In situ preservation of “Pholas tzayi Hu, 1992” (Mollusca, Pholadidae) from Chinshui Shale (Pliocene) in western Taiwan

  • First report of in situ preservation of Pliocene bivalves from Taiwan
  • Preferred fossil orientation contains data on stratigraphic facing
  • New index fossil for Chinshui Shale with hypothesized deposition model
Abstract

A total of 30 specimens of “Pholas tzayi Hu, 1992” were collected from the Chinshui Shale (Upper Pliocene) at the Laotinliao Creek, Touwu Township, Miaoli County, Taiwan. Among them, 18 specimens are preserved with both valves and 8 specimens are preserved with diagnostic characters. Field observation shows that many specimens are preserved with unbroken valves and buried with the posterior end pointing upward and perpendicular to the bedding; thus, stratigraphic facing can be inferred based on pholadid bivalves buried in situ. This is important because Chinshui Shale consists of intensely bioturbated, muddy sandstone and organic-rich mudstone without obvious bedding surfaces. The layer of intact pholadid bivalves not only provides clues about original seafloor conditions but also contains information about original bedding orientation. Furthermore, this layer has been noted in more than one locality and can be used as a key layer for correlation. Rapid sedimentation events, such as obrution, can contribute to the death of organisms. Organisms found in obrution deposits tend to have a better chance to get preserved in the fossil records because that rapid burial implies anoxic or dysoxic conditions within the sediment layers preventing further bioerosion due to scavenging activities. This study helps us to understand the invertebrate faunal evolution near the Plio-Pleistocene boundary in Taiwan. Due to the short range of occurrence of this endemic species “P. tzayi” and has been reported from several localities, it is recognized as one of the key fossils for Chinshui Shale.

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