Ward, Peter D. Department of Geological Sciences, University of Washington, Seattle, Washington.
Teichert, Curt Department of Geological Sciences, University of Rochester, Rochester, New York.
Russell-Hunter, W. D. Department of Biology, Syracuse University, Syracuse, New York.
Last reviewed:January 2020
- Fossil record
- Description and behavior
- Related Primary Literature
- Additional Reading
A group of externally shelled cephalopods, represented by the two living genera, Nautilus, known since the 1500s, and the more recently defined Allonautilus, which was seen alive for the first time only in 1986. Allonautilus differs from Nautilus not only in specific anatomical differences, but in the shell itself: unlike Nautilus, the shell of the living Allonautilus is covered with a thick blanket of periostracum (a thin membrane-like organic layer covering the outer shell surface), giving the shell a curious “bearded” appearance. The formal designation of this group as a subclass is now generally used only for those externally shelled cephalopods that resemble Nautilus in having completely coiled shells (thus the subclass includes Nautilus and Allonautilus). In living forms, the basic structural plan includes a shell consisting of a septate phragmocone, a living chamber, and a siphuncle. In fossil nautiloids, this simple pattern is modified in great variety with respect to shell form and size, structure and size of the siphuncle, and the large number of devices to counteract the buoyancy of the phragmocone. The shape of fossil nautiloids may deviate in many ways from the simple Nautilus model (Fig. 1); the length of straight or slightly curved shells varies from less than 2.5 cm (1 in.) to more than 10 m (33 ft). Few coiled shells are larger than Nautilus. Also, the siphuncle may vary considerably in size and shape (Fig. 1). The aperture of the living chamber may be constricted or contracted into various shapes, and the interiors of siphuncle and camerae may be partially filled by layers of aragonite and conchiolin. See also: Cephalopoda
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