Cephalopod Mollusks: Squid and Octopus

Cephalopods are a group of swimming mollusks, including the living squid, octopus, and the chambered Nautilus. Although most living cephalopods have somewhat reduced shells, fossil shells were well developed. Cephalopod shells have evolved into many astonishing and beautiful forms, exhibiting a variety of shapes, such as straight, slightly curved, crescent, and coiled.

Two diagrams showing the anatomy of different types of cephalopod shells
Cephalopod mollusk anatomy: coiled and straight cephalopod shells (after Boardman and others, 1987).

The shell of a cephalopod is normally tube- or cone-shaped with many dividers. These dividers are called septa, and they partition the inside of the shell into chambers. Each septum intersects the shell wall at a suture, which can be seen as a pattern on the outside of the shell. Sutures are characteristic of cephalopod groups and are therefore helpful for identification. The body chamber, which lacks sutures, is the area where the animal lived. A small tube called the siphuncle runs the length of the shell and passes through the septa. The siphuncle contains liquid that helps maintain the buoyancy of the animal in the water.

Like their clam and snail relatives, cephalopods possess typical molluscan features: a shell, a muscular foot, and a mantle. They also have highly developed sensory organs. The eye of a cephalopod, for example, is similar to that of a human. Unlike the brachiopods and clams, cephalopods are mobile predators, and some can swim at speeds of approximately 64 kilometers (40 miles) per hour by jetting water from the mantle cavity through a fleshy funnel.

Cephalopods originated during the Cambrian period and are common as fossils in Ordovician and Silurian rock in Wisconsin. Fossil cephalopods from Wisconsin can exceed 4 meters (13 feet) in length. They are exciting to find because of their large size, but they can be preserved as molds and casts and as a result can be difficult to identify.

Group of 12 drawings showing different types of cephalopods
Cephalopod mollusk fossils. A) Spyroceras, a common Ordovician and Silurian cephalopod with a straight and ridged shell [10 cm]. B) Richardsonoceras, common in Ordovician rock [11 cm]. C) Crescent-shaped Richardsonoceras [5.5 cm]. D) and E) Examples of Whitfieldoceras, an Ordovician cephalopod with a constricted body chamber [2.5 and 5 cm, respectively]. F) Gyroceras, a Silurian crescent-shaped cephalopod with a ribbed shell [6 cm]. G) Actinoceras, a large, straight cephalopod common in Ordovician rock [35 cm or greater]. H) Enlarged siphuncle, associated with buoyancy, of Actinoceras [4.5 cm]. I) Beloitoceras, a short and stout Ordovician cephalopod [5 cm]. J) Trocholites, a coiled cephalopod occasionally found in Ordovician rock [8.5 cm]. K) Kionoceras, an Ordovician cephalopod possessing longitudinal ribs [8.5 cm]. L) Cyrtorizoceras, an Ordovician cephalopod possessing a large body chamber [4 cm].
A slab of rock that has been cut to show its numerous fossilized cephalopod mollusks
Cephalopod mollusk death assemblage. (Photo by Madeleine Haberman)

Cephalopod mollusk fossil photos

A cephalopod mold fossil with visible shell chambers
A cephalopod cast fossil
A cephalopod mold fossil
A cephalopod cast fossil

(Photos courtesy the Milwaukee Public Museum.)