The cephalopod Spirula spirula and its interesting relatives

This post, and the following two posts, concern the coleoid cephalopods.

Cephalopods, which are the most specialized of the mollusks, are characterized by having a distinct head, arms, and tentacles. They are exclusively marine, and most are distinguished by their chambered shell although some of the more modern ones, however, have a reduced internal shell or no shell at all (octopus).

The main divisions of cephalopods are: ammonoids (including ammonites), nautiloids, and coleoids.

The main divisions of coleoids are belemnoids, spirulids (genus Spirula), sepiids (genus Sepia), squids, and octopods (octopus). In an earlier post, I focused on the subject of the belemnoids (an extinct group, which was dominant during the Jurassic and Cretaceous). The other coeloids are highly diversified and because they have a pair of gills are referred to as dibranchiates, which are mainly Cenozoic forms and their fossil records are poor. The following forms exemplify them.  


Spirula spirula (Linnaeus) is the only living member of this genus of spirulid coleoids. It lives as a free-drifting animal in deep seas of the world's warm oceans. Dead shells can be found on beaches. Spirula has a coiled chambered shell (phragmocone), but the walls of each coil do not touch the previous coils. The closely spaced chambers (septa) of the shell have a thin tube (siphuncle) extending through them via a ventral perforation. This tube allows for transport a gas used for buoyancy. The shell is carried internally, near the posterior end of the animal. Unlike the pearly Nautilius shell (see my August, 2016 post), there is no large, final body chamber.  

The internal chambered shell of Spirula spirula from Florida.  Maximum diameter of this shell is 2.3 cm.

Same shell as shown above;  interior of the last chamber (6 mm width), showing the perforation for the siphuncle. 

The fossil record of the early coleoids is sparse, therefore, there are many challenges in trying to unravel the evolution of this group because the degree of coiling differs dramatically.  Two very rare Eocene specimens are shown below: 

This first specimen consists of two fragments of the same specimen from the Llajas Formation in southern California is middle Eocene. The two fragments together are 23 mm in length.

This schematic diagram shows an inferred reconstruction of the Llajas Formation specimen shown above and its tapering but straight (orthoconic) phragmocone with chambers. For more information, see Squires, R.L., 1983. New mollusks from the lower middle Eocene Llajas Formation, southern California. Journal of Paleontology 57(2):354-362.

This other specimen is of late Eocene age from the Hoko River Formation in the extreme northwest corner of Washington State (in the Strait of Juan de Fuca. The image shows the dorsal view of the 20 mm length of the hoof-like shell. In its posterior area, the chambers of the phragmocone are much smaller and are tightly coiled, versus the later chambers.  For more information, see Squires, R.L. 1988. Cephalopods from the late Eocene Hoko River Formation, northwestern Washington. Journal of Paleontology 62(1):76-82.