A non-marine <i>Actinocyclus</i> species from a volcanic ash layer in the middle Miocene Otogawa Formation in the Yatsuo area, Toyama, central Japan

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  • Saito-Kato Megumi
    Department of Geology and Paleontology, National Museum of Nature and Science
  • Yanagisawa Yukio
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)

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Other Title
  • 富山県八尾地域の中部中新統音川層中の火山灰層からの非海生<i>Actinocyclus</i>属の産出報告
  • A non-marine Actinocyclus species from a volcanic ash layer in the middle Miocene Otogawa Formation in the Yatsuo area, Toyama, central Japan

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Abstract

<p>A fossil Actinocyclus species is reported from a volcanic ash layer (OT1 Tuff Bed) within middle Miocene marine deposits of the Otogawa Formation, Yatsuo area, Toyama Prefecture, central Japan. The co-occurrence of this species with exclusively freshwater Aulacoseira species suggests a freshwater origin for the Actinocyclus. The freshwater diatoms were present in the water or sediment of a lake on land and were caught up in a pyroclastic flow that was deposited to form the OT1 Tuff bed. The Actinocyclus specimens are neither identified with certainty as a published species nor described as a new species because they have been affected by diagenesis and the valves are poorly preserved; however, the specimens are similar to Actinocyclus krasskei. Their valve morphology is as follows: discoidal; 20–75 µm in diameter; radial or vaguely fasciculated areola rows on valve face; areola density 11–14 in 10 µm; rimoportulae on short hyaline stripes; external pores of rimoportulae are larger than areolae and appear as openings on the mantle just below the valve face/mantle junction; broad fan-shaped rimoportulae expand parallel to the valve face with short stalks that appear as ovals in cross-section; a pseudonodulus is observed under the light microscope on the inside of the valves as a simple pore or a pore with depression; thick silicification with modified areola openings at the external side of the valve face/mantle junction; and large areola chambers with a thin valve wall without spongy structure. These morphological characters resemble those of Actinocyclus krasskei, A. tubulosus, and other non-marine Actinocyclus species. This occurrence of Actinocyclus highlights the importance of volcanic ash for preservation of diatoms in lacustrine settings, and fills a gap in the diatom fossil record of Japan.</p>

Journal

  • Diatom

    Diatom 36 (0), 69-79, 2020-12-25

    The Japanese Society of Diatomology

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