Fossil of the month: Dunbarella
This month's fossil of the month is Dunbarella. Is it a gold scallop fossil? Not really, but you might think it is. The fossil is an ancient ancestor of scallops, and the gold color is the mineral pyrite, sometimes referred to as "fool's gold."
Description. Dunbarella is a small, fossil bivalve. It is a pectanid type of bivalve, which means it shares some similarities with the modern scallop (Pecten). The main similarity is its ear-like or wing-like extensions of the shell (called auricles) along the hinge. The auricle on the anterior side of the valve is more pronounced than the auricle on the posterior side. Dunbarella's outline is subrounded to subquadrate toward the front, whereas modern scallops are rounded to subrounded. Dunbarella's valves are ornamented with fine ribbing, called costae, which is different from the coarser ribbing or plications in modern scallops.
Slight differences in (1) outline shape, (2) the shape and size of auricles, and (3) ornamentation, are used to differentiate different pectinid genera and species (Newell, 1937; Cox and others, 1969). Other fossil pectinid bivalves found in Kentucky, which have slightly different shapes but could be confused with Dunbarella, include Acanthopecten, Aviculopecten, Deltopecten, Limapecten, and Pernopecten (Chesnut, 1991).
Species. Two species of Dunbarella are reported from Kentucky. Dunbarella knighti occurs in Middle and Upper Pennsylvanian rocks of eastern Kentucky and the central Appalachian Basin. Dunbarella rectilaterarea occurs in Middle (and Upper?) Pennsylvanian rocks in western Kentucky and the Illinois Basin. Dunbarella rectilaterarea is distinguished from D. knighti by a more subquadrate (rather than rounded) shape and greater height (dorsal to ventral distance along the midline of the shell) (Newell, 1937). Specimens of Dunbarella that cannot be identified to specific species are described as Dunbarella sp. (without a second name following the genus name).
Range. The oldest modern scallop ancestors are from the early Mesozoic Era (Hautmann, 2010), but earlier pectinid forms, including Dunbarella, lived in the Paleozoic Era. Dunbarella fossils are found in Mississippian and Pennsylvanian strata worldwide (Cox and others, 1969), but in Kentucky, Dunbarella is only found in Middle and Upper Pennsylvanian strata. In the Western Kentucky Coal Field, the species D. rectilaterarea is reported from black shales above the Springfield (W. Ky. No. 9) coal bed of the Carbondale Formation and Herrin (W. Ky. No. 11) coal bed of the Shelburn Formation. The Springfield and Herrin coal beds were historically two of the most heavily mined coals in the state. The shales above these coals are the horizons from which pyritized fossils have most commonly been found. In eastern Kentucky, nonpyritized Dunbarella sp. have been reported from silty gray shales above many other coal beds. The species D. knighti has been reported from the Kendrick Shale Member of the Hyden Formation (above the Amburgy coal) and Magoffin Shale Member of the Four Corners Formation (above the Taylor/Copland coals) (Chesnut, 1991). The assorted shales in which Dunbarella have been found in Pennsylvanian rocks of Kentucky are 300 to 310 million years old.
Why are shells pyritized? Not all Dunbarella shells are pyritized, only those in black shales of marine origin. The dark color of these shales means they are organic-rich. Organic material is preserved if conditions were anoxic (no oxygen) to dysoxic (oxygen poor) in the seafloor or lower part of the water column during deposition. In the presence of decaying organic matter, sulfate-reducing bacteria can thrive. Sulfate-reducing bacteria transform sulphates (SO2-) in sea water to hydrogen sulfide (H2S). Sulfides can react with free iron (Fe2+) in the sediment to form the mineral pyrite (FeS2) (Berner, 1970; Raiswell and Berner, 1985; Maples, 1986). Examination of trace elements in the black shales above the Springfield and Herrin coals shows they were deposited in marine anoxic to dysoxic conditions (Eble and Greb, 2018) in which sulfate-reducing bacteria could thrive.
Bivalve shells (like modern scallops and ancient Dunbarella) are composed of the mineral aragonite. Aragonite is stable at the Earth's surface, but breaks down quickly when buried. In the organic-rich, oxygen-poor substrates of some Pennsylvanian seafloors, pyrite replaced Dunbarella's aragonite shells during burial. The shales in which Dunbarella are found in Kentucky also contain other pyritized mollusks, including snails (gastropods) and cephalopods (squids with shells). Gastropods and cephalopods also had original aragonite shells. 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 can happen soon or many years after specimens are collected.
Paleoecology. Dunbarella is considered to have lived a similar lifestyle to modern scallops. Modern scallops are filter-feeding, marine bivalves. They have tiny tentacles and many eyes along the margin of their commissure (outer ventral edge). Such features are soft and not preserved as fossils, but ancestral pectinids likely had similar features.
Scallops live in mostly shallow seas, although different species have different ranges of depths and temperatures they prefer, including deeper waters (Brand, 2016). All scallops are epifaunal as juveniles, which means they live on the seafloor or attach to other objects such as old shells, algae, and sea grasses. Scallops can swim to escape predators, and some have a free-swimming lifestyle as adults. Scallops propel themselves through the water by squirting water through their valves and clapping their valves together rapidly (Stanley, 1972; Serb, 2016).
In life, modern scallops are oriented with the right valve (slightly larger) on the seafloor and attached by thread-like features called "byssae" or "byssal threads." These threads extend from a small notch adjacent to the anterior auricle. Swimming scallops lose the byssal notch and ability to attach as adults (Stanley, 1972; Serb, 2016). Byssal threads are soft, so are not usually preserved as fossils, but Dunbarella has a small byssal notch, so likely had byssal threads for attachment (Hoare and others, 1979).
In the Pennsylvanian rocks of Kentucky, D. rectilaterarea and D. knighti usually occur with other mollusks (snails, cephalopods), but sometimes are found with marine brachiopods. Where Dunbarella occurs with other marine fauna, it is interpreted as a marine bivalve (Williams, 1960; Boardman and others, 1984; Burk and others, 1987; Chesnut, 1991; Lebold and Kammer, 2006). In the Midcontinent and other parts of the Appalachian Basin, however, Dunbarella sp. and at least the species D. striata are sometimes found in strata with tidal origins and are interpreted as brackish-water fauna (Hoare and others, 1979; West and others, 2003). Hence, different species of Dunbarella may have tolerated different salinities and different water depths.
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- Serb, J.M., 2016, Reconciling morphological and molecular approaches in developing a phylogeny for the Pectinidae (Mollusca; Bivalvia), in Shumway, S.E., and Parsons, G.J., eds., Scallops: Biology, ecology, aquaculture, and fisheries [3d ed.]: Elsevier Developments in Aquaculture and Fisheries Science, v. 40, p. 1-30.
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- Williams, E.G., 1960, Marine and fresh water fossiliferous beds in the Pottsville and Allegheny Groups of western Pennsylvania: Journal of Paleontology, v. 34, no. 5, p. 908-922.
Text and illustrations by Stephen Greb (KGS).