Fossil of the month: Tentaculites
Some fossils are so small that they are easily overlooked. An example found in Kentucky is Tentaculites. Generally the size of a small eyeglass screw, this fossil has no apparent modern ancestor and scientists are uncertain what type of organisms made these tiny shells.
Description. Tentaculites are tiny (1-40 mm), thin, ribbed, conical shells. Called “conchs,” their shells may be straight to slightly curved and have radial symmetry. The widest end of the conch has an opening (aperture) in which some type of invertebrate organism lived. Conchs are ornamented with thin ring-like ribs, which are raised from the surface, making the fossil look like a tiny screw, or sometimes like a tiny crinoid (a different kind of fossil). Spaces between rings may have even smaller transverse markings called “lirae.” Under a microscope, cut specimens show the conch is separated into tiny chambers called “camerae” by thin walls called septa. These structures are similar to those of cephalopods, but they are different because the chambers are not connected by a tube (Fisher, 1962; Bergström, 1996).
Tentaculites sterlingensis from the Ordovician Bull Fork Formation in Mason County, Kentucky. Sample from the Kentucky Geological Survey paleontological collection.
Species. Three species of Tentaculites are known from Kentucky. Two occur in Upper Ordovician strata and one occurs in the Devonian. Each Kentucky species is differentiated by size and the spacing of rings on the conchs. These features require good specimens and a hand lens or magnifying glass to see. The Upper Ordovician species are T. richmondensis and T. sterlingensis. Tentaculites richmondensis is 25 to 30 mm (2.5 to 3 cm) long and has relatively widely spaced, thin ribs (rings); 7 to 10 ribs per 1 cm (Bergström, 1996). Tentaculites sterlingensis is smaller (10 to 15 mm, 1 to 1.5 cm) and tends to have more closely spaced ribs (rings); 20 to 25 ribs every 1 cm (Bergström, 1996), Some specimens may have more than 30 ribs per cm. The Devonian species, T. scalariformis, is 20-45 mm (2-4.5 cm) long. Its ribs (rings) are separated by spaces, which may exhibit many finer transverse lines (called lirae); although these are not preserved in all specimens. T. scalariformis has 10 to 15 rings per 1 cm (Bergström, 1996). Some specimens may have as many as 20 rings per cm.
Other similar small conical shells. Tentaculites can look like tiny crinoid stems, except that they are pointed at one end. Two other tiny conical shells can be found in Kentucky, which might be confused with Tentaculites; Cornulites and Styliolina. Cornulites can be similar in size to Tentaculites but is commonly shorter. Like Tentaculites, it is ribbed, although individual ribs may be relatively wider than for Tentaculites. Cornulites tends to have a larger angle of widening from its base than Tentaculites so it may appear more conical than tubular (Fisher, 1962). Cornulites also sometimes group or cluster from a single or similar point of attachment, while Tentaculites doesn’t. Cornulites are commonly found attached to other shells such as brachiopods, crinoids, and gastropods (Morris and Felton, 1993), while Tentaculites are not.
Styliolina is generally smaller in size than Tentaculites, has a smooth shell rather than a surface with raised rings, and has a small, tear-dropped shaped bulb, rather than a point, at its narrow end (Fisher, 1962; Bergström, 1996).
Range and geographic occurrence. The genus Tentaculites ranges from the Upper Ordovician to Upper Devonian and is found around the world (Fisher, 1962; Bergström, 1996). In Kentucky, Tentaculites sp. has been reported from the Upper Ordovician Camp Nelson, Kope, and Bull Fork Formations in the central part of the state. The species, T. richmondensis and T. sterlingensis are most common in the Bull Fork Formation (Bergström, 1996; Meyer and Davis, 2009; Holland, 2016). Tentaculites scalariformis has been found in the Devonian Jeffersonville and Sellersburg Limestones near Louisville. Cornulites, which can be confused with Tentaculites, also occurs in the Kope and Bull Fork Formations, as well as the Silurian Drakes and Bisher Formations, but does not occur in Devonian strata in Kentucky. Styliolina has been reported, but is rare, in Devonian black shales (Ohio Shale, New Albany Shale) in Kentucky (Savage, 1930; Barron and Ettensohn, 1981).
Tentaculites and bryozoan and brachiopod shell debris, from the Bull Fork Formation, Kentucky Geological Survey paleontological collection.
