More than 300 million years ago during the Carboniferous Period, the landscape of what is now West Virginia was dominated by vast forests of unusual plants around shallow swamps, and because bacteria and fungi had not yet developed the ability to break down lignin, layers of woody plant material accumulated over time and formed the coal beds for which the state is known. Studying these prehistoric plants and animals helps place the evolution of modern species in context, and some well-preserved fossils even retain cellular structures. Among the extinct plants were Cordaites, relatives of coniferous trees that grew in swamps and on dry land and are commonly found in West Virginia as strap-shaped leaf fossils; giant lycopsids such as Lepidodendron and Sigillaria, whose root structures are known as Stigmaria and whose fossils include diamond-shaped leaf scars and the tree stumps called kettlebottoms in regional coal mines; Calamites, tree-like horsetail relatives with hollow stems that often filled with sand or mud and preserved distinctive nodes; and several kinds of fern-like plants, including true ferns, tree ferns, and seed ferns. West Virginia also preserves Carboniferous fossils of aquatic life, including brachiopods, crinoids, trilobites, corals, cartilagenous fish, and other species that lived amid changing environments, and the coral fossil Lithostrotionella from the Mississippian sub-period was named the official State Gem in 1990 and has been found only in Greenbrier and Pocahontas counties.