Fossil of the month - OctoberArachnophyllum

This month's fossil is a distinctive fossil coral composed of small sun-burst patterns. It is called Arachnophyllum, which means, spider group, in latin; possibly from the web-like sunburst patterns. Sometimes erosion causes successive layers of sunbursts to be exposed like small flowers inside each polygonal cup of this coral fossil.

Description. Arachnophyllum is a colonial or compound rugose coral Most rugose corals are solitary and sometimes called horn corals. Horn corals have only one cup atop their structure. The most common colonial corals are called tabulate corals which have different interior structures than rugose corals. Rugose corals, however, can also be colonial. Colonial rugose corals have many adjoining cups that formed mound-like structures.

Whole Arachnophyllum coral colonies (called corallums) generally were small (10-40 cm), with low-lying, mound (semi-hemispherical) shapes. Individual corallites (tubes or chambers capped by cups) within the colony are connected or joined to neighboring corallites. Each corallite has a broad calyx (cup or platform) with a small pit near the center on the outer surface of the colony. Calices (plural of calyx) are usually pentagonal, polygonal, or quadrangular in shape, but one species has subcircular shapes. Individual calices range from 0.7 to 6 cm in diameter (Foerste, 1931; Stumm, 1964; Laub, 1979). Very fine lines called septa radiate from the axial pits towards the margins of each calyx, forming the small, adjacent sunburst patterns.

General terms used to describe different parts of an Arachnophyllum coral (modified from Stumm (1964) and Hill (1981). Specimen is from the Louisville Limestone in Jefferson County, and is on display at the Kentucky Geological Survey.

Arachnophyllum calices have two general growth habits, which are distinctive for different species of Arachnophyllum. Cerioid calices have distinct boundaries, usually with a small wall (sometimes expressed as a raised ridge at the surface) between adjoining calices. Astreoid calices lack a distinct boundary or ridge between calices (Hill, 1981).

Cerioid and astreoid growth habits of coral calices, with examples of species of Arachnophyllum showing these habits (A. striatum from Davis, 1887, plate 121, no. 1. A. sinemurum from Davis, 1887, plate 121, no. 4).

Other features that define this genus and separate it from other colonial rugose corals are based on internal features, which can sometimes be seen in the side of broken specimens, but which require more explanation and generally some expertise to differentiate. See the Coral Treatise on Invertebrate Paleontology (Hill, 1981) for more details.

Strombodes vs. Arachnophyllum. Historical reports of fossils in Kentucky list the fossil coral Strombodes from Kentucky (e.g., Davis, 1887; Foerste, 1931; McFarlan, 1943). Most of those examples were from Silurian-age rock units. All of the Silurian species of Strombodes (at least 7) reported in Kentucky were subsequently reassigned to Arachnophyllum [see for example listings from Stumm (1964) and Laub (1979)]. These types of reassignments happen when a fossil genera is subdivided into two or more genera based on some differences that were not used to subdivide the genera previously. Strombodes is still a valid genera, but has some subtle internal differences from Arachnophyllum. More importantly, what would now be called Strombodes only occurs in Devonian strata (Hill, 1981). One species of Devonian-age Strombodes exists in Kentucky, but any Silurian reference of Strombodes from Kentucky, would now be considered Arachnophyllum.

More examples of Arachnophyllum corals from the Louisville Limestone in Jefferson County, Ky. Fossils are from the Kentucky Geological Survey paleontological collection.

Other types of Silurian colonial rugose corals. Many types of tabulate coral fossils occur in Silurian limestones with Arachnophyllum. Tabulate corals differ from rugose corals (like Arachnophyllum) in having much smaller (usually less than 0.3 cm) calices (as well as other details). The only other compound colonial rugose coral reported from Silurian strata in Kentucky is Entellophyllum, and it has a very different appearance than Arachnophyllum. Entellophyllum consists of groups of narrow tubes or cylinders (corallites) which are slightly separated or only partly touching. This type of growth habit is called phaceloid or fasiculate, and is easily distinguished from the joined (cerioid, asteroid growth) polygonal cups of Arachnophyllum.