An uncertain affinity. Tentaculites is scientifically defined as a genus with species and placed in a family and order. However, as scientists are uncertain what type of organism made these shells, its class and placement of its class into a higher phylum is uncertain. For the most part, Tentaculites used to be interpreted as a pteropod (free-swimming sea snail), but this affinity was later questioned. In the Treatise on Invertebrate Paleontology (Fisher, 1962), Tentaculites and similar conical shells, were placed in a new class called Cricoconarida, which was placed in the phylum Mollusca, inferring Tentaculites had an affinity with snails, clams, and squids. Not everyone agreed with this placement, and subsequent researchers, have commonly placed Tentaculites in its own class (Tentaculitoidea) and left its phylum placement as uncertain or at least questionable. Currently, Tentaculites is debated as some type of brachiopod relative, without valves (e.g., Towe, 1978); a phoronid (“horseshoe”) worm (e.g., Towe, 1978; Vinn and Motus, 2008); or like Cornulites, a tiny tube worm (e.g., Vinn, 2010) or some type of jellyfish- or coral-like cnidarian with a shell (e.g., Herringshaw and others, 2007). Because shells are small, no soft parts are known, and no living relative exists, the true affinity of these shells remains a scientific mystery. Historical discussions about the possible phylum affinities of these creatures hinge on details of the shells, which can be found in Fisher (1962), Larsson (1979); Wittmer and Miller (2011), Taylor and others (2012),and Vinn and Zatoń (2012).
Life and paleoecology. Tentaculites (and other tentaculitoids) are only found in marine strata so scientists infer they lived in marine seas. The tiny invertebrate organism within the shell may have had tentacles like some marine worms and jellyfish, but that is uncertain. Some species have thicker conchs than others. Thick conchs may have lived on (benthic), or in (epifaunal) the sea floor, while thinner conchs may represent swimming (nektic) or floating (planktic) forms (Bergström, 1996). How they may have swam or floated, and the orientation of the shell if swimming or floating, is uncertain (Fisher, 1971; Bergström, 1996; Cornell and others, 2003; Wittmer and Miller, 2011)? Perhaps, future fossil finds will help to unravel some of the mysteries of this enigmatic little fossil.
Barron, L.S., and Ettensohn, F.R., 1981, Paleoecology of the Devonian-Mississippian black shale sequence in eastern Kentucky with an atlas of some common fossils: Morgantown, West Virginia, U.S. Department of Energy, Morgantown Energy Technology Center, Final report to U.S. Department of Energy contract DE-AC21-76ET12040, DOE/ET/12040-151, 75 p.
Bergström, S.M., 1996, Tentaculitoids, in Feldmann, R.M., and Hackathorn, M., eds., Fossils of Ohio: Ohio Division of Geological Survey, Bulletin 70, Chapter 20, p. 282–283.
Cornell, S.R., Brett, C.E., and Sumrall, C.D., 2003, Paleoecology and taphonomy of an edrioasteroid dominated hardground association from tentaculitid limestones in the Early Devonian of New York: a Paleozoic rocky peritidal community: Palaios, v. 18, p. 212–224.
Fisher, D.W., 1962, Small conoidal shells of uncertain affinities, in Hass, W.H., Häntzchel, W., Fisher, D.W., Howell, B.F., Rhodes, F.H.T., Muller, K.J., and Moore, R. C., eds., Part W–Miscellanea, Treatise of Invertebrate Paleontology: Geological Society of America and University of Kansas Press, p. 98–143.
Herringshaw, L.G., Thomas, A.T. and Smith, M.P., 2007, Systematics, shell structure and affinities of the Palaeozoic Problematicum Cornulites: Zoological Journal of the Linnean Society, v. 150, no. 4, p. 681–699.
Holland, S., 2016, UGA Stratigraphy Lab website: https://strata.uga.edu/cincy/fauna/tentaculitoidea/Tentaculites.html, accessed 4/12/2022.
Larsson K., 1979, Silurian tentaculitids from Gotland and Scania: Fossils and Strata 11: p. 1–180.
Meyer, D. L., and Davis, R.A., 2009, A sea without fish: Life in the Ordovician sea of the Cincinnati region: Indiana University Press, 346 p.
Morris, R.W. and Felton, S.H., 1993, Symbiotic association of crinoids, platyceratid gastropods, and Cornulites in the Upper Ordovician (Cincinnatian) of the Cincinnati, Ohio region: Palaios, v. 8, no. 5, p. 465–476.
Savage, T.E., 1930, The Devonian rocks of Kentucky: Kentucky Geological Survey, ser. 6, v. 33, p. 1–161.
Towe, K.M., 1978, Tentaculites: evidence for a brachiopod affinity?: Science, v. 201 (4356), p. 626–628.
Vinn, O. and Mõtus, M.A., 2008, The earliest endosymbiotic mineralized tubeworms from the Silurian of Podolia, Ukraine: Journal of Paleontology, v. 82, no. 2, p. 409-414.
Vinn, O., and Zatoń, M., 2012, Phenetic phylogenetics of tentaculitoids–extinct, problematic calcareous tube-forming organisms: GFF, v. 134 (no. 2), p. 145–156.
Wittmer, J.M. and Miller, A.I., 2011, Dissecting the global diversity trajectory of an enigmatic group: The paleogeographic history of tentaculitoids: Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 312 (no. 1-2), p. 54–65.
Text and illustrations by Stephen Greb