Fossil of the month: Calamites

This month’s fossil is one of the most common fossils in the Eastern Kentucky Coal Field. It is the fossil horsetail rush, Calamites.

Description. Calamites is a fossil “horsetail” or “scouring” rush. Rushes are reed-like plants with jointed stems. They belong to a class of plants called sphenopsids. Modern sphenopsids include Equisetum. The scientific name, Calamites, is used to describe trunks, stems, and branches of an extinct type of sphenopsid plant. It is also the name given to the whole plant. Stem and branch fragments are commonly preserved as cast, mold, impression, compression, and adpression types of fossils. Adpressions are combinations of impressions and compressions that are common in plant fossils. Calamites fossils are found in shales, siltstones, sandstones, and sometimes inside of siderite (iron carbonate) nodules. Stem and branch fragments can be several inches in diameter to more than a foot in length. Stems and branches are ornamented with very thin ribs oriented parallel to the long axis. Ribs terminate along regularly spaced transverse indentations called joints or nodes.

Learn more information about Calamites form genera

The ribs are external ornamentation on Calamites fossils, but there is some scientific debate as to whether or not these were external or actually an internal feature of the original plant. Most scientists have inferred that the thin ribs on Calamites stems and branches are internal sediment casts of indentations on the hollow inside of the original plants, called pith casts. When the plants died, the hollow pith filled with sediment, which became the fossils. The pith was originally surrounded by soft plant tissues (cortex, etc.), which were compacted and coalified or rotted, and not preserved in fossils (e.g., Gillespie and others, 1978). Other scientists think that the ribbing is an external impression, called a stem cast (see discussion in DiMichele and Falcon-Lang, 2012; Taylor, 2014; Falcon-Lang, 2015).

Both pith casts and stem casts likely occur as fossils. For example, leaf-bearing fossil stems are external stem casts, impressions, or adpressions. In either case, most Calamites fossils are not petrified remains of the original plant, but rather are sediment infills or impressions of the plants. Thin coal rims or streaks on the outside of some fossils represent all that is left of the original plant tissue. Permineralized Calamites which include original plant details are preserved in rare deposits called coal balls, but these are usually only found in active coal mines, so are not found by collectors.

The segmented joints (nodes) of Calamites fossils are where branches grew outward from the main axes. Circular branch scars are sometimes preserved in fossils. Different species have different numbers and spacing of branches. In some Calamites, branches consisted of several single stems at each whorl. In others, branches higher on the trunk were single stems, but branches lower on the trunk, sometimes had secondary branches.

Calamites had roots called rhizomes, which could bud and grow into other Calamites. Root casts are sometimes preserved as fossils. The roots have similar ribbing and jointing to the main axial stem or trunk, but may curve and narrow. The curved, conical shape and external ribbing can give them the false appearance of a fossil horn coral!

Parts of the plant. Typically, only parts of whole plants are preserved as fossils, and each part can be given a different genus name, called a form genus. This is sometimes confusing for amateur collectors. In fossils formed as compressions, impressions, adpressions, and pith casts, the common trunks, stems, and branches of the Calamites plant are called Calamites. Each branch bore spiral arrangements of leaves (whorls), at regularly spaced intervals. Leaves of the Calamites plant are called Annularia or Asterophyllites, depending on shape. Different types of leaves were on different types of Calamites plants. Calamites leaf whorls are sometimes mistaken for fossil flowers by amateur collectors, but they are actually leaves; Calamites was not a flowering plant.

Cone-like reproductive organs of the plant include Calamostachys, Calamocarpon, Cingularia, Macrostachya, Mazostachys, and Palaeostachya. Like leaves, different cones were likely on different types (species) of Calamites plants. Some cones contain microspores and megaspores. Calamites microscopic spores are called Calamospora, Laevigatosporites, Reticulatisporites, and Vestispora. Microspores generally require special techniques for extraction.

Large casts and impressions of main roots are called Calamites, similar to branch and trunks specimens, but thinner roots, rootlets, and root traces are called Pinnularia (Gillespie and others, 1978; Stewart, 1981; Cichan and Taylor, 1983; Thomas, 2014).

In coal balls where Calamites plant parts are permineralized (original structures replaced by minerals) plant parts sometimes have different names to show that they preserve internal anatomical structures. Some even show cellular structures (Stewart, 1981; Cichan and Taylor, 1983)!

