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.

A 50-million year old chambered nautiloid shell

The living "pearly nautilus," also called the "chambered nautilus," is a favorite seashell of many collectors. Today, the biodiversity (number of species) of these animals is very low, and they are confined to tropical waters in the equatorial region of the western Pacific. As certain times in the geologic past, however, when warm oceans extended north and south of where they are now, chambered shells similar to the "pearly nautilus" had high biodiversity, and their distribution was widespread (cosmopolitan). These chambered shells are commonly referred to as coiled nautiloids.

On August, 2016, I created a post about the "pearly nautilus," and two of my pictures are shown again here for comparative purposes. I encourage you to use the "search box" at the top right-hand side of this blog page to find this post and read it again. I also give some interesting details about the life habits of this animal.

Exterior of a modern-day "pearly nautilus."

            Maximum diameter is 14 cm

Interior of same specimen shown above 

This present post concerns one of these ancient widespread groups of coiled nautiloids, namely, an extinct genus belonging to genus Aturia Bronn, 1838.  It was widespread (cosmopolitan) and its geologic time range was Paleocene to Miocene (approximately 40 million years long).

In particular, this post is about Aturia myrlae Hanna, 1927, an early to middle Eocene species of genus Aturia. The ancient geographic distribution of this species covered an area now referred to as central California, southern California (including Ventura, Los Angeles, and San Diego counties), and Baja California Sur, Mexico. Aturia lived in subtropical to tropical ancient environments. So, if you are lucky enough to find one of these fossils, you can be certain that it represents a warm-water ancient environment. Specimens are not that common because, like other coiled nautiloids, Aturia was a predator, thus, their numbers were few.

The next three pictures show a partial specimen of Aturia myrlae from Simi Valley, southern California. The widest dimension (diameter) of this incomplete specimen is 14 cm. 

Side view showing the complex outlines (septal pattern or suture pattern) of the chamber walls (septa). The suture pattern of Aturia is a very distinctive character of this genus and is in sharp contrast to the simple-curved suture pattern of the "pearly nautilus," which belongs to genus Nautilus.

Back side view of the same specimen. 

Front view of the same specimen. Notice the presence of the siphuncle (see a reference picture at the beginning of this post), which was a hollow tube that connected all the empty chambers and allowed for nitrogen gas to be dispersed to all the chambers. In so doing, the shell achieved buoyancy when the gas was pumped in, and the shell sank when the gas was pumped out. The shell could, therefore, move up and down in the water column.