of nautiluses survive to this day. Unlike the modern nautilus, most fossil nautiloids had straight external shells

Some nautiloids became partially coiled, such as these Lituites sp., or even completely coiled.

Sanggzhi, China

Middle Ordovician Period

Black Hills Institute Museum

Both ammonites (below) and Nautiloid shells are subdivided into a series of chambers that can be filled with gas to regulate buyoancy. During fossilization, water seeping in deposits minerals in these chambers that can crystalize. This Cymatoceras sakalavus nautiloid has been cut in half to reveal the internal chambers. Note the tube, the siphuncle, that connects the chambers. The presence of this feature is one way to distinguish nautiloids from Ammonites (below).

Sitampiky, Madagascar

Early Cretaceous Period, Albian Stage

Black Hills Institute Museum

Goldringia cyclops, a nautiloid

Late Devonian Period

Iowa

Museum of Ancient Life, Utah

Endoceras sp., a nautiloid

Middle-late Ordovician Period

Museum of Ancient Life, Utah

Cooperoceras texanum, a nautiloid

Middle Permian Period of Texas

Museum of Ancient Life, Utah

Mooreoceras sp. nautiloids

Carboniferous Period

Museum of Ancient Life, Utah

Amphicyrtoceras orcas, a nautiloid

Middle Silurian Period, 425 Ma; Wisconsin

Denver Museum of Science & Nature

Uranoceras hercules nautiloid capturing a Gravicalymene trilobite

Middle Silurian Period, 425 Ma; Wisconsin

Denver Museum of Science & Nature

A typical straight-shelled nautiloid, Dawsonoceras annulatum (top) and the unusual coiled nautiloid, Phragmoceras nestor, atop the blue tabulate coral. Other animals shown include a trilobite, brachiopods, and several echinoderms.

Middle Silurian Period, 425 Ma; Wisconsin

Denver Museum of Science & Nature

Michelinoceras normale, a nautiloid

Devonian Period, 370 Ma

Museum of the Rockies

Ammonites are an extinct subgroup of cephalopod mollusks. Like the nautiloids, from which they are derived, they had an external shell. Their shell was usually tighly coiled but there were various exceptions.

Ammonite shells were often covered in bumps and ridges, perhaps to reduce predation, such as in these two Hoploscaphites spedeni shells

Black Hills Institute Museum

The original aragonite (type of calcium carbonate) mineral is often replaced during fossilization. In some cases, the replacement mineral can be irridescent and colorful and is considered a gemstone (ammolite). This specimen is Hoploscaphites nebrascensis.

Black Hills Institute Museum

This Kosmoceras sp. shows the internal chambers of ammonite shells (only the last chamber, in this fossil where there is solid rock, is occupied by the living animal). The walls (septa) of ammonite chambers are convoluted and complex, unlike the much simpler walls of the nautiliods (note also the lack of a siphuncle).

Russia

Late Jurassic Period, 160 Ma

Wyoming Dinosaur Center

The shell is often eroded away during fossilization, revealing the complex suture patterns made by the internal septa, as in this Sphenodiscus sp. (only a small bit of shell is present near the main body cavity). The patterns made by these sutures are used to classify ammonites.

Fox Hills Formation, South Dakota

Late Cretaceous Period, Maastrichtian Stage

Black Hills Institute Museum

Placenticeras meeki ammonite, showing intricate suture patterns.

Late Cretaceous, Campanian Stage

Pierre Shale, Meade Co., SD

Black Hills Institute Museum

Ammonites that have shapes other than the traditional coiled shape are called heteromorph ammonites. Didymoceras cheyennense is on the left and Exiteloceras jenneyi is on the right.

Late Cretaceous, Campanian Stage

Pierre Shale, Montana

Black Hills Institute Museum

The heteromorph ammonite Ancyloceras waageni

Volga River, Russia

Early Cretaceous, Albian Stage

Black Hills Institute Museum

Exiteloceras jenneyi heteromorph ammonite.

Carter Co., Montana

Late Cretaceous, Upper Campanian Stage

Black Hills Institute Museum

Titanites sp., an ammonite

Jurassic Period of Europe

Museum of Ancient Life, Utah

Placenticeras sp., an ammonite

Late Cretaceous Period, 70 Ma

Museum of Ancient Life, Utah

Baculites grandis, a straight-shelled ammonite

Late Cretaceous Period, 66 Ma; Wyoming

Museum of Ancient Life, Utah