Orthoceras Extinct Marine Mollusc


Orthoceras are molluscs that lived more than 400 million years ago. The name means straight horn, referring to the characteristic long, straight and conical shell. The preserved shell is all that remains of this ancestor of our modern squid. The soft body lived in the last open segment at the broad end of the conical shell.

As the body grew and the housing segment became too small, a dividing wall, called septa, grew to separate the old chamber from the new one. Differences in the composition of these shell parts have allowed for differences in fossilization so that the parts can be seen. The siphon is a tube that runs the entire length of the shell, through each of the chambers.

This tube had two functions. Once filled with water, the nautiloid could expel the water, propelling itself backwards with a kind of jet propulsion. By releasing the water and leaving the airspace, the tube could act as a flotation device that allows the animal to ascend and descend to different depths. The siphon is the line that runs down the center from head to tail.

The septa are the short curved lines that cross or side to side. These straight-shelled nautiloids ranged in size from less than an inch to more than 14 feet in length! All living relatives of these nautiloids, squids, octopuses, cuttlefish and nautiluses are predators, and we can assume that Orthoceras was also a hunter of the Paleozoic seas.