Elimia tenera: A commonly misidentified Eocene freshwater snail

Over the years, as I have viewed various collections of fossils, I have come across specimens of a small fossil gastropod that occur in great abundance. Rocks containing these shells can be found for sale in rock shops or online, and the shells are commonly and incorrectly called “Turritella agate.” These rocks do not consist of Turritella; rather they consists of specimens of the freshwater gastropod Elimia tenera (Hall, 1845), which have been preserved in chalcedony. Actual specimens of Turritella can be much larger, possess only spiral ribs, and are known only from shallow-marine deposits. Elimia tenera has both radial and spiral ribs, and the aperture of Elimia is quite unlike that of Turritella.

This rock (77 mm width) is fully packed with specimens of E. tenera.

This polished slab (37 mm width) shows only the
cross section of shells of E. tenera.

These three specimens of E. tenera are internal casts (i.e., each one shows
only the interior of a shell, which was filled with chalcedony).
The largest specimen is 14 mm height. 

There has been considerable disagreement in blogs and websites as to whether or not the E. tenera specimens, found in rock shops, have been replaced by chalcedony or agate. Technically speaking, chalcedony is the “culprit.” It is a microcrystalline form of silica, and chalcedony has many varieties, including agate, which commonly has multi-colored curved or angular banding. The specimens of E. tenera that I have seen were replaced by a fairly uniform brown or gray color of “ordinary” chalcedony and not replaced by the more eye-catching, beautiful colors typically associated with agate.

Elimia tenera: Specimen on the left (19 mm height) shows the spiral ribs, and the
specimen on the right (14 mm height) shows both spiral and radial ribs.

Elimia tenera, which used to be (and incorrectly) called Goniobasis tenera, is a freshwater snail that lived in shallow subtropical lakes with intermittent volcanic eruptions nearby. This gastropod is of Eocene age and is commonly found in the Laney Member of the Green River Formation in Utah. This is the same formation that famously has many very well preserved fish, insect, leaf, and other fossils.

Genus Elimia belongs to family Pleuroceridae, and, as currently defined, this family  today is confined entirely to North American fresh waters: The eastern United States and into Texas and from southern Canada to Florida. Pleurocerids might be relicts (“living fossils”) from earlier geologic times (Paleozoic?) in eastern North America.

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).