Species. At least seven species of Calamites (stem and branch parts) have been reported from Kentucky and surrounding states, although most fossils are not identified to species level. Calamites species reported from Kentucky include C. carinatus, C. cisti, C. cruciatus, C. gracilis, C. ramosus, C. suckowii, and C. undulatus. The most common from the Middle Pennsylvanian are C. carinatus, C. cisti, C. suckowii, and C. undulatus (Lesquereux, 1857; Jennings, 1981; Gillespie and others, 1978; Spurgeon and Jennings, 1985; Cross and others, 1996). Species are differentiated based on the shape of infranodal canals, shape of internodes, spacing and density of branch scars, relative rib width and shape, shape of the end of the ribs, and other features. Many more species names are applied to the other form genera (leaves, etc.) of the whole plant.

Range. Calamites lived from the Late Mississippian to Permian. They are common fossils in Middle Pennsylvanian coal-bearing rocks in Kentucky and can be found in 52 counties, including both of Kentucky’s coal fields. Calamites fossils are more common in the Eastern Kentucky Coal Field because of topography and exposure of plant fossil-bearing rocks. Late Mississippian samples are 325 to 330 million years old. Pennsylvanian samples, which are the most abundant in Kentucky, are 300 to 320 million years old.

Paleoecology. Calamites grew in a variety of habitats but are mostly found in rocks originally deposited in wetlands or marginal to wetlands, and along lakes, rivers, and estuaries. They could grow on mineral (mud, silt, sand) and peat soils. Calamites grew as short groundcover, medium-sized to tall shrubs, and even trees (called arborescent forms). Most Calamites were probably less than 10 ft tall, but fossil trunks more than 30 ft long have been reported (Scott, 1920; Rössler, 2012; Taylor, 2014), and some may have been as much as 100 ft tall (Gillespie and others, 1978)!

Calamites fossils are particularly common in sedimentary rocks formed in “disturbance-prone” environments characterized by periods of rapid flooding and sedimentation (Gastaldo, 1992; DiMichele and Phillips, 1994; DiMichele and others, 2009; Thomas, 2013; Falcon-Lang, 2015). The rhizome roots of Calamites made them well-adapted to these types of environments. When Calamites plants were knocked over by floods, new plants could sprout from their buried rhizomes. Fossils found as upright casts in growth position represent plants buried by flood sediments. Although larger lycopod plant fossil trunks garner more attention as buried in place fossil trees, in place Calamites stands may be more common.

Although upright Calamites are probably more common than upright lycopod trunks, neither are common fossils. The most abundant Calamites fossils are impressions and casts found parallel to bedding in layered shales and siltstones. These types of Calamites fossils represent parts of plants that were knocked over and buried in sediment. In shales, Calamites impression, compression, and adpression fossils are commonly flattened from burial and post-burial compaction.

Calamites stem and branch fossils are not found in coals, but microscopic fossil spores of the plants remain to show they lived in the ancient coal-forming swamps. In Kentucky’s coal swamps Calamites spores generally account for less than ten percent of the total spores in the coal beds (Eble, 1996; Eble and Greb, 2017). In peat swamps, Calamites likely lived on wet forest floors and along lake and creek margins in the peat swamps.

Calamites plants are usually reconstructed as a slender reed-like bush, or small bushy tree (arborescent forms), similar in appearance to its distant modern relative Equisetum or bamboo. Bamboo is also a jointed plant with branches that grow from rhizome roots, and it can grow to tree-like stature. Although useful for illustrating what Calamites may have looked like, modern bamboo is a variety of different grasses, so are physiologically different than rushes like modern Equisetum and ancient Calamites. Different forms of Calamites may have lived in different habitats. One research paper suggests the common illustration of Calamites as an Equisetum or bamboo-like plant may only apply to juvenile and younger growth stages of Calamites. Mature forms may have had tall trunks without branches on the lower part of their trunks. Branches at the top may have formed a bushy crown, so that mature plants looked more like trees than bushy reeds and rushes (Daviero and Lecoustre, 2000).

References Cited

• Cichan, M.A., and Taylor, T.N., 1982, Structurally preserved plants from southeastern Kentucky: Stauropteris biseriata sp. nov.: American Journal of Botany, v. 69, no. 9, p. 1491-1496.