Although it would be difficult to confuse the Silurian-age Arachnophyllum with Entellophyllum, several types of Devonian-age colonial rugose corals could easily be confused with Arachnophyllum, including Billingsastrea, Disphyllum, Hexagonaria, Prismatophyllum, and Strombodes. Differences between these various genera are based on details of their calices, septa within their calices, and internal structures. Some of these details are very subtle and require good specimens and usually some paleontological expertise to discern. See Stumm (1964) and Hill (1981) for more details. Knowing the location in which your specimen was found can aid in determining if it is a Silurian- or Devonian-age fossil, which is information that can help with identification.

Species. Several species of the fossil genus Arachnophyllum can be found in Kentucky. The most common species is A. striatum, which is known from the Louisville Limestone, in the greater Louisville area. At least five less common and less well known species, however, also occur in the Louisville Limestone. The seven known species reported in Kentucky occur in two broad shapes or groups. The first and most common (5 species) are forms that have a broad base and a sub-hemispherical, low-lying mound shape composed of many abutting calices (cups). The second grouping contains corals with only a few calices and an overall conical shape when viewed from the side. These expand upward from a common base (two species). In some ways, members of this second group look a little like a solitary horn coral that has two or three calices, rather than the usual single calyx. Differences in species are based on external and internal features, but external features can be used in well-preserved specimens for species assignment. Different species are based on the shape and size of the calices, calyx ornamentation, shape and size of axial pits, and presence or absence of a surface ridge or wall along the border of calices (cerioid vs. astreoid shape), among other details. See table for more information.

Images of some of the different species of Arachnophyllum found in Kentucky. These were originally described as species of Strombodes. Images of A pentagonum from Davis (1887), plate 121, no. 3; A. striatum from Davis (1887), plate 122, no. 1; A. quadrangulare from Davis (1887), plate 122, no. 3; A. mamillare from Davis (1887), plate 123, no. 4. See table for more details.

Range and geographic occurrence. Arachnophyllum has a global distribution but only occurs in Silurian strata. In Kentucky, it has been reported (historically as Strombodes) in the Brassfield Formation (Laub, 1976); Waco Member of the Crab Orchard Formation (Foerste, 1906; 1931, 1936; Ettensohn and others, 2013); and the Louisville Limestone (Foerste, 1931; McFarlan, 1943; Conkin and others, 1972; 1976). Although reported from the Brassfield, it is most common in the Waco Member of the Crab Orchard Formation and Louisville Limestone. Several different beds in the Louisville Limestone contain Arachnophyllum (Conkin and others, 1992; Brett and others, 2012).

Known ranges of different species of Arachnophyllum in Kentucky showing the units in which they have been found (based on data in Foerste (1931), Stumm (1964), and Laub (1979)). See species images and the species description table for more information on distinguishing the different species of Arachnophyllum shown in the chart. Arachnophyllum striatum and A. pentagonum are probably the most commonly reported species, and both are found in the Louisville Limestone in the greater Louisville area.

Silurian strata occur in a narrow belt on the outer margin of the Bluegrass Region, near the inner edge of the Knobs region in Kentucky. The Brassfield Formation is a dolostone that occurs at the base of Silurian strata in at least 16 counties in Kentucky. The Waco Member is a dolostone that occurs within the Crab Orchard Formation on the eastern side of central Kentucky. The Crab Orchard Formation occurs in at least 11 counties. The Louisville Limestone is exposed at the surface in the greater Louisville area (four counties).

Paleoecology. The Brassfield Dolomite (dolostone), Waco Member of the Crab Orchard Formation (also a dolostone), and Louisville Limestone (dolostone and limestone) are all rock units deposited in marine conditions when shallow seas covered what is now Kentucky during the lower and middle part of the Silurian Period. Arachnophyllum was a marine coral. Coral colonies appear to have been small, and were mostly low-lying mounds. Each calyx on the outer surface of the coral would have been home to a coral polyp. The tiny arms of the polyps would have collected tiny food particles from sea water similar to corals today. In the Louisville Limestone seas, small Arachnophyllum corals shared the sea floor with other tabulate and solitary rugose corals, stromatoporoid sponges, brachiopods such as Pentamerus, crinoids, gastropods, bryozoa, and trilobites.

