Sunday, August 14, 2016

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considerable disagreement in blogs and websites as to whether or not the E. tenera specimens, found in rock shops, have been replaced by chalcedony or agate.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.

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