Taxonomy, Morphology and Phylogeny of the Neogene Diatom Zonal Marker Species in the Middle-to-High Latitudes of the North Pacific

Abstract

Based on both light microscopic and electron microscopic observations, taxonomic refinements of most of the Neogene diatom zonal markers of the middle-to-high latitudes of the North Pacific produce a firmer basis for its zonal frameworks. Denticulopsis (defined by Simonsen, 1979, and by far the most important genus biostratigraphically) is subdivided into Crucidenticula n. gen., Denticulopsis emend., and Neodenticula n. gen., based on submicroscopic features and phylogenetic relationships. All species belonging to the three genera recognized in Japan and its environs are discussed and illustrated in detail. The plicated members of genus Coscinodiscus, including C. temperei and C. yabei, are transferred to Thalassiosira. In total, 38 species are described in this paper.

Stratigraphic framework of the late Miocene Pisco Formation at Cerro Los Quesos (Ica Desert, Peru)

“…Preliminary SEM investigations have revealed their affinities with Thalassiosira flexuosa (Brun) (Akiba & Yanagisawa, 1986), as described by Tanimura (1996). Although the range of this species in DSDP site 173 is significantly older (LO ca.…”

Morphological and phylogenetic comparisons ofNeodenticula seminae(Bacillariophyta) populations between the subarctic Pacific and the Gulf of St. Lawrence

“…General valve outline and the deck and basal ridges associated with solid-walled costae (reported as primary pseudosepta in Akiba & Yanagisawa, 1986;Yanagisawa & Akiba, 1990;Semina, 2003) are common to three closely related genera, Neodenticula, Crucidenticula and Denticulopsis. However, basal ridges connected to the deck and the solid-walled costae on the internal valve were only observed in natural subarctic Pacific material , never in natural Gulf of St. Lawrence material, nor in cultures from both regions grown in natural Gulf of St. Lawrence waters enriched with F/2 culture medium plus silicate (Figs 28,32,35); vestigial-walled costae are barely distinguishable from the other costae.…”

“…However, careful attention to the type of colony formation, shape and colour of the chloroplast, and degree of silicification of valve ornamentation allows their differentiation. Moreover, all cleaned cells showed the distinctive valve features of Neodenticula, with its structural asymmetry along the apical axis, marginal fibulae from branching transapical costae, delicate and reduced areolation on the valve face, and the continuous raphe fissure from one pole to the other (Akiba & Yanagisawa, 1986;Yanagisawa & Akiba, 1990). These features, as well as the presence of walled costae and closed valvocopulae, allow N. seminae to be distinguished from Fragilariopsis species.…”

“…Later, Simonsen (1979) separated off the marine species of Denticula and erected a new genus, Denticulopsis, for species in which the pseudosepta exhibited peculiar, apically oriented, ridge-like thickenings near the margins. This new genus is by far the most important for the Neogene biostratigraphy of high latitudes in the Northern Hemisphere because it contains many abundant, short-ranging, widely distributed species (Akiba, 1986;Akiba & Yanagisawa, 1986;Yanagisawa & Akiba, 1990). Since its erection, many new taxa have been described, but only one extant species, Denticulopsis seminae (Simonsen et Kanaya) Simonsen.…”

“…Since its erection, many new taxa have been described, but only one extant species, Denticulopsis seminae (Simonsen et Kanaya) Simonsen. The increased number of species within Denticulopsis led Akiba & Yanagisawa (1986) to undertake a taxonomic re-examination of the described species. They showed and explained the diverse structure of the valves, which was also correlated with stratigraphic and geographic distributions.…”