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CATASTROPHICALLY BURIED MIDDLE PENNSYLVANIAN SIGILLARIA AND CALAMITEAN SPHENOPSIDS FROM INDIANA, USA: WHAT KIND OF VEGETATION WAS THIS?

¹ U.S. National Museum of Natural History, Smithsonian Institution, Department of Paleobiology, Washington, D.C. 20560, USA
² Illinois State Geological Survey, Coal Section, 615 E. Peabody Drive, Champaign, Illinois 61820, USA
³ Black Beauty Coal Company, Evansville, Indiana 47715, USAdimichel{at}si.edu

A catastrophically buried stand of calamitean sphenopsids and sigillarian lycopsids is reported from the Middle Pennsylvanian of southwestern Indiana, in the Illinois Basin. The plants were exposed in the highwall of a small surface mine and were rooted in a thin bed of coal (peat), thus representing a flooded and buried swamp surface. Coarse, floodborne silts and sands buried the forest to a depth of <3 m or more, before further incursions of water and sediment truncated the deposit. The rocks are part of the Staunton Formation. Taking up >250 linear meters of exposed highwall surface, the vegetation appears to have been a patchwork of calamitean thickets, with stems perhaps as tall as 3–5 m, within which scattered, but much larger, emergent Sigillaria trees grew, possibly reaching heights of 10–15 m. No ground cover was observed, nor were foliage or reproductive organs attributable to the dominant plants found. The growth of this vegetation in a peat-forming swamp indicates conditions of high water availability, likely in a humid, high-rainfall climate. This kind of plant assemblage, however, cannot be characterized as a rain forest, given that it consisted of medium-height thickets of horsetails with scattered, emergent, and polelike, giant lycopsids, thus lacking a closed upper canopy and possibly only partially shading the ground.