Diatoms from the Upper Miocene Hot Springs Limestone, Snake River Plain, Idaho (U.S.A.)

Abstract

The taxonomy, ultrastructure and systematic relationships of gomphonemoid diatoms found in the non-marine upper Miocene Hot Springs Limestone (Idaho, USA) are considered. A total of 15 taxa are identified and of these 13 are described as new. The new taxa include 2 species and 1 variety of Gomphoneis and 8 species and 2 varieties of Gomphonema. Lectotypes for two Gomphonema species, G. grovei M. Schm. and G. occidentale var. abbreviata M. Schm., are designated. Intraand intergeneric similarities and differences in valve ultrastructure are described and used to suggest systematic affinities of the taxa. In Gomphoneis, a species is described that appears to represent a lineage separate from G. elegans and its allies and G. herculeana and its relatives. Several Gomphonema species are described that have chambers around the central area. A specimen of G. grovei is shown to possess recurved, not anchor-shaped, internal proximal raphe ends which suggests a closer affinity to the freshwater gomphonemoid lineage than previously demonstrated. Most of the Gomphonema and Gomphoneis species considered possess stigmata and finer, more radiate striae near the footpole. The high degree of morphological diversity exhibited in both Gomphonema and Gomphoneis suggests the Late Miocene may have been a period of rapid evolutionary change in these genera. INTRODUCTION Despite a long history of reports on the freshwater fossil diatom flora of westem North America (e.g. Bailey 1845; Ehrenberg 1854, 1871; Schmidt 1899; Lohman in Moore 1937; Lohman 1938; Lohman and Andrews 1968; VanLandingham 1964, 1985; Bradbury 1984; Bradbury et al. 1985), many new taxa are still being discovered and described (e.g. Theriot and Bradbury 1987). An area of great interest has been the Snake River Plain (Bradbury and Krebs 1982), for which a wealth of corresponding geological and paleontological information has been published (Berggren and Van Couvering 1974; Armstrong et al. 1975; Kimmel 1979; Smith et al. 1982). Although most of the recent taxonomic treatments (utilizing electron microscopy) have been concerned with centric diatom taxa (Bradbury and Krebs 1982; Theriot and Kociolek 1986; Theriot and Bradbury 1987), Kociolek and Stoermer (1987, 1988b) described the ultrastructure of new pennate species from the Hot Springs Limestone, located along the southeastern portion of the Snake River Plain. The flora of this deposit is dominated by raphid diatoms and initial observations indicated several poorly-known or apparently new gomphonemoid diatoms occur in the material. The purpose of the present report is to consider the taxonomy, ultrastructure and systematic relations of these forms. MATERIALS AND METHODS The Hot Springs Limestone is located in Owhyee County, Idaho, north of Bruneau Canyon. Littleton and Crosthwaite (1957) discussed the geology of the region in detail and assigned to this deposit an approximate age of 8 million years BP. The stromatolite-like carbonates of the Hot Springs Limestone contain abundant freshwater fossils, including diatoms. Diatoms occur in intercolumnar sediment of the algal reef carbonate columns. The structure, formation and paleoecological environment of the Hot Springs Limestone has been described by Staccia et al. (in press). Diatom-bearing sediment was extracted from portions of the limestone designated DO-51X-095, Hot Springs Limestone, Snake River Plain, Miocene Chalk Hills Formation, down side of rock next to chalk-like inclusion, 46?46.2'N, 115?44.5'W, Owyhee Co., Idaho provided to us by Dr. Bruce Wilkinson (Dept. Geology, The University of Michigan). The extracted sediment was rinsed in HC1, then diluted in distilled water and boiled in HNO3. The acid treated sediments were repeatedly washed and settled to remove oxidation by-products. Cleaned material was air-dried onto coverglasses which were mounted onto glass slides with Hyrax?. For scanning electron microscopy, coverglasses with the air-dried material were mounted onto aluminum stubs. Stubs were sputter-coated with approximately 20nm of gold-palladium and viewed with either an ISI DS-130 SEM operated at 10kV or a JEOL T-100 SEM operated at 25kV. Nomenclature of ultrastructural features follows the proposals of Anonymous (1975), Ross et al. (1979), Kociolek and Rosen (1984) and Krammer and Lange-Bertalot (1986). RESULTS Genus GOMPHONEIS Cleve Gomphoneis magna Kociolek & Stoermer, n. sp. Plate 1, figures 1-6; plate 3, figures 1-6; holotype specimen is plate 1, figure 2. Description: Valves linear-lanceolate, length 120-250gm, breadth 15-21ugm. Axial area broadly lanceolate, narrow at poles, widening progressively to the elliptical central area. The micropaleontology, vol. 36, no. 4, pp. 331-352, pls. 1-12, 1990 331 This content downloaded from 157.55.39.67 on Mon, 28 Nov 2016 04:00:16 UTC All use subject to http://about.jstor.org/terms J. P. Kociolek and E. F. Stoermer