Fossil of the month: Anthraconaia

 

This month’s fossil is commonly mineralized, so it may look like small copper, golden or iridescent shells in dark gray to black shale. It’s a small, easy-to-misidentify shell fossil found in Kentucky’s coal fields. This month’s fossil is the bivalve, Anthraconaia.

 

Description. Anthraconaia (An-THRAC-oh-NYE-uh) is a fossil bivalve. Bivalves are shelled invertebrate animals (those without backbones) like clams and scallops. Fossil bivalves have a variety of shapes often called profiles. Anthraconaia has a sub-oval to mytiliform shape (similar to the modern mussel, Mytilus). It is broadly elliptical with two rounded ends. One end of the ellipse, the posterior, is slightly wider than the other end, the anterior. Valves are commonly ornamented with fine, concentric growth lines.

 

Description of Anthraconaia bivalve parts. Bivalves have two valves that are mirror images of each other.

Why are shells gold and copper colored?  The color of the fossil shells at the top of the page and following examples is caused by the mineral pyrite, commonly called Fool’s gold. Pyrite is often found in dark gray to black shales. The dark color of these shales means they are organic-rich. In the presence of decaying organic matter, sulfate-reducing bacteria can transform sulphates to sulfides, which can react with free iron in the sediment to form the mineral pyrite (FeS₂) (Berner, 1984). Although pretty to look at, pyritized fossils can be fleeting once collected. Upon contact with air or moisture, some forms of pyrite will oxidize and disintegrate into a yellow, white, and gray powder called a "sulfur bloom," destroying the fossil. Disintegration does not always happen but if it does, the process can happen soon or many years after specimens are collected. Luckily, it has not happened with these specimens from the Kentucky Geological Survey’s collection.

 

Pyritized fossil Anthraconaia from the Kentucky Geological Survey’s paleontological collection. These (and the picture at the top of the page) are from a shale above the River Gem coal bed, Grundy Formation, in McCreary County, Kentucky.

 

Fossil shells that can easily be confused with Anthraconaia

Anthraconaia is similar in appearance to several other Pennsylvanian-age bivalves including Anthraconauta and Naiadites. Both Anthraconauta and Naiadites have longer hinge lines, and both are proportionally wider towards the posterior side of their valves. Naiadites also has a pronounced posterior ridge (between the umbo and posterior side of the valve), which is more subdued in Anthraconauta, and is subdued or lacking in Anthraconaia (depending on species).

Anthraconaia is similar in appearance to several other small, fossil shells that can be found in similar Pennsylvanian-age dark gray to black shales.

Anthraconaia is also similar in appearance to the brachiopod Lingula, which is relatively common in many Pennsylvanian-age shales. Brachiopods are another type of fossil sea shell. Brachiopods have two dissimilar valves while bivalves have two valves which are mirror images of each other. Determining if two valves are symmetrical or not is not always possible when the shells are preserved in matrix rock. This is especially true in shales, which are more compacted than other rock types, and tend to have fossils that may be flattened.

Both Lingula and Anthraconaia are (1) small (often <1.5 cm long), (2) broadly elliptical, (3) ornamented with concentric growth lines, and (4) relatively common in Pennsylvanian-age, dark gray to black shales. Anthraconaia and Lingula have subtly different profile shapes (especially towards the posterior and anterior ends) and may be confused with each other, especially where shells are partly fragmented, or the shell is preserved at an angle to bedding and partly hidden in the matrix rock.  Chesnut (1981) noted the similarity of the two shells and the possibility that many shells identified in Pennsylvanian-age shales of Kentucky’s coal fields as lingulids or Lingula, might be misidentified Anthraconaia.

Anthraconaia fossils in dark gray to black shales are not always easy to identify. Gray fossils in gray rock can also be difficult to see! These examples are from a location in the Grundy Formation, McCreary County.

Species. Several species of Anthraconaia are known in Pennsylvanian rocks (Eager and Belt, 2003), but only one has been reported in Kentucky, A. ohioensis. Species identification requires very well-preserved specimens and expertise in Pennsylvanian bivalves. For that reason, most of the identified Anthraconaia found in Kentucky, are only identified to genus, rather than species (written as Anthraconaia sp.).

 

Range and geographic occurrence. Anthraconaia lived from the Early to Late Pennsylvanian worldwide (Cox and others, 1969), which lasted from 299 to 323 million years ago. In Kentucky, Anthraconaia is found in Lower and Middle Pennsylvanian rocks. Most of the recorded specimens are from the Eastern Kentucky Coal Field, but they can also be found in the Western Kentucky Coal Field.

Life and paleoecology. Dark gray and black shales in Kentucky’s coal fields were originally deposited as organic-rich muds. Many dark gray to black shales represent muds that were deposited in saline, marine environments; others represent brackish-estuarine or bay environments; and others represent freshwater lake and pond environments. Although different criteria are used to interpret the environment in which a specific shale layer or unit was originally deposited, fossil evidence is often an important component. Anthraconaia, Anthraconauta, and Naiadites are commonly interpreted as freshwater bivalves (e.g., Rogers, 1965; Henry and Gordon, 1979). Some species of Anthraconaia appear to have favored well-oxygenated freshwater environments (lakes, rivers, etc.) while others favored plant-rich, stagnant lakes and ponds (Eager and Belt, 2003). Eagar (1973) suggested some species of Anthraconaia may also have lived in brackish waters (including A. ohioensis found in southeastern Kentucky). Anthraconaia is not found in marine shales, so it can be used to help differentiate Pennsylvanian-age shales deposited in fresh- or brackish-water environments, from those deposited under marine conditions.

