A NEW FOSSILIZED TREE FERN STEM, NISHIDACAULIS-BURGII GEN-ET-SP-NOV, FROM NEBRASKA, SOUTH-DAKOTA, USA

A permineralized tree fern stem assigned to the new genus and species Nishidacaulis burgii was collected from northwestern Nebraska. Although collected in the basal conglomerate of the Upper Eocene-Lower oligocene Chadron Formation, it was most likely reworked from sediments of Early Cretaceous age in the Black Hills, South Dakota. The stem appears to have had radial symmetry and was surrounded by persistent leaf bases and adventitious roots. Uniseriate, multicellular epidermal trichomes are also persistent on and around the stem. A sclerenchymatous sheath around the solenostelic stele is absent. Leaf and petiolar traces are undivided throughout and are modified reverse omega- or U-shaped with incurved ends. Mucilage cells or cavities occur in the outer phloem of the petiolar vascular strand, but are lacking in the fundamental tissue of the petiole. This new genus may represent a transitional form in a possible lineage from Upper Jurassic-Lower Cretaceous ''Cibotium'' tasmanense to Upper Cretaceous C. iwatense and extant members of Cibotium. The basal pinna trace being derived from the adaxial corner only of the petiolar vascular strand in N. burgii indicates the original plant had a small or reduced basal pinna and a short petiole. Its entire frond was more or less rhombic, suggesting a habit with an erect stem with radiating fronds forming a crown.