Ammonite vs. Fossilized Nautilus

How do you distinguish between an ammonite and a fossilized nautilus?

Both the ammonite and nautilus are members of the cephalopod family, whose living members include the octopus, squid, and cuttlefish. The ammonites are extinct, having lived from the Devonian to the Cretaceous eras (~400 to ~65 million years ago). The nautiluses first appeared during the late Cambrian era (~500 million years ago) and some species still remain today.

Examine the siphuncle tube in the shell:

Both ammonites and nautiluses lived in an external chambered shell, usually in the shape of a coiled spiral. Depending on the species, it could also be straight, partially bent, or corkscrewed. Their body was in the last and largest chamber. As they grew they added progressively bigger compartments to the open end of their shell, walling off the old ones.

This chambered shell is called a phragmacone (“frah-muh-cone”), from the Greek phragmo, “fence, enclosure, partition”, + konos, "cone".  A thin organ called a siphuncle (“sigh-fung-kuhl”) connected each segment to the body chamber like a strand of beads. The word comes from the Latin sīphunculus, meaning “a little siphon”. They used this tube to maintain their buoyancy, regulating the mixture of water and gas in each compartment through their blood.

A: Nautilus: In most nautilus species the siphuncle passes through the middle of each chamber. It may be visible when cut in a cross section.

B: Ammonite: In most ammonite species the siphuncle is found near the bottom of each chamber, along the inside surface of the outer edge of the shell. It may be visible when cut in a cross section and in polished but uncut specimens.

C: Nautilus vs. Ammonite: The siphuncle is in the middle of each chamber in a nautilus and the bottom in an ammonite. There are species exceptions for both animals. Early nautiluses had the siphuncle at the bottom of the chamber and one branch of ammonites, the Clymeniida, had them in the middle.

Examine the septa and sutures of the shell:

The chamber walls in both animals are called septa, pronounced “sep-tuh” from the Latin saeptus, meaning “enclosure, fence, wall”. If the shell has been broken, exposed to the elements, or polished, patterns may be visible on the surface where the septa connected to the outer shell. These are called sutures, pronounced “sue-tchers”, from the Latin sutura, meaning “a seam”.

A: Nautilus: A nautilus has a thicker shell and its septa are concave, curving away from the mouth of the shell when cut in a cross section. Its sutures are a simple rounded line.

B: Ammonite: An ammonite has a thinner shell and its septa are convex, curving towards the mouth of the shell when cut in a cross section. Some species had folded septa, producing intricate leaf like sutures. These distort their convex orientation.

C: Nautilus vs. Ammonite: Nautilus sutures are concave, ammonite sutures are convex. Complex sutures may not appear convex. Simple sutures are found in both nautiluses and ammonites, complex sutures are only found in ammonites.

This article is part of a series for Enter the Earth, located in Asheville, North Carolina.  To find both ammonites and fossilized nautiluses, check out their retail and online stores:

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© 2012, C. L. Matthews

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To read about the metaphysical properties of ammonites, see: The Metaphysical Properties of Ammonites and Ammolite