• Cross, A.T., Gillespie, W.H., and Taggart, R.E., 1996, Chapter 23-Upper Paleozoic vascular plants, in Feldman, R.M., and Hackathorn, M., eds., Fossils of Ohio: Division of the Ohio Geological Survey, Bulletin 70, p. 396-479.


• Daviero, V. and Lecoustre, R., 2000, Computer simulation of sphenopsid architecture. Part II. Calamites multiramis Weiss, as an example of Late Paleozoic arborescent Sphenopsids: Review of Palaeobotany and Palynology, v. 109, no. 2, p.135-148.


• Dimichele, W.A. and Phillips, T.L., 1988, Paleoecology of the Middle Pennsylvanian-age Herrin coal swamp (Illinois) near a contemporaneous river system, the Walshville paleochannel: Review of Palaeobotany and Palynology, v. 56, no. 1-2, p.151-176.


• DiMichele, W.A. and Phillips, T.L., 1994, Paleobotanical and paleoecological constraints on models of peat formation in the Late Carboniferous of Euramerica: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 106, no. 1-4, p. 39-90.


• DiMichele, W.A., Falcon-Lang, H.J., 2012, Calamitalean pith casts reconsidered: Reviews of Palaeobotany and Palynology, v. 173, p. 1-14.


• DiMichele, W.A., Nelson, W.J., Elrick, S., Ames, P.R., 2009, Catastrophically buried Middle Pennsylvanian Sigillaria and calamitean sphenopsids from Indiana, USA: What kind of vegetation was this? Palaios, v. 24, p. 159-166.


• Eble, C.F., 1996, Lower and lower Middle Pennsylvanian coal palynofloras, southwestern Virginia, in Hower, J.C. and Eble, C.F. (editors), Geology and Petrology of Appalachian Coals: International Journal of Coal Geology, v. 31, nos. 1-4, p. 67-114.


• Eble, C.F. and Greb, S.F., 2017, Compositional variability of Middle Pennsylvanian coal beds near the north-west margin of the Eastern Kentucky Coal Field, Central Appalachian Basin, USA: Palynology, v. 41, p. 221-246.


• Falcon-Lang, H.J., 2015, A calamitalean forest preserved in growth position in the Pennsylvanian coal measures of South Wales: implications for palaeoecology, ontogeny and taphonomy: Review of Palaeobotany and Palynology, v. 214, p. 51-67.


• Gastaldo, R.A., 1992, Regenerative growth in fossil horsetails (Calamites) following burial by alluvium: Historical Biology, v. 6, p. 203-220.


• Gillespie, W.H., Clendening, J.A., and Pfefferkorn, H.W., 1976, Plant fossils of West Virginia: West Virginia Geological and Economic Survey, Educational Series ED-3A, 172 p.


• Greb, S.F., Eble, C.F., Chesnut, D.R., Phillips, T.L. and Hower, J.C., 1999, An in situ occurrence of coal balls in the Amburgy coal bed, Pikeville Formation (Duckmantian), central Appalachian Basin, USA: Palaios, v. 14, no. 5, p. 432-450.


• Rössler, R., 2006, Two remarkable Permian petrified forests: correlation, comparison and significance, in Lucas, S.G., Cassinis, G., and Schneider, J.W., eds., Non-marine Permian biostratigraphy and biochronology: Geological Society, London, Special Publications, v. 265, no. 1, p. 39-63.


• Scott, D.H., 1920, Studies in fossil botany, vol.1, Pteridophyta: A & C Black, London, 434 p.


• Spurgeon, P.A., and Jennings, J.R., 1985, Pennsylvanian plants of eastern Kentucky: A flora from the Breathitt Formation near Grannies Branch and Rocky Branch of Goose Creek, Clay County, Kentucky: Kentucky Geological Survey, Ser. XI, Report of Investigations, no. 3, 34 p.


• Stewart, W.N., 1983 (1985), Paleobotany and the evolution of plants: Cambridge, Cambridge University Press, 405 p.


• Thomas, B.A., 2014. In situ stems: preservation states and growth habits of the Pennsylvanian (Carboniferous) calamitaleans based upon new studies of Calamites Sternberg, 1820 in the Duckmantian at Brymbo, North Wales, UK: Palaeontology, v. 57, p. 21-36.

Text by Stephen Greb (KGS).

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