Fossil of the Month: Worthenia
This month’s Fossil of the Month is the Pennsylvanian gastropod Worthenia. Gastropods are a class of mollusk that includes snails and slugs.
Description. Worthenia is a moderately spired fossil snail with a trochoid, also called a trochiform shape. Trochiform shapes are similar to "top" shells of the modern snail family Trochidae. The term “top” shells comes from a shape that is similar to a toy spinning top. Worthenia has four to five whorls, with well-developed sutures (grooves) between each whorl. It has a distinctive ornamentation along the edge of a medial band along each whorl (termed the “selenizone”) consisting of closely spaced, tiny nodes. If you look at the main whorl from below, the lower part of the whorl is ornamented with intersecting raised, thin lines. Spiral lines (also called threads) are oriented longitudinally (parallel to direction of spiral). Transverse lines or threads are oriented across whorls. The main whorl has a broad, flat medial profile bounded by two ornamented, chord-like rims. The aperture of Worthenia has a semicircular shape.
Species. Two species of Worthenia are reported from Pennsylvanian strata in the Appalachian Basin: Worthenia parvula and Worthenia tabulata. These species differ slightly in size and ornamentation, but Worthenia tabulata is the more common in Kentucky.
Range and geographic occurrence. Worthenia has been reported from Mississippian through Pennsylvanian strata. In eastern Kentucky, Worthenia has been found in the Middle Pennsylvanian Crummies Member, Dwale Member, and Elkins Fork Member, all of the Pikeville Formation; the Kendrick Shale Member of the Hyden Formation; the Magoffin Shale Member of the Four Corners Formation; and the Stoney Fork Member of the Princess Formation (Chesnut, 1991). Worthenia has also been reported from the Pennsylvanian Brush Creek Limestone Member (Martino and others, 1996) and Portersville Member (Hoare and others, 1997), both of the Conemaugh Formation.
Life and paleoecology. Worthenia is a marine snail, found in association with other marine fossils. When most people think of fossils from the coal fields, they think of plant fossils associated, but fossils such as Worthenia show that in between deposition of some of the coal beds, the seas rose and covered the coal-forming swamps.
Worthenia is sometimes preserved in the Middle Pennsylvanian Kendrick Shale Member, Hyden Formation, and Magoffin Shale Member, Four Corner Formation, in Kentucky with its original aragonite shell (Dennis and Lawrence, 1979; Brand, 1987). Modern gastropod shells are also composed of the mineral aragonite, which is stable at the earth’s surface, but unstable when buried. Most fossil gastropods are molds and casts of the original shells, or have their original aragonite shells replaced with other minerals. In Pennsylvanian-age rocks, however, original aragonite shells are sometimes preserved as meta-stable aragonite. Oxygen isotope (δ18O) analysis of aragonite can be used to interpret salinity of the waters the mineral in the shell was formed from. Jiminez and others (2019) have recently used δ18O isotope values from Pennsylvanian Worthenia (and other fossils) from the Magoffin Shale Member, Four Corner Formation, near Hazard, Ky. Results indicate seasonally variable salinities for the seas in which the Magoffin was deposited, possibly from the influx of fresh water from seasonal rivers.
Text and illustrations by Stephen Greb
- Brand, U., 1987, Depositional analysis of the Breathitt Formation’s marine horizons, Kentucky, USA: Trace elements and stable isotopes: Chemical Geology, Isotope Geoscience Section, v. 65, p, 117–136.
- Chesnut, D.R., Jr., 1991, Paleontological survey of the Pennsylvanian rocks of the Eastern Kentucky Coal Field: Kentucky Geological Survey, ser. 11, Information Circular 36, 71 p., DOI: doi.org/10.13023/kgs.ic36.11.
- Dennis, A.M., and Lawrence, D.R.,1979, Macro-fauna and fossil preservation in the Magoffin marine zone, Pennsylvanian Breathitt Formation of eastern Kentucky: Southeastern Geology, v. 20, p. 181–192.
- Hoare, R.D., Sturgeon, M.T., and Anderson, J.R., Jr., 1997, Pennsylvanian marine gastropods from the Appalachian Basin: Journal of Paleontology, v. 71, no. 6, p. 10191039.
- Jimenez, M.Y., Ivany, L.C., Judd, E.J., and Henkes, G., 2019, Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin: Earth and Planetary Science Letters, v. 519, p. 182–191, DOI: 10.1016/j.epsl.2019.04.051.
- Martino, R.L., 1996, Glenshaw Formation (Late Pennsylvanian) in central Wayne County, West Virginia: Southeastern Geology, v. 36, no. 2, p. 65–83.