Fossil of the month: Bellerophon
This month’s fossil is an unusually coiled shell, which may or may not be a snail, and which preserves hints to its original color patterns! Meet Bellerophon (Pharkidonotus).
Description. Bellerophon is a coiled shell. Bellerophon and its close relatives (bellerophontids) have planispiral (also termed isotrophic) coiled shells, which means a plane of symmetry divides the aperture and whorl in half, and the two sides are mirror images of each other. This type of coiling makes the shells bilaterally symmetrical. Bellerophon’s shell widens dramatically towards its aperture (opening) and flares outward. The outer lip has a medial sinus (indentation) or slit, which generates and is attached to a narrow, medial ridge, termed a selenizone (Knight and others, 1960).
Traditionally, bellerophontids are classified as gastropods (snails), but the planispiral coiling of Bellerophon has made it questionable as a snail (Knight and others, 1960; Wahlman, 1992; Frýda and others, 2008). Most snail shells are coiled around a central axis, and the aperture (opening) is to the side of the axis, resulting in a lack of bilateral symmetry across the shell. Bellerophon, however, has planispiral coiling and bilateral symmetry, which is more common in nautiloid cephalopods, such as the modern chambered Nautilus, than in gastropods. In fact, from the side, Bellerophon could easily be mistaken for a coiled cephalopod. Inside the shell, however, Bellerophon lacks the internal chambers that characterize cephalopods, so it is not a cephalopod. Also, some details of its structure, such as patterns of muscle scars on the inside of its shell are more typical of gastropods than other mollusks (Knight and others, 1960; Wahlman, 1992), with the possible exception of a lesser known class of mollusks called monoplacophores (Frýda and others, 2008). In a recent revision of fossil gastropod classification, Bouchet and others (2017) listed Bellerophon and similar planispiral shells as uncertain gastropods or monoplacophores. Hence, at this time whether Bellerophon is a snail or not is uncertain, but the soft-bodied organism inside the shell was probably snail-like.
Why does this fossil have a second name in parentheses? Bellerophon is divided into two subgenera: Bellerophon (Bellerophon) and Bellerophon (Pharkidonotus). The name before the parentheses is the genus name; the names in parentheses are subgenus names. Subgenus designations are used when organisms have features that are slightly different from each other, but are not distinct enough to be considered separate genera. In the case of Bellerophon, both genera are planispirally coiled, and both have a broad aperture that flares outward, with a medial slit on its outer lip.
Bellerophon (Pharkidonotus) is differentiated from Bellerophon (Bellerophon) by its ornamentation, the development of a medial ridge and selenizone, and details of its inductura, which is a smooth shell layer on the inner lip of the aperture formed by the mantle. Bellerophon (Pharkidonotus) has a distinctive elevated ridge (carina, or selenizone, along the plane of symmetry generated by its medial slit on its outer lip, and is ornamented with growth lines (collaboral undulations) curving toward the ridge. It also has a thicker inductura layer than Bellerophon (Bellerophon) (Knight and others, 1960).
Since most specimens of Bellerophon (Bellerophon) and Bellerophon (Pharkidonotus) found in Kentucky have at least partially broken outer lips, the characteristic outward flaring of these shells and medial slit are not always preserved. Rather, the support structures of the outer lip, termed the columella, are usually preserved, sometimes as strut-like projections or wings on either side of the inner whorl.
Species. In Kentucky, two species of Bellerophon (Pharkidonotus) have been reported from Middle Pennsylvanian strata: B. (P.) crassus and B. (P.) percarinatus (Morse, 1931; Chesnut, 1991). Of the two, B. (P.) percarinatus is more common. A third species, B. (P.) girtyi, has been reported from the Late Pennsylvanian Brush Creek Limestone in Boyd and Lawrence Counties (Martino and others, 1996). In neighboring Ohio and West Virginia, additional species, including B. (P.) breveri, B. (P.) labioreflexus, and B. (P.) tricarinatus, have been reported from Middle and Late Pennsylvanian marine units (Hoare and others, 1997). The species of Bellerophon (Pharkidonotus) differ mostly in their ornamentation (shape and size of the midline ridge [selenizone] and growth lines [lirae]) and in the size and amount of flaring in their apertures (which unfortunately, are not commonly preserved). Bellerophon (Bellerophon) has many more species than Bellerophon (Pharkidonotus) does because it has a much longer range.
Preserved color banding. Chesnut and Slucher (1990) reported preserved color banding in Bellerophon (Pharkidonatus) cf. B. percarinatus. Twenty specimens found in the Kendrick Shale Member near Wheelwright, Kentucky, had the same pattern of darkening along the aperture and along the axis of their shells. The similarity of the patterns suggests they are related to original patterns in the shells rather than some type of darkening formed during fossilization. This does not mean the current color of the shells was the original color of the shells, because color pigments and other sources of color in shells can fade or change with time. It does, however, show where patterns of different colors were on the original shells. Although not mentioned in the paper, the position of the darker patches on the inner whorl of these specimens is at the inductura, which is where the soft, fleshy mantle of the snail-like organism would likely have been in life.
