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.

The Nautilus shell

The chambered pearly shell of Nautilus pompilius, named by C. Linnaeus in 1758, is not only pretty but its spiral growth is an excellent example of logarithmic spiral growth, similar to the spiral bands of clouds in a hurricane or the arms of a spiral galaxy (i.e., Google "logarithmic spiral" for more information on this subject).

The Nautilus is a cephalopod, and this group of animals also includes the squid, cuttlefish, and octopus. Living specimens of N. pompilius can only be viewed in their natural state at a few locales in tropical waters in the southwest Pacific Ocean, or in controlled environments in public or private aquariums. Nautilus shells can be found as beach drift on some beaches. 

Adult shell of Nautilus pompilius (swimming mode orientation);
 maximum diameter 5.5 inches (14 cm).
 

Juvenile shell of Nautilus pompilius shell, 
maximum diameter 2.9 inches (7.3 cm) 

Notice that the juvenile Nautilus is fully covered with stripes, whereas the adult shell only has the stripes on its early part. The stripes provide camouflage for the juvenile because it spends its time on or near the ocean floor. The stripes allow it to blend in. The adult spends most of its time swimming or floating in the water column, and stripes are not needed, at least, on the ventral part of its shell. If a predator looks at the adult Nautilus shell from below, the shell looks like the sun-lit waters near the surface of the ocean.

Cut-away (median-longitudinal) section of adult Nautilus pompilius shell
 showing interior structures; diameter 6.3 inches (16 cm).

As shown in the above picture, the early part of the shell has numerous, closely spaced chambers called camera (single chamber = camerum), which provide great strength to the shell when the animal sinks into the depths (several hundred feet deep) of the ocean during the day. If the shell did not have this added strength, it would implode.

The camera are filled with nitrogen gas, which gives buoyancy to the shell. The siphuncle is a fleshy tube that connects all the camera and serves as a conduit for the transfer of the gaseous contents. The buoyancy also affects the shells after death of the animal. The empty shells can drift long distances. If you submerge an empty Nautilus pompilius shell in a bucket of water, the shell will bop up, rather than sink.

The interior of the N. pompilius shell consists of "mother-of-pearl" shelly material, which is the biomineral aragonite. This mineral  was secreted by the animal as the shell grew, and that is why the term "biomineral" is used here. 

Nautilus is one of only two genera of extant (living) cephalopods known as nautiloids. Fossil nautiloids have a geologic record that goes back to the Cambrian Period, 550 million years ago, although shells did not become common until the subsequent Ordovician Period. These early nautiloids had a straight shell and are called orthocone nautilioids, as opposed to the more modern, coiled nautiloids, like N. pompilius.

Example of Eutrephoceras, shell incomplete (dorsal margin partially missing).
 Maximum diameter 1 inch (2.54 cm).

Eutrephoceras, an extinct coiled nautiloid whose geologic range is Late Jurassic to Miocene, differs in its morphology from Nautilus by having straighter septa (also called sutures).