Examples of Anthraconaia fossils (dash outlines) from Kentucky with fossil plant debris (green dash lines). Some specimens look like they may possibly be attached to the plants. Specimens are from a shale in the Stearns coal zone, Alvy Creek Formation, McCreary County, Kentucky.

Modern freshwater mussels live burrowed in sediment or attached to other objects, like rocks and plants. Anthraconaia fossils from the Eastern Kentucky Coal Field commonly are found in association with plant fossils. Chesnut (1981) noted an occurrence of Anthraconaia with debris of the Carboniferous reed, Calamites which grew in Kentucky’s ancient coal swamps and along river, pond, and lake margins. Anthraconaia at another location were found with calamitid debris, and stems or shoots of the plant fossil, Rhodea. In shales, it can be difficult to determine if the shells were actually attached to the plants in life and died together, or the plants fell to the lake bottom next to the shells and then were buried together.

The dark color of the Anthraconaia-bearing shales is from organic matter preserved in the original muddy sediment. In the presence of normal oxygen levels, organic matter would be consumed or oxidize. Hence, preservation of organic matter means the muds were deposited under oxygen-limited conditions. Such conditions can happen in lakes and bays (in the sediment or overlying lower parts of the water column) if the water is relatively stagnant and becomes oxygen stratified. Low-oxygen conditions limit bacterial activity, which not only helps to preserve microscopic organic matter but also larger organic matter (plants and organisms) buried in the mud. If preserved after initial burial, the plants and organisms have a better chance of becoming fossils!

Acknowledgements.  The fossil of the month and many of the Pennsylvanian-age fossils in the KGS paleontological collections were collected by Don Chesnut, Jr. (retired, Kentucky Geological Survey). Thanks Don!

References Cited

• Berner, R.A., 1984, Sedimentary pyrite formation: an update: Geochimica et cosmochimica Acta, v. 48, no. 4, p. 605.
• Bless, M.D.M., 1970, Environments of some upper Carboniferous coal-basins (Asturias, Spain; Limburg, Netherlands): Compte Rendu, 6th Congress International Stratigraphie et Geologie Carbonifere, Sheffield, 1967, v. 11, p. 503–516
• Chesnut, D.R., 1981, Marine zones of the Upper Carboniferous of eastern Kentucky, in Cobb, J.C., Chesnut, D.R., Hester, N. and Howard, J.C., eds., Coal and coal-bearing rocks of eastern Kentucky: Kentucky Geological Survey, Annual Geological Society of America Coal Division Field Trip, Nov. 5-8, 1981, p. 57–66.
• 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.
• Cox, L.R., Newell, N.D., Boyd, D.W., Branson, C.C., Casey, R., Chavan, A., Coogan, A.H., Dechaseaux, C., Fleming, C.A., Haas, F., Hertlein, L.G., Kauffman, E.G., Keen, A.M., LaRocque, A., McAlester, A.L., Moore, R.C., Nuttall, C.P., Perkins, B.F., Puri, H.S., Smith, L.A., Soot-Ryen, T., Stenzel, H.B., Trueman, E.R., Turner, R.D., and Weir, J., 1969, Part N—Mollusca 6, in Teichert, C., ed., Treatise on invertebrate paleontology: Geological Society of America and University of Kansas Press, 952 p.
• Eagar. R.M.C., 1970, Preliminary notes on some near Pennsylvanian marine and non-marine faunas in the eastern U.S.A.: Compte Rendu, 6th Congress International Stratigraphie et Geologie Carbonifere, Sheffield, 1967, v. 11, p. 679–694.
• Eagar, R.M. C., 1973, Variation in shape of shell in relation to paleoecological station in some non-marine Bivalvia of the coal measures of south-east Kentucky and of Britain: Compte Rendu, 7e, Congress lnternational Stratigraphie et Geologie Carbonifere, Krefeld, 1971, v. 2, p. 387–416.
• Eager, R.M.C., and Belt, S., 2003, Succession, palaeoecology, evolution, and speciation of Pennsylvanian non-marine bivalves, Northern Appalachian Basin, U.S.A.: Geological Journal, v. 38, p. 109–143.
• Henry, T. W., and Gordon, M., Jr., 1979, Late Devonian through Early Permian (?) invertebrate faunas in proposed Pennsylvanian System stratotype area, in Englund, K.J., Arndt, H.H., and Henry, T.W., eds., Proposed Pennsylvanian System stratotype, Virginia and West Virginia: American Geological Institute, Ninth International Congress of Carboniferous Stratigraphy and Geology, Field Trip No. 1, p. 97–103.
• Rogers, M.J., 1965, A revision of the species of nonmarine Bivalvia from the upper Carboniferous of eastern North America: Journal of Paleontology, v. 39, p. 663–686.
• Williams, E.G., 1960, Marine and fresh water fossiliferous beds in the Pottsville and Allegheny Groups of western Pennsylvanian: Journal of Paleontology, v. 34, no. 5, p. 908–922.