Orthocerida

Orthocerida are an order of extinct nautiloid cephalopods that lived from the Early Ordovician to the Late Triassic (about 500 to 200 million years ago), but were most common and diverse from the Ordovician to the Devonian.

The shell is usually long, and may be straight ("orthoconic") or gently curved. In life, these animals may have been similar to the modern squid, except for the long shell. The internal structure of the shell consists of concavo-convex chambers linked by a centrally-placed tube called a siphuncle. There is a tendency for the chambers to develop cameral deposits, which were used as ballast to balance the long gas-filled shell. Depending on the family, the siphuncle has orthochoanitic or cyrtochoanitic septal necks. The shell surface may be (depending on the species or genus) smooth, transversely ribbed, or ornamented by a network of fine lirae. Fossils are common and have been found on many continents, including the Americas, Africa, Europe, and Asia.

Orthocerids may have swum near the sea bed with their buoyant shell resting horizontally in the water. Like modern cephalopods they would have used jet-propulsion for locomotion, although the long bulky shell makes it unlikely that they were as agile as modern cephalopods. They most likely fed on trilobites and other arthropods.

References

• Sweet, Walter C., (1964), Nautiloidea — Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3. (Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas)

Links

• Palaeos