A FOSSIL EQUISETUM SP. (FAMILY EQUISETACEAE, SUBGENUS HIPPOCHAETAE) FROM THE LATE TERTIARY ASH HOLLOW FORMATION OF NEBRASKA

Silicified leaf-sheath fragments of Equisetum sp. are described from Late Tertiary strata in west-central Nebraska. The close affinity of these fossils to the subgenus Hippochaetae is supported by the presence of sunken stomata arranged in two regular rows on each leaf-sheath segment. This is the first report of the genus Equisetum from the late Tertiary Ash Hollow Formation of central North America. ALTHOUGH the genus Equisetum is abundantly represented in the fossil record from the Carboniferous to the Pleistocene, there are few reports in the literature of such fossils exhibiting well preserved micromorphological or anatomical features. Harris (1961) used epidermal features to distinguish certain Jurassic fossil Equisetum and Gould (1968) described the Jurassic E. bryanni from stems and leaves showing micromorphological and anatomical details. Gomez (1978) reported a fossil Equisetum impression from late Tertiary strata in Costa Rica. He referred the fossil to the subgenus Hippochaetae on the basis of poorly preserved stomatal features. The only account of a fossil Equisetum from North American deposits showing preservation of anatomical details is that of Brown (1975). He described E. clarnoii from the Eocene of Oregon based on petrified root and stem fossils and concluded that the fossils could be assigned to the subgenus Hippochaetae. In the present paper silicified fossils of Equisetum exhibiting micromorphological detail are described from Miocene-Pliocene strata in Nebraska. STRATIGRAPHY-The fossils were collected by the author during the summer of 1978 from a freshwater marl deposit approximately midway in a vertical exposure of 55 meters of fluvial silts, sands, and conglomerates. The strata 1 Received for publication 23 April 1979; revision accepted 20 August 1979. This study was supported by grants from the National Geographic Society (1883), the National Science Foundation (DEB-7809150), and the Black Hills Foundation. The author thanks Drs. N. R. Lersten, C. N. Page, R. W. Pohl, and F. Rich for reviewing the manuscript and Mr. Sherman Fuerniss and Ms. Debbie McBride for their able assistance in the field. The Bessey SEM facilities at Iowa State and the metallurgical engineering SEM facilities at South Dakota School of Mines and Technology were used in the preparation of this report. from which the fossils were recovered belong to the Ash Hollow Formation (Nebraska sense) of Late Miocene to Pliocene age. The locality has yielded numerous and diversified plant (angiosperm and algae) and animal (vertebrate and invertebrate) remains which are currently the subject of separate studies. Detailed locality information is available to qualified investigators in the files of the Black Hills State Herbarium or from the author. MATERIALS AND METHODS-Eleven specimens were found in a sandy-silty, freshwater marl deposit at JRT site 50 in Garden County, Nebraska. Fossils were recovered in situ from the sediment by degaging. Herbarium and fossil specimens selected for scanning electron microscopy were mounted on brass discs with high purity silver paint and coated with carbon and 28 cm of . 008 in. diam 99.9% gold in a Varian vacuum evaporator. The specimens were then observed at 20 Kv in a JEOL JSM35 (Fig. 1-3, 5-6) or at 10 Kv in a Coates and Welter Qwickscan 100 (Fig. 4) scanning electron microscope. Specimens were numbered in a BP (brass plate) series. A photographic record was kept on Polaroid 665 positive/negative film. All specimens are deposited in the herbarium at Black Hills State College, Spearfish, South Dakota. DESCRIPTION-The fossils consist of fragments of leaf-sheaths devoid of scale leaves (Fig. 1, 2) and derive probably from aerial stems (C. N. Page, personal communication, 1979). The fragmentary leaf-sheaths generally consist of 2 to 4 segments, each 0.5-0.6 mm wide and 2.4-3.5 mm long. The individual segments, which are slightly convex in the center, are separated by a furrow about one-tenth the width of each segment. One nearly complete leaf-sheath fragment is 2.5 mm wide and con-