Fossil of the month: Modiolodon
This month’s fossil, Modiolodon, is a type of mussel (clam) found in the Ordovician rocks of central Kentucky. Several similar-shaped shells are known from the Ordovician and Modiolodon can be easily confused with some of its similarly shaped cousins.
Description. Modiolodon is a type of fossil bivalve (also called a pelecypod). Its shell is thin, sub-oval (subovate), with a broader (wider) posterior than anterior end. The anterior end is the side with the “beaks” (McFarlan, 1931; Cox and others, 1969). The outline shape is somewhat similar to the shape of the modern bivalve Modiolus, sometimes called horse mussels. The shape of Modiolus is used to define a general bivalve shape called “modioloform.”
As with most bivalve fossils, Modiolodon is generally preserved as internal molds and casts, but external molds and casts, and recrystallized and silicified valves, are also found. Modiolodon valves may be ornamented with concentric growth lines, but they are only preserved on external casts and molds, and recrystallized or silicified specimens. Silicified specimens sometimes preserve details of original shell structures. Specimens collected by Pojeta (1971) provide evidence of byssal threads for these shells when they were living. Byssal threads (also called byssus) are strong hair-like filaments that some bivalves use to attach to the substrate or other organisms. Silicified specimens from Kentucky also sometimes preserve muscle scars and other internal shell details. The U.S. National Museum and Kentucky Geological Survey collection have several silicified specimens from Kentucky, which were removed from their limestone matrix with acid.
Historically, many elliptical Ordovician bivalves and especially those with a modioloform shape, were called Modilopsis. Many Ordovician bivalves, however, have elliptical, sub-ovate, and modioloform shapes. Modiolodon has a suboval to slightly modioloform shape. Modiolodon is distinguished from Modiolopsis by a slightly shorter length and teeth inside the shell, but the teeth are only visible where the interior of the shell is preserved and visible. This is problematic, because most fossils of bivalves are internal molds and casts, which only preserve the outline shape of the shell, and not internal details. The similarity of many Ordovician bivalve shapes makes them difficult to identify. Most modioloform fossils should only be described as “modioloform” unless being identified by someone with expertise in these types of fossils or with exceptionally well preserved fossils.
Species. Only one species of Modiolodon is known from Kentucky, M. oviformis. Exceptional silicified examples that preserve internal details of M. oviformis are known from central Kentucky (Pojeta, 1971).
Range. Modiolodon is found in Upper Ordovician to Permian strata worldwide (Cox and others, 1969). In Kentucky, it is only found in Upper Ordovician strata. Specimens of Modiolodon can be found in parts of the Bluegrass Region of central Kentucky in the Lexington Limestone, Bull Fork, and Drakes formations.
Life and Paleoecology. Modiolodon was a filter-feeding, marine bivalve. It was probably epifaunal, which means it lived on the seafloor or on other organisms, such as bryozoan colonies. Silicified specimens from the Loganna Member of the Lexington Limestone in central Kentucky preserve byssal attachments (Pojeta, 1971), so these shells could attach to the sea floor or other shells with byssal threads. The Loganna Member was deposited in relatively deeper waters of the Lexington Limestone seas (Cressman, 1973), during a rise in sea level (Pope and Read, 1997; Coates and others, 2010). In the Loganna, Modiolodon oviformis is found with the bivalves Deceptrix and Whiteavesia, and the gastropods (snails) Liospira and Lophospira.
- Atlas of Ordovician Life, 2021, Modiolopsis: www.ordovicianatlas.org/atlas/mollusca/bivalvia/modiomorphida/modiomorphidae/modiolopsis/ (accessed, 6/15/2021)
- Coates, J.W., Ettensohn, F.R., Rowe, H.D., Finney, S.C. and Berry, W.B.N., 2010, Correlations across a facies mosaic within the Lexington Limestone of central Kentucky, USA, using whole-rock stable isotope compositions, in Finney, S.C., and Berry, W.B.N., eds., The Ordovician Earth System: Geological Society of America Special Papers, no. 466, p. 177–193.
- 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.
- Cressman, E.R., 1973, Lithostratigraphy and depositional environments of the Lexington Limestone (Ordovician) of central Kentucky: U.S. Geological Survey, Professional Paper 768, 61 p.
- McFarlan, A.C., 1931, Ordovician strata, in Jillson, W.R., ed., Paleontology of Kentucky: Kentucky Geological Survey, ser. 6, v. 36, p. 47–165.
- Pojeta, J.J., 1971, Review of Ordovician pelecypods: U.S. Geological Survey, Professional Paper 695, 46 p., 20 plates.
- Pope, M.C., and Read, F.J., 1997, High-resolution surface and subsurface sequence stratigraphy of the Middle to Late Ordovician (late Mohawkian-Cincinnatian) foreland-basin rocks, Kentucky and Virginia: American Association of Petroleum Geologists Bulletin, v. 81, p. 1866–1893.