What is the producer of "trace fossil of Excirolana chiltoni"? : Tracemaking mechanism of Macaronichnus segregatis

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  • Nara Masakazu
    Department of Geology and Mineralogy, Faculty of Science, Kyoto University

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Other Title
  • "ヒメスナホリムシの生痕化石" の形成者は何か? : 生痕化石 Macaronichnus segregatis の形成メカニズム
  • ヒメスナホリムシ ノ セイコン カセキ ノ ケイセイシャ ワ ナニカ セイコン

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Abstract

Macaronichnus segregatis, which is recognizable as white colored spots having 2 to 5mm in diameter, occurs in the Middle to Late Pleistocene foreshore deposits distributed in the Kanto plain, central Japan. It is a long, sinuous, unbranched burrow without apparent wall or burrow lining. This fossil has been regarded in Japan as trace of locomotion of a small isopod, Excirolana chiltoni. The microscopic observation reveals that the particles in the burrow filling are composed of sand grade colorless minerals such as quartz and feldspar, rich in poorly rounded particles as compared to the surrounding sediments. A deformed lamination, which may have been synchronously formed with the burrow, is discernible in the laminated sand of the burrow-bearing horizon. The tracemaker seems to penetrate the sediments with peristaltic movement, called intrusion. According to my field observation and experimental study E. chiltoni never make a trace like M. segregatis. The isopod moves through sediments by backfilling but not by intrusion. Taking all things into consideration, the tracemaker is not E. chiltoni, but a probable vermiform animal, and the present trace fossil is probably of fecal origin. The tracemaker may have selectively ingested particles in relation to their shape and specific gravity, though the exact mechanism of particle selection is yet unknown.

Journal

  • Fossils

    Fossils 56 (0), 9-20, 1994

    Palaeontological Society of Japan

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