Fossil of the month: Stigmaria
Is it a snake? A dinosaur tail? No, this month’s fossil is Stigmaria, and it’s a fossil tree root rather than a fossil reptile.
Description. Stigmaria is a form genera name for the roots of Carboniferous lycopod (scale) trees. Form genera are genera defined for a part of an organism or plant, rather than the whole organism or plant. Because plants, especially large plants such as trees, are commonly fossilized as parts (leaves, bark, cones, trunks, roots, etc.), which are found separate from each other, the different parts are given distinct names. Stigmaria are the roots of extinct lycopod trees, including the lycopods Lepidodendron and Sigillaria.
Stigmaria fossils are generally elongate and tubular. They range from almost circular to flat in cross section. Their outer surface is smooth to crenulated or rope-like, and commonly ornamented with many circular or oval depressions a centimeter or less in diameter, arranged in spiraling patterns. The rims, and sometimes center of the depressions, may be slightly raised. These depressions are where rootlets attached to the main roots. In cross section, some specimens will preserve an axial core, which is the pith cast of the original root. Some specimens are rounded or convex on one side, and depressed or concave on the other, with a groove or depression oriented along the long axis of the fossil root. In shales and coaly shales, the elongate tubular parts of Stigmaria may be surrounded by thin, hair-like rootlets.
Stigmaria fossils were preserved in many different ways. Most are internal casts and molds of rotted tree roots rather than petrified fossils of the roots themselves. When lycopod trees died, the tree and roots rotted. If a hollow space was left in the sediment where the roots rotted, it could be filled in with sediment, forming a cast and mold of the root. Stigmaria with an external coal rind had some outer organic material remaining when the axis of the root was infilled with sediment. In some cases, preservation may have been aided by the deposition of minerals around the roots while they rotted (Thomas and Seyfullah, 2015). Roots play a key role in element cycling in swamps, and minerals can build up around them because of chemical changes in the microenvironment around the roots (Pye and others, 1990; Mendelson and others, 1995).
Some Stigmaria fossils are found attached to fossil tree trunks, but most are found as isolated roots in underclays (ancient soils) and beneath exposure surfaces in coal-bearing rocks, without attached trunks. Roots are more likely to be preserved as fossils than the overlying tree because they are already buried. Burial is the first step in fossilization.
Stigmaria fossils range in length from less than 10 centimeters to more than 6 meters! Long specimens taper in thickness from one end to the other. Most fossils are less than 60 centimeters long and 6 to 8 centimeters wide, but wider specimens are known, especially in specimens found near or still attached to fossil tree stumps. Specimens preserved in sandstone and siltstone generally are more circular to rounded in cross section than specimens preserved in shale, coaly shale, and claystone, which tend to be more compacted and flatter. The appearance of Stigmaria varies depending on how and where it was formed, the type of sediment it formed in, and when sediment was injected into decayed roots relative to how much of the root had decayed, and how much the root hollow had collapsed or been compacted.
Although most Stigmaria fossils are preserved as casts or molds, Stigmaria has also been preserved in coal balls (carbonate concretions and masses in coals) that represent in-place mineralization (termed permineralization) of the actual roots. Permineralized specimens of Stigmaria have preserved microscopic details of the original root structures. Coal balls containing Stigmaria have been found in both the Western (Phillips and others, 1985) and Eastern (Schopf, 1961; Greb and others, 1999) Kentucky Coal Fields.
Species. Many species of Stigmaria have been proposed, mostly based on differences in the arrangements or patterns of rootlet bases. Three species names have been used in Kentucky and surrounding areas. All specimens from Pennsylvanian strata and most Mississippian forms are Stigmaria ficoides. Late Mississippian strata contain two additional species, Stigmaria stellata and Stigmaria wedingtonensis. Stigmaria stellata and S. wedingtonensis exhibit stellate (star or asterisk shape) markings around their root bases (circular to oval depressions), which S. ficoides lack (Pfefferkorn, 1972; Jennings, 1973).
Range and geographic occurrence. Stigmaria ranges from the Late Devonian through Pennsylvanian globally. In Kentucky, it is widely reported from the Pennsylvanian in both of Kentucky's coal fields. Pennsylvanian strata are approximately 280 to 320 million years old. Some Stigmaria have also been reported from Late Mississippian (approximately 325 to 330 million years ago) units in Kentucky (Pfefferkorn, 1972; Jennings, 1973), but are less common in the Mississippian than Pennsylvanian strata. Stigmaria has not been reported from Devonian rocks in Kentucky. Devonian Stigmaria represents the root systems of Protolepidodendron and Archeosigillaria, the predecessors of Carboniferous lycopods such as Lepidodendron and Sigillaria.
Life and paleoecology. Lycopod trees had shallow root systems. Lycopods had five or more main roots, which spread out horizontally from the trunk. Roots branched dichotomously (split into two parts) outward from the tree multiple times. Horizontal rather than vertical rooting was likely an adaptation to the wet, swampy substrates in which lycopod trees thrived (DiMichele and Phillips, 1985; Philips and others, 1985). Studies of Stigmaria roots preserved in coal balls show they also contained special tissues with air chambers (called aerenchymatous tissues), which allowed them to function under water in wet substrates and in low-oxygen conditions, both common features in swamp environments (Phillips and others, 1985).
