Characteristics
The subclass nautiloidea, in the broad original sense, is distinguished by two main characters, simple concave septa, concave in the forward direction, that produce generally simple sutures, and a siphuncle in which the septal necks point to the rear, i.e. is retrosiphonate, throughout the ontogeny of the animal.
The septa between the chambers (camerae) of the phragmocone (the chambered part of the shell) are formed during growth spurts of the animal. At that time the rear of the mantle secretes a new septum adding another chamber while the more forward part adds on to the shell. The body of the animal, its viscera, continues to occupy the last chamber of the shell – the living chamber.
The septa are perforated by the siphuncle, which runs through each of the internal chambers of the shell. Surrounding the fleshy tube of the siphuncle are structures made of aragonite (a polymorph of calcium carbonate – which during fossilisation is converted to calcite): septal necks and connecting rings. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers (called cameral deposits) or within the siphuncle (endosiphuncular deposits), a process which may have been connected with controlling buoyancy. The nature of the siphuncle and its position within the shell are important in classifying nautiloids.
Sutures (or suture lines) are visible as a series of narrow wavy lines on the surface of the shell, and they appear where each septum contacts the wall of the outer shell. The sutures of the nautiloids are simple in shape, being either straight or slightly curved. This is different from the "zigzag" sutures of the goniatites and the highly complex sutures of the ammonites.
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