Fossil of the month:  Platycrinites penicillus

 A picture containing invertebrate, rock, arthropod, stone

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Some fossils represent organisms which only lived for a short time. If these fossils are distinctive and widespread enough, they may be useful guide fossils for the rock unit(s) in which they are found, as well as the relative age of the unit(s). The small, football- or lens-shaped fossils with tiny spines around the edge in the picture above are examples of a guide fossil from western Kentucky and surrounding areas. This month’s fossil of the month is Platycrinites penicillus.

Description. Platycrinites is a genus of crinoid. Crinoids are sometimes called “sea lilies” because they look like underwater flowers. They are, however, small animals related to starfish, rather than plants. Most Paleozoic crinoids consisted of a cup or calyx, comprised of many plates, to which many arms were attached. The arrangement of plates is specific for each genus. The arms are composed of even smaller, stacked, ring-like interlocking plates. The cup and arms together are called a crown. The cup is attached to a stem or column comprised of smaller columnals. Platycrinites has a small (1 cm or less), bowl-shaped cup composed of two rows of plates; a basal circle overlain by radial plates. The arms branch upward from the radial plates (Ubaghs and others, 1978; Ausich and Kammer, 2000, 2009).

Illustration of a Platycrinites
crinoid showing its parts, and fossil of Platycrinites penicillus (a
species of the genus) with a preserved cup and partial arms.

Illustration of a Platycrinites crinoid showing its parts, and fossil of Platycrinites penicillus (a species of the genus) with a preserved cup and partial arms. Fossils of crinoids usually consist of isolated plates and fragments.

Platycrinites penicillus basal circlet plates from the base of the crinoid’s cup.

Platycrinites penicillus basal circlet plates from the base of the crinoid’s cup. The first three are loose basal circlets. The fourth image is a basal circlet in bedrock. The circular depression in the center of the specimen is where the crinoid column attached to the base of the cup. These specimens are from the Kentucky Geological Survey paleontological collection and collected from the Ste. Genevieve Limestone in western Kentucky. Scale in centimeters.


Many crinoids have columns composed of circular, ring-like columnals, which are not generally distinctive. Platycrinites and a few close cousins have distinctive, helically twisting columns. Platycrinites has circular columnals towards the top of its column, but most of its column is composed of lens-shaped columnals. Columnals are generally less than half a centimeter in width, and often are only tenths of a centimeter wide. When viewed from the top, each columnal has a small ridge along the central long axis and a slightly raised ridge along the outer margin. The ridge along the axis sometimes looks like a tiny airplane propeller.  When viewed from the side, P. penicillus columnals commonly have small spines or broken bases of spines preserved along their outer edges. The columnals are not always as well preserved as the example shown here, but they are distinctive enough to make this crinoid a useful guide fossil, when it can be found. Because each crinoid column consists of hundreds of columnals, and columns commonly broke apart into individual columnals when the crinoids died, hundreds of fossils from each crinoid could be spread around the Ste. Genevieve seas. The individual columnals of P. penicillus are widespread and useful guide fossils, if you have a sharp eye and know what to look for!

Tiny, loose, Platycrinites
penicillus column fragments.

Tiny, loose, Platycrinites penicillus column fragments. Note the characteristic twisting of the columns, and the tiny spines on the outer edge of each columnal. Specimen from the Kentucky Geological Survey paleontological collection. Collected from the Ste. Genevieve Limestone in Christian County.

Platycrinites penicillus column
fragments and lens-shaped columnals with tiny spines in Ste. Genevieve Limestone bedrock.

Platycrinites penicillus column fragments (white arrows) and lens-shaped columnals with tiny spines (blue arrows) in Ste. Genevieve Limestone bedrock. Note twisting of some of the column fragments. Specimen from the Kentucky Geological Survey paleontological collection. Collected from the Ste. Genevieve Limestone in Christian County.