Importance in the history of understanding coal. Stigmaria was originally described in the 1800s, when scientists were first describing many fossils and considering the origins of coal. They were something of a curiosity and were illustrated and described in many research papers about fossils in the 1800s. Steinhauer (1818) named his specimen Phytolithus. Sternberg (1820) named a specimen Variolaria. Both names had problems, so the famous French scientist Alexandre Brogniart gave the genus its currently accepted name, Stigmaria, in 1822. At first, these fossils were considered a strange type of plant trunk, but as more coal was mined in Europe, specimens were found attached to lycopod tree trunks. The first tree trunks Stigmaria was associated with were the lycopod Sigillaria, but later they were found attached to the lycopod Lepidodendron as well (Brown, 1848). Now they are recognized as the roots of several different types of extinct lycopod trees.
During the late 1800s, scientists debated whether coal was deposited as transported organic debris or represented in-place accumulations of organic peat. Once the common Stigmaria fossils in underclays of coals were recognized as tree roots in an ancient soil (Lyell, 1841, 1845; Brown, 1848; Binney, 1859; Gresley, 1887; Williamson, 1887), coal beds were proved to have been in-place peat accumulations.
- Binney, E.W., 1859, Some observations on Stigmaria ficoides: Quarterly Journal of the Geological Society, v. 15, nos. 1-2, p. 76–80.
Brongniart, A., 1822, Sur la classification et la distribution des végétaux fossiles en général, et sur ceux des terrains de sediment supérieur en particulier: Memoires du Museum d’Histoire Naturelle, Paris, v. 8, p. 297–348.
- Brown, R., 1848, Description of an upright Lepidodendron with Stigmaria roots, in the roof of the Sydney Main Coal, in the island of Cape Breton: Quarterly Journal, Geological Society of London, v. 4, p. 46–50.
- Dimichele, W.A., and Phillips, T.L., 1985, Arborescent lycopod reproduction and paleoecology in a coal-swamp environment of late Middle Pennsylvanian age (Herrin Coal, Illinois, USA): Review of Palaeobotany and Palynology, v. 44, nos. 1-2, p. 1–26.
- Greb, S.F., Eble, C.F., Chesnut, D.R., Jr., 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, U.S.A.: Palaios, v. 14, p. 433–451.
- Gresley, W.S., 1887, Notes on the formation of coal-seams, as suggested by evidence collected chiefly in the Leicestershire and South Derbyshire Coalfields: Quarterly Journal of the Geological Society, v. 43, nos. 1–4), p. 671–674.
- Jennings, J.R., 1973, The morphology of Stigmaria stellata: American Journal of Botany, v. 60, no. 5, p. 414–425.
- Lyell, C., 1841, On the stigmaria clay in the Blossberg coalfield of Pennsylvania, in Skinner, H.C., ed., Charles Lyell on North American geology: New York, Arno Press, p. 225–228.
- Lyell, C., 1845, Travels in North America, in the years 1841-2: With geological observations on the United States, Canada, and Nova Scotia: New York, Wiley and Putnam, 221 p.
- Lyell, C., 1854, Principles of geology: New York, Appleton and Co., 834 p.
- Mendelssohn, I.A., Kleiss, B.A. and Wakeley, J.S., 1995, Factors controlling the formation of oxidized root channels: a review: Wetlands, v. 15, no. 1, p. 37-46.
- Pfefferkorn, H.W., 1972, Distribution of Stigmaria wedingtonensis (Lycopsida) in the Chesterian (Upper Mississippian) of North America: American Midland Naturalist, v. 88, no. 1, p. 225–231.
- Phillips, T.L., Peppers, R.A., and DiMichele, W.A., 1985, Stratigraphic and interregional changes in Pennsylvanian coal-swamp vegetation: Environmental inferences: International Journal of Coal Geology, v. 5, p. 43–109.
- Pye, K., Dickson, J.A.D., Schiavon, N., Coleman, M.L. and Cox, M., 1990, Formation of siderite-Mg-calcite-iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England: Sedimentology, v. 37, no. 2, p. 325–343.
- Schopf, J.M., 1961, Coal-ball occurrences in eastern Kentucky: U.S. Geological Survey Professional Paper 424-B, no. 95, p. 228–230.
- Steinhauer, H., 1818, On fossil reliquia of unknown vegetables in the coal strata: Transactions of the American Philosophical Society, New Series, v. 1, p. 265–297.
- Thomas, B.A., and Seyfullah, L.J., 2015, Stigmaria Brongniart: A new specimen from Duckmantian (Lower Pennsylvanian) Brymbo (Wrexham, North Wales) together with a review of known casts and how they were preserved: Geological Magazine, v. 152, no. 5, p. 858–870.
- Williamson, W.C., 1887, A monograph on the morphology and histology of Stigmaria ficoides: Journal of the Palaeontographical Society, v. 40, p. 189–193.