Boreholes in the middle Miocene Echinarachnius (Clypeasteroid Echinoid) from Japan and their implications for cassid gastropods drilling

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  • 中期中新世のカシパンウニ化石Echinarachniusに見られるトウカムリ科巻貝による捕食痕
  • チュウキ チュウシンセイ ノ カシパンウニ カセキ Echinarachnius ニ ミラレル トウカムリカ マキガイ ニ ヨル ホショクコン

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Abstract

An assemblage of Echinarachnius microthyroides collected from the middle Miocene Daishoji Formation in Ishikawa Prefecture, was examined for drill holes as the predation track. The cylindrical holes were observed of 120 of collected 241 tests. The ratio of drill hole is affected by urchin test length, as less than 28mm are 22.0%, and more than 28mm are 69.5%. The feature of the drill holes as cylindrical, 2 to 3mm in diameter and unrepaired acid-etching edge, and of small percentage (6.7%) of tests of multiple holes suggest that they were produced by cassid gastropods, estimated to be several centimeters in length, such as Liracassis japonica. Almost all of the holes are complete, and opened on the aboral side, that is the predator attacked the test from the top. The tests of Echinarachnius microthyroides eaten by cassid gastropods were accumulated by wave action and buried in the sandy sediments under shallow marine environment.

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