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Orthoceras-Straight Shelled Nautiloid

Orthoceras was a straight shelled nautiloid that lived during the Paleozoic Era. The name means straight horn, ortho=straight ceras= horn. It is this long conical shell that is commonly preserved as a fossil.

Orthoceras Classification


Orthoceras are classified like this:

  • Phylum Molluska - along with snails, slugs, and clams
  • Class Cephalopoda - with today’s chambered nautilus, squid and octopus
  • The Order is Orthocerida

What Did Orthoceras Hunt?

All the living relatives of orthoceras, squid, octopus, cuttlefish, and nautilus are predators, and we can assume that orthoceras was also a hunter of the Paleozoic seas, possibly having trilobites for breakfast!

How Big Were Orthoceras

There are huge variations in the size of these fossils. The have been found as small as a centimeter to more than 14 feet long!

When Did They Live

The fossils of this group are found from the early Ordovician Period up to the Triassic period. This time covers about 300 million years. They were most prolific during the Ordovician and Devonian Periods.

The soft body of the orthoceras lived in the last open-ended segment at the large end of the conical shell. As the body grew it added on to the shell. Eventually a dividing wall, called the septa grew to separate the old “home chamber” from the new one.

The siphuncle is a tube that runs the entire length of the shell, through each of the chambers. This tube could serve as a bouyancy device allowing the animal to rise and lower itself to different depths.

Where are Orthoceras Fossils Found?

Orthoceras fossils have been found in North America, Europe, Africa, Asia, and Australia.

polished orthoceras

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