Preserved color banding is rare in fossils but has been recorded in Bellerophon and many other Paleozoic gastropods, including many examples from Middle and Late Pennsylvanian strata. Sturgeon (1964) noted color banding in specimens of Bellerophon (Pharkidonotus) percarinatus and Bellerophon (Bellerophon) graphicus from the Late Pennsylvanian Cambridge Shale and Columbiana Limestone in Ohio. The specimens of B. (P.) percarinatus from Ohio had two dark bands on either side of the raised ridge along the axis of the shell.
Range and geographic occurrence. In Kentucky and surrounding areas, Bellerophon is a long-ranging fossil. It has been reported from Late Ordovician through Late Pennsylvanian time. The subgenus Bellerophon (Pharkidonotus) is more restricted, being found only in Pennsylvanian strata of the coal fields. In Kentucky, Bellerophon (Pharkidonotus) has been found in the Middle Pennsylvanian Kendrick Shale Member of the Hyden Formation, and Magoffin Shale Member of the Four Corners Formation (Chesnut, 1991), and the Late Pennsylvanian Brush Creek Limestone of the Conemaugh Formation (Martino and others, 1996). In neighboring Ohio and West Virginia, Bellerophon (Pharkidonotus) has been reported from these units and several additional Middle to Late Pennsylvanian marine units (Hoare and Sturgeon, 1978; Hoare and others, 1997). These Pennsylvanian shales and limestones are 310 to 320 million years old.
Life and paleoecology. Bellerophon (Pharkidonotus) was an unusual snail or snail-like mollusk (either a gastropod or monoplacophoran). Many Paleozoic bellerophontids appear to have inhabited nearshore, shallow subtidal and intertidal marine environments (Wahlman, 1992). Bellerophon (Pharkidonotus) is found in dark gray shales of the Kendrick and Magoffin Shale Members, which were deposited in subtidal, offshore, muddy marine environments. Wahlman (1992) inferred that bellerophontids which lived in shallow waters likely fed by browsing on algae. Those which lived in deeper marine waters with soft muddy bottoms and in turbid delta-front areas (possibly like those inferred for parts of the Kendrick and Magoffin Members) may also have been detritus feeders and scavengers. They shared the Pennsylvanian seafloor with other gastropods, bivalves (clams), and brachiopods.
- Bouchet, P., Rocroi, J.-P., Hausdorf, B., Kaim, A., Kano, Y., Nützel, A., Parkhaev, P., Schrödl, M., and Strong, E.E., 2017, Revised classification, nomenclator and typification of gastropod and monoplacophoran families: Malacologia, v. 61, nos. 1–2, p. 1–526, https://doi.org/10.4002/040.061.0201.
- Chesnut, D.R., Jr., 1991, Paleontological survey of the Pennsylvanian rocks of the Eastern Kentucky Coal Field: Part 1, Invertebrates: Kentucky Geological Survey, ser. 11, Information Circular 36, 71 p., https://doi.org/10.13023/kgs.ic36.11.
- Chesnut, D.R., Jr., and Slucher, E.R., 1990, Color-banded gastropods from the Kendrick Shale Member (Middle Pennsylvanian, Westphalian B) of eastern Kentucky: Journal of Paleontology, v. 64, no. 3, p. 475–477.
- Frýda, J., Nützel, A., and Wagner, P.J., 2008, Paleozoic gastropoda, in Ponder, W.F., and Lindberg, D.R., eds., Phylogeny and evolution of the Mollusca: University of California Press, p. 239–270.
- Hoare, R.D., and Sturgeon, M.T., 1978, Color patterns on Pennsylvanian gastropods from Ohio: Ohio Journal of Science, v. 78, no. 1, p. 3–10.
- Hoare, R.D., Sturgeon, M.T., and Anderson, J.R., Jr., 1997, Pennsylvanian marine gastropods from the Appalachian Basin: Journal of Paleontology, v. 76, p.1019–1039, https://doi.org/10.1017/S0022336000036003.
- Knight, J.B., Cox, L.R., Batten, R.L., and Yochelson, E.L., 1960, Systematic descriptions (Gastropoda), in Moore, R.C., Treatise on invertebrate paleontology. Part I, Mollusca 1: Lawrence, University of Kansas Press, p. 1169–1324.
- Martino, R.L., McCullough, M.A., and Hamrick, T.L., 1996, Stratigraphic and depositional framework of the Glenshaw Formation (Late Pennsylvanian) in central Wayne County, West Virginia: Southeastern Geology, v. 36, no. 2, p. 65–83.
- Morse, W.C., 1931, The Pennsylvanian invertebrate fauna of Kentucky, in Jillson, W.R., ed., The Paleontology of Kentucky: Kentucky Geological Survey, ser. 6, p. 295–348.
- Sturgeon, M.T., 1964, Allegheny fossil Invertebrates from eastern Ohio: Gastropoda: Journal of Paleontology, v. p.189–226.
- Wahlman, G.P., 1992, Middle and Upper Ordovician symmetrical univalved mollusks (Monoplacophora and Bellerophontina) of the Cincinnati Arch Region: U.S. Geological Survey Professional Paper 1066-O, 213 p., 45 plates, https://doi.org/10.3133/pp1066O.