RHABDOXYLON AMERICANUM SP. N., A STRUCTURALLY SIMPLE FERN-LIKE PLANT FROM THE UPPER PENNSYLVANIAN OF ILLINOIS

A new species of the genus Rhabdoxylon Holden (1960), an anatomically simple plant of presumed fern or fern-like affinities, is described from a coal ball petrifaction found in the Upper Pennsylvanian of southern Illinois. The new species, R. americanum, is based upon five specimens consisting of stems bearing spirally arranged leaves and numerous randomly distributed adventitious roots. The haplostelic stems branch by equal dichotomies and bear foliar traces which arise as unequal dichotomies of the stele. Leaf traces possess a circular outline in cross section and one adaxial protoxylem strand. The characteristics of exclusively primary tissues, diarch adventitious roots, centrarch haplosteles with simple scalariform pitting, and the nature and arrangement of the leaf traces, suggest that Rhabdoxylon represents a fern or fern-like plant rather than a representative of the Rhyniophytina or Trimerophytina. At present it is not possible to determine whether the simple structure of Rhabdoxylon has come about through phyletic reduction or represents a primitively simple condition.