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disagreement in blogs and websites as to whether or not the E. tenera specimens, found in rock shops, have been replaced by chalcedony or agate.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

Dinosaur gastroliths

Gastroliths are literally “stomach stones.” It has long been postulated that they were swallowed voluntarily by giant sauropods (herbivorous dinosaurs) for the purpose of aiding digestion by grinding food material. 

Gastroliths look just like highly polished river gravel, and this fact has helped convince some paleontologists that gastroliths are nothing more than sedimentary gravel and never used by dinosaurs to aid their digestion process. It would follow, therefore, that if you believe you have found "stomach stones," it would strengthen your hypothesis if you could show that they came from inside of a dinosaur skeleton in the stomach area.

Many years ago, a reputable vertebrate paleontologist gave me a stomach stone (see the red stone shown below), which he said that he had collected from the stomach area of a large sauropod of Late Jurassic age in the San Rafael Swell area of Utah.

A gastrolith 9 cm long (= a small cobble) from Utah. The rock, which is heavy, is an iron-bearing (reddish) quartzite.

The other picture (shown below) is of two other gastroliths, also from Utah. These specimens were part of a private collection that was sold.

Two gastroliths (small one is 1.8 cm long = large pebble; large one is 3.8 cm long = very large pebble) from Utah. The rock type, which is lightweight, is a hard but highly polished detrital sedimentary rock (probably coarse-grained sandstone).

Jasperized (red) horn coral

Some of the most eye-catching fossils I have ever seen are the blood-red solitary rugose corals (horn corals) belonging to genus Lophophyllidium of Pennsylvanian age (approx. 308 m.y. old) from a locale in the Morgan Formation in the Wasatch Mountains, east of Salt Lake City, Utah. Many of these horn corals from this locality have had their calcareous exoskeletons replaced, in part or nearly completely, by jasper. Thus these fossils are commonly referred to as “jasperized horn corals.” Another common moniker is “agatized corals.” These specimens are coveted by collectors and especially those who make jewelry.

Lophophyllidium sp., Pennsylvanian, Utah,
8.5 cm length, 4.5 cm width.

6 cm length, 4 cm width.

Top view of specimen shown to the left;
width 4 cm.

Jaspar is an opaque, impure variety of silica consisting of an aggregate of microquartz and/or chalcedony. Jasper is commonly red (due to iron inclusions), but it can be brown, green, yellow, and rarely blue.

A nugget of jasper, length 5 cm.

"Blind" trilobite Itagnostus from Utah

Trilobites like the one shown above are well known to most collectors of fossil invertebrates (animals without backbones). This trilobite, which is 4 cm long, has a cephalon or head (with eyes), a thorax (body with many segments), and a short pygidium ("tail"). This particular specimen is an Elrathia kingi of Middle Cambrian age (see time diagram below) from the Wheeler Shale near Delta, Utah.

Another type of trilobite found alongside E. kingi is Itagnostus interstictus, until recently known as Peronopsis interstrictus, which belongs to one group of so-called "blind" trilobites. Technically speaking, "blind trilobites" were not blind because they never had eyes. Itagnostus is an agnostid trilobite, which are characterized by having a thorax consisting of only two segments and a cephalon and pygidium of approximately the same size.

This is a "cluster" of two specimens of Itagnostus interstices. The largest specimen is nearly 1 cm  long.