At least six species of Platycrinites are known from Kentucky, and several additional species are known which were originally considered Platycrinites, but were later placed in other similar genera. It’s important to recognize that there are many species of Platycrinites, because not all Platycrinites are the guide fossil, P. penicillus. Species were defined based on differences in the relative size and details of the cup, ornamentation or lack of ornamentation on plates, proportions (relative heights and widths) of basal circlets and radial plates, and the number of arms on preserved crowns (Ausich and Kammer, 1990). Some units have multiple species. Some species can be identified by single plates. Others require more parts. This is problematic, since most crinoids are not preserved whole, but rather are found as isolated parts and column fragments, or columnals. Subtle differences between species can be difficult for non-specialists to identify and many fossil specimens can’t be identified to a specific species. Plates, columnals, and fragments of columns which are distinctive enough to be identified as the Platycrinites genera, but not to a species of Platycrinites, should be labeled as “Platycrinites sp.” Luckily, Platycrinites penicillus is the only species of this crinoid found in rocks younger than the Salem Limestone, so if you find a Platycrinites in rocks above the Salem Limestone, it is likely P. penicillus. Also, the spines around the edge of its columnals are distinctive. Having only one species of a genus in a rock unit with distinguishing characteristics helps to make it a useful guide fossil. Click here for more information about Platycrinites species.

Platycrinites penicillus was historically called Platycrinus penicillus (slightly different genus spelling). Also, P. penicillus is the same species as P. huntsvillae, which was the name used for a common crinoid fossil found near Huntsville, Alabama. The crinoids were described separately a long time ago. Later, comparisons of fossils showed they were the same species. In paleontology, when two fossils with different names are determined to be the same species, the species which was named first has priority. So now P. huntsvillae is called P. penicillus. Scientifically, P. huntsvillae is called a junior synonym of P. penicillus.

Platycrinites species table

Species of the crinoid genus, Platycrinites, and related crinoids reported from Kentucky. The black bars show the rock units in which specific species have been reported. Dashed lines between bars show range of species through units in which that species has not been reported.  The red bar shows the unit in which Platycrinites penicillus is most abundant. Data from Weller (1931), Ausich and Kammer (1990; 2009), and other references. The image is a twisting Platycrinites sp. column from the Fort Payne Formation in south-central Kentucky.



Platycrinites (the genus) ranges from the Lower Mississippian to Permian periods worldwide and is most common in the Mississippian (Ubaghs and others, 1978; Ausich and Kammer, 2009). In Kentucky, the genus Platycrinites is found only in Mississippian strata, from the New Providence Shale (or Member of the Borden Formation depending on region) to the lower part of the Renault Limestone. In western and south-central Kentucky, the species Platycrinites penicillus ranges from the uppermost Warsaw-Salem into the lower Renault Limestone. In eastern Kentucky, it ranges from the Upper St. Louis Limestone through the Ste. Genevieve Limestone (Ettensohn and others, 2003). The St. Louis and Ste. Genevieve are formations in western Kentucky, but are members of the Slade Formation (and were previously members of the Monteagle and Newman Limestones) in parts of south-central and eastern Kentucky (e.g., Ettensohn and others, 2004). Although the species, Platycrinites penicillus, can occur in several units, it is only common in the Ste. Genevieve Limestone. The Ste. Genevieve Limestone is 330-335 million years old. The Ste. Genevieve Limestone occur throughout the Mississippian Plateaus region of Kentucky in at least 24 counties. It is the rock unit in which most of the caves in the region have formed.

Platycrinites penicillus in Ste. Genevieve bedrock sample.

Platycrinites penicillus in Ste. Genevieve bedrock sample. Dashed white circle highlights basal circlet plate. Blue arrows point to individual lens-shaped columnals with distinctive spines or bumps around the edges. White arrows point to column fragments with spines. Specimen is from the Kentucky Geological Survey paleontological collection and was collected in western Kentucky.


