OSMUNDA WEHRII, A NEW SPECIES BASED ON PETRIFIED RHIZOMES FROM THE MIOCENE OF WASHINGTON

Silicified rhizomes from Miocene strata near Yakima, Washington represent a new species of Osmunda. The stems are 8-13 mm in diameter and are surrounded by a thick sheath of adherent leaf bases, each of which shows stipular expansions typical of the Osmundaceae. The new species has an ectophloic siphonostele in which the xylem cylinder is dissected by leaf gaps with 12-14 strands being visible in a given stem cross section. Such sections also show 12-16 leaf traces in the cortex. The xylem of each leaf trace diverges from the xylem cylinder of the stem as an adaxially concave strand with its protoxylem organized into a single medial adaxial cluster. Initial bifurcation of the leaf-trace protexylem occurs as the leaf trace passes through the outer cortex of the stem. In the basal part of the stipular region of the petiole base, thick-walled fibers form an arch on the abaxial side of the sclerenchyma ring around the petiolar bundle. This arch persists throughout most of the length of the stipular region, with the thickwalled fibers becoming reorganized into two lateral masses in the distal part of the stipular region. Similar thick-walled fibers form an elongate strip of tissue in each wing of the stipule along with several small clusters scattered near the sclerenchyma ring. The new species belongs to the subgenus Osmunda and shows that during the Neogene, the latter existed as a group of closely related species much as it does today. Furthermore, Osmunda wehrii combines features of the modern 0. regalis, 0. japonica, and 0. lancea with those of 0. claytoniana and thus supports the inclusion of the latter species in the subgenus Osmunda. OSMUNDA L. STANDS out among genera of modern ferns because of its extensive fossil record based mostly on petrified rhizomes with adhering leaf bases and adventitious roots. The composition and arrangement of tissues in these fossils and in their modern counterparts provide features that permit classification to subgenus and comparison with closely related species within subgenera. These subgenera have been distinct throughout the Tertiary and probably originated during the Cretaceous. One species of fossil foliage like that of the modern Osmunda regalis L. is known from the Early Cretaceous, and several additional species of similar foliage have been reported from the Late Cretaceous (Miller, 1967, 1971). The present report describes a new species of the subgenus Osmunda based on two specimens of silicified rhizomes from the Late Miocene of Washington. This is the first species of petrified rhizomes belonging to this subgenus recorded from Neogene sediments in North America even though the subgenus is represented here by two Paleogene forms and the modern Osmunda regalis L. , 0. claytoniana L. , and 0. x ruggii Tryon. The new species, in combination with the other known I Received for publication 26 September 1980; revision accepted 10 December 1980. Neogene forms, shows that during the Neogene the subgenus Osmunda existed as a complex of closely related species much as it does at present. MATERIALS AND METHODS-The new species is based on two specimens of silicified rhizomes that came from the Late Miocene sediments near Yakima, Washington. The rhizomes occurred near the contact of the Roza Basalt and the overlying Priest Rapids Basalt. Their study is based mainly on the examination of thin sections by transmitted light microscopy. To enchance contrast between the plant tissues and the petrifying mineral, certain pieces were boiled for 8-10 hr in a reflux system containing Bismark Brown stain dissolved in 10% ethyl alcohol acidified with a few drops of concentrated HCl per liter of solution. Stain concentration was not critical. Other pieces of the rhizome were polished and examined under reflected light. SYSTEMATIC PALEOBOTANY Osmunda wehrii sp. nov., Fig. 1-6. Diagnosis-Rhizomes about 4 cm in diameter, consisting of a stem about 12 mm in diameter surrounded by a mantle of closely adhering stipular petiole bases; xylem cylinder