Fossil of the month: Acrocyathus
This month’s fossil looks like a strange honeycomb but is actually a colonial coral. This particular coral is also a guide fossil. Guide fossils are fossils that are characteristic of a named rock unit, so when they are found, they are useful for identifying the unit. This month’s fossil, Acrocyathus, is common in the Mississippian St. Louis Limestone.
Description. Acrocyathus is a colonial rugose coral. Acrocyathus colonies (corallums) are hemispherical to massive and slightly irregular in shape. They generally are 5 to 20 centimeters wide and 5 to 15 centimeters tall. Individual corallites (tubes) within the colonies average 15 millimeters in width and are capped by polygonal (honeycomb-shaped) or semicircular cups, which have a central raised bump (axial boss), and 23 to 32 vertical spoke-like plates (septa) (Easton, 1973; Sando, 1982). Details of internal structures such as tabulae (plates inside corallites) are also important for distinguishing Acrocyathus from other colonial rugose corals. Each calyx was where a coral polyp lived in life. Specimens in Kentucky are commonly silicified, which means they have been replaced with quartz (chert). The amount and type of replacement dramatically influences the details preserved in specimens.
Species. Two species of Acrocyathus are reported from Kentucky, and both are common in the St. Louis Limestone: A. floriformus and A. proliferum. The main difference between the two is the shape of the calyx and corallites, and how the individual corallite tubes are connected. Colonies with polygonal calices and corallites in which corallites are fused together (all sides are touching, termed cerioid), are A. floriformus. Colonies in which corallites are more rounded, and partly touching (termed pseudo-cerioid or pseudo-phacelioid) or separated with thin connecting tubes (phaceloid) are A. proliferum. (Easton, 1973; Sando, 1982; Ettensohn and Johnson, 2009).
Acrocyathus floriformus was historically called by several different names, which were subsequently determined to be the same as A. floriformus (Easton, 1973; Sando, 1982); Lithostrotion canadense (in part), Lithostrotion basaltiforme (because of its polygonal corallites, which were reminiscent of the shape that the igneous rock basalt sometimes forms), Lithostrotionella castelnaui, and Lithostrotion mamillaris (in part). Acrocyathus proliferum was historically called Lithostrotion mamillare (in part), Lithostrotion canadense (in part), and Lithostrotion proliferum (Sando, 1982). These older names appear on many Kentucky geologic quadrangle maps.
Range. Acrocyathus is found nearly worldwide in Mississippian strata, although it is most common in the Midwest and eastern United States (Sando, 1982). In Kentucky and the Midwest, Acrocyathus is most common in the St. Louis Limestone, which is middle Mississippian in age. The top of the St. Louis is the top of the Meramecian Stage of the Mississippian. The St. Louis Limestone is approximately 330 million years old. Both A. floriformus and A. proliferum are considered guide fossils for the St. Louis (Weller, 1926, 1931; McFarlan, 1943). The St. Louis Limestone occurs in at least 30 counties in Kentucky. It can be found in southern Kentucky along part of the Tennessee border, in south-Kentucky, and in a narrow belt along the edge of the Mississippi Plateaus physiographic region in east-central and west-central Kentucky.
Paleoecology. The St. Louis Limestone was deposited in shallow seas under subtidal conditions. Acrocyathus colonies are common in several layers of the St. Louis, commonly occurring with colonies of the pipe-organ coral Syringopora virginica. The St. Louis also contains other marine fossils such as crinoids, brachiopods, bryozoans, echinoid plates, and gastropods (Ettensohn and others, 1984; Dever and Moody, 2002; Ettensohn and Johnson, 2009).
- Dever, G.R., and Moody, J.R., 2002, Bronston and Burnside Members: Subdivision of the St. Louis Limestone in south-central Kentucky: Kentucky Geological Survey, ser. 12, Report of Investigations 9, 18 p., https://doi.org/10.13023/kgs.ri09.12.
- Easton, W.H., 1973, On the tetracorals Acrocyathus and Lithostrotionella and their septal morphology: Journal of Paleontology, v. 47, no. 1, p. 121–135.
- Ettensohn, F.R., and Johnson, W.K., 2009, Carboniferous coral succession in the Appalachian basin, in Greb, S.F., and Chesnut, D.R., Jr., eds., Carboniferous of the Appalachian and Black Warrior Basins: Kentucky Geological Survey, ser. 12, Special Publication 10, p. 94–97.
- Ettensohn, F.R., Rice, C.L., Dever, G.R., Jr., and Chesnut, D.R., 1984, Slade and Paragon Formations—New stratigraphic nomenclature for Mississippian rocks along the Cumberland Escarpment: U.S. Geological Survey, Bulletin 1605–B, 37 p.
- McFarlan, A.C., 1943, Geology of Kentucky: Lexington, University of Kentucky, 531 p.
- Sando, W.J., 1982, Revision of the rugose coral genus Lithostrotionella Yabe and Hayasaka: Journal of Paleontology, v. 56, no. 1, p. 236–239.
- Weller, J.M., 1926, Faunal zones in the standard Mississippian section: Journal of Geology, v. 34, p. 320–335.
- Weller, J.M., 1931, The Mississippian fauna of Kentucky, in Jillson, W.R., ed., Paleontology of Kentucky: Kentucky Geological Survey, ser. 6, v. 36, p. 251–290.