Guide fossil

In western Kentucky, the St. Louis Limestone and overlying Ste. Genevieve Limestone often look very similar, which can make it difficult to pick the boundary between the two units in the field. Where they have similar appearance, certain guide fossils are used to help differentiate the two units. The corals Acrocyathus floriformus (previously Lithostrotion basaltiforme) and Acrocyathus proliferum (previously Lithostrotion proliferum) are common in the St. Louis Limestone, but not the Ste. Genevieve Limestone. Platycrinites penicillus is common in the Ste. Genevieve Limestone. It sometimes occurs in the St. Louis Limestone, but it is not common. Hence, P. penicillus was commonly used to help pick the base of the Ste. Genevieve Limestone (Butts, 1922; Weller, 1931; McFarlan, 1943; Swann, 1963; Willman, 1975; Rice and others, 1979; Sable and Dever, 1990; Dever, 1999; Ettensohn and others, 2003). The small brachiopod, Pugnoides ottumwa, and the coral, Scoenophyllum aggregatum, are also used as guide fossils in combination with Platycrinites penicillus for the Ste. Genevieve Limestone.

Not only was the base of the Ste. Genevieve Limestone sometimes identified by the stratigraphically lowest (oldest) occurrence of common Platycrinites penicillus, but the highest (youngest) occurrence of the crinoid, coupled with the lowest occurrence of two different crinoids, Agassizocrinus and Talarocrinus, traditionally was used by field geologists to define the top of the Meramecian and base of the Chesterian series (now stage) of the Mississippian in the Illinois Basin. The highest occurrence of P. penicillus in western and west-central Kentucky is in the upper Ste. Genevieve Limestone or Levias Member of the Renault Limestone (depending how the units are mapped) (e.g., Weller, 1931; Sable and Dever, 1990).

In historical publications, P. penicillus and the Ste. Genevieve Limestone were considered to be part of the uppermost Meramecian series (now stage) of the Mississippian. In 1987, however, the Chesterian-Meramecian boundary was moved from near the top or just above the Ste. Genevieve Limestone down to the base of the Ste. Genevieve Limestone based on other kinds of fossils. The Ste. Genevieve Limestone is now considered the lowest part of the Chesterian stage (Maples and Waters, 1987). This change doesn’t mean P. penicillus is no longer a useful guide fossil. On the contrary, now this little crinoid is a guide to help pick the Ste. Genevieve Limestone (a rock unit), as well as the base of the Chesterian (a time subdivision).

Platycrinites penicillus crowns and arm fragments from the Kentucky Geological Survey paleontological collection.

Platycrinites penicillus crowns and arm fragments from the Kentucky Geological Survey paleontological collection. Specimens are from the Ste. Genevieve Limestone in Christian County, western Kentucky.



During much of the Mississippian Period, Kentucky was covered by shallow seas. The Ste. Genevieve Limestone was deposited in shallow tropical seas, because the area that is now Kentucky was closer to the equator during the Mississippian than it is today (e.g., Ettensohn and others, 1984; Sable and Dever, 1990). The limestone contains common oolites, which are tiny, spherical, carbonate grains. Oolites only form in shallow, high-energy shoals similar to those found in the Bahamas today (Rao and Mann, 1972; Carr, 1973). Platycrinites penicillus was a marine crinoid, which lived between the shoals. Crinoids still live today, so we know that when Platycrinus penicillus was feeding, its arms curved outward into a large umbrella shape to capture tiny particles of food from the water. The lens-shaped columnals of its column allowed the column to twist and be more flexible than other crinoids that were living at the time (Riddle and others, 1989; Baumiller and Ausich, 1996). This may have allowed the crinoid to adjust the orientation of its arms in the shifting currents around the shoals. Platycrinites penicillus shared the Middle Mississippian sea floor with other marine creatures like corals, brachiopods, bryozoans, and bivalves, which are also found in the Ste. Genevieve Limestone.



Ausich, W.I. and Kammer, T.W., 1990, Systematics and phylogeny of the late Osagean and Meramecian crinoids Platycrinites and Eucladocrinus from the Mississippian stratotype region: Journal of Paleontology, v. 64, no. 5, p. 759–778.