Example of Arachnophyllum striatum from the Louisville Limestone in Jefferson County, Kentucky (top view, side view, and detail of calyx). Kentucky Geological Survey's paleontological collection.


Brett, C.E., McLaughlin, P.I., Schramm, T.J., Sullivan, N.B., and Thomka, J.R., 2012, Middle Paleozoic sequence stratigraphy and paleontology of the Cincinnati Arch: Part 1. Central Kentucky and Southern Ohio, in

Brett, C.E., Cramer, B.D., and Gerke, T.L., eds., International Geoscience Programme (IGCP) Project 591 Second Annual Meeting and First Foerste Symposium, 54 p.

Butts, C., 1915, Geology and mineral resources of Jefferson County, Kentucky: Kentucky Geological Survey, Series 4, v. 3, no. 2, 210 p.

Conkin, J.E., and Conkin, B.M., 1972, Guide to the rocks and fossils of Jefferson County, Kentucky, southern Indiana and adjacent areas: Louisville, Kentucky, University of Louisville Reproduction Services, 331 p.

Conkin, J.E., and Conkin, B.M., 1976, Guide to the rocks and fossils of Jefferson County, Kentucky, southern Indiana and adjacent areas: Louisville, Kentucky, University of Louisville Reproduction Services, second edition, 238 p.

Conkin, J.E., Conkin, B.M. and Kubacko, J., 1992, Ordovician, Silurian, and middle Devonian stratigraphy in northwestern Kentucky and southern Indiana: some reinterpretations: Ohio Department of Natural Resources, Division of Geological Survey, Field trip guidebook, Field trip 8 for the Geological Society of America meeting, Cincinnati, Ohio, October 26-29, 1992, 43 p, plus 19 plates.

Davis, W.J., 1887, Kentucky fossil corals, a monograph of the fossil corals of the Silurian and Devonian rocks of Kentucky, part 2 [part 1 was never published]: Kentucky Geological Survey, ser. 2, Monograph 2, 139 plates.

Ettensohn, F.R., Lierman, R., Mason, C.E., Andrews, W.M., Hendricks, R., Phelps, D.J. and Gordon, L.A., 2013, The Silurian of central Kentucky, USA: stratigraphy, palaeoenvironments and palaeoecology: Memoirs of the Association of Australasian Palaeontologists, v. 44, p.159-189.

Foerste, A.F., 1906, The Silurian, Devonian and Irvine Formations of east-central Kentucky: Kentucky Geological Survey, Bulletin 7, 369 p.

Foerste, A.F., 1931, The Silurian fauna of Kentucky, in Jillson, W.R., ed., The paleontology of Kentucky: Kentucky Geological Survey, Series 6, v. 36, p. 167-213.

Hendricks, R.T., 1996, The stratigraphy and sedimentology of the Silurian (Wenlockian) Laurel Member of the Salamonie Dolomite in Kentucky and adjacent southeastern Indiana: Lexington, University of Kentucky, M.S. thesis, 322 p.

Hill, D., 1981, Part F-Coelenterata-Supplement 1-Rugosa and Tabulata, in Teichert, C., ed., Treatise of Invertebrate Paleontology: Geological Society of America and University of Kansas Press, two volumes, 762 p.

Laub, R.S., 1975, The ancestry, geographical extent, and fate of the Brassfield coral fauna (middle Llandovery, North America): Bulletins of American Paleontology, v. 67, no. 287, p. 273-286.

Laub, R.S., 1979, The corals of the Brassfield Formation (mid-Llandovery; lower Silurian) in the Cincinnati Arch region: Bulletins of American Paleontology, v .75, no. 305, p. 1-457.

McFarlan, A.C., 1943, Geology of Kentucky: University of Kentucky, Lexington, 531 p.

Phelps, D.J., 1990, Characterization and paleoenvironmental interpretation of the Louisville Limestone (Silurian), west-central and western Kentucky: University of Kentucky, Lexington, M.S. thesis, 211 p.

Stumm, E.C., 1964, Silurian and Devonian corals of the Falls of the Ohio: Geological Society of America Memoirs, v. 93, 184 p.

Text and illustrations by Stephen Greb

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