Ausich, W.I., and Kammer, T.W., 2009, Generic concepts in the Platycrinitidae Austin and Austin, 1842 (class Crinoidea): Journal of Paleontology, v. 83, no. 5, p. 694–717.

Baumiller, T.K. and Ausich, W.I., 1996, Crinoid stalk flexibility: theoretical predictions and fossil stalk postures: Lethaia, v. 29, no. 1, p. 47-59.

Butts, C., 1922, The Mississippian Series of eastern Kentucky: Kentucky Geological Survey, Series 6, v. 7, 188 p.

Carr, D.D., 1973, Geometry and origin of oolite bodies in the Ste. Genevieve Limestone (Mississippian) in the Illinois Basin: Indiana Geological Survey, Bulletin 48, 80 p.

Dever, Jr., G.R., 1999, Tectonic implications of erosional and depositional features in upper Meramecian and lower Chesterian (Mississippian) rocks of south-central and east-central Kentucky: Kentucky Geological Survey, Series 9, Bulletin 5, 67 p.

Ettensohn, F.R., Rice, C.L., Dever, Jr., G.R. & Chesnut, D.R., 1984, Slade and Paragon formations – new stratigraphic nomenclature for Mississippian rocks along the Cumberland escarpment in Kentucky. U.S. Geological Survey Bulletin 1605, 37 p.

Ettensohn, F.R., Ausich, W.I., Kammer, T.W., Johnson, W.K., and Chesnut, D.R., Jr., 2003, Carboniferous echinoderm zonation in the Appalachian Basin, eastern USA, in Wong, Th. E. (ed.): Proceedings of the XVth International Congress on Carboniferous and Permian Stratigraphy. Utrecht, the Netherlands, 10–16 August 2003. Royal Netherlands Academy of Arts and Sciences, p. 177-189.

Maples, C.G., and Waters, J.A, 1987, Redefinition of the Meramecian/Chesterian boundary (Mississippian): Geology, v. 15, no. 7, p. 647–651.

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

Rao, C.P. and Mann, C.J., 1972, Quantitative environmental analysis of Ste. Genevieve Limestone (Mississippian), southern Illinois and eastern Missouri: The Journal of Geology, v. 80, no. 4, p. 464480.

Rice, C.L., Sable, E.G., Dever, Jr., G.R. & Kehn, T.M., 1979. The Mississippian and Pennsylvanian (Carboniferous) systems in the United States – Kentucky. U.S. Geological Survey Professional Paper 1110-F, p. F1–F32.

Sable, E.G., and Dever, G.R., Jr., 1990, Mississippian rocks in Kentucky: U.S. Geological Survey Professional Paper 1503, 125 p.

Swan, D. H., 1963, Classification of Genevievian and Chesterian (Upper Mississippian) rocks of Illinois: Illinois Geological Survey Report of Investigations, v.  216, 91 p.

Ubaghs, G., Moore, R.C., Rasmussen, W., Lane, N.G., Breimer, A., Strimple, H.L., Brower, J.C., Jeffords, R.M., Sprinkle, J., Peck, R.E., Macurda, D.B., Jr.,Meyer, D.L., Roux, M., Sieverts-Doreck, H., Fay, R.O., and Robsion, R.A., 1978, Part T—Echinodermata 2, in Moore, R.C., ed., Treatise of Invertebrate Paleontology: Geological Society of America and University of Kansas Press, v. 2-3, p. 403–1027 (v. 2 is p. 404–812, v. 3 is p. 813–1027).

Webster, G.D., 1999, Bibliography and Index of Paleozoic Crinoids, Coronates, and Hemistreptocrinoids 1758–1999: Geological Society of America on-line database,, accessed 2006.

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.

Willman, H.B., 1975, Handbook of Illinois stratigraphy: Illinois State Geological Survey, Bulletin 95, 261 p.


Text, photographs, and illustrations by Stephen Greb

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Last Modified on 2022-08-26
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