軟体動物化石にみられる巻貝による穿孔現象
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概要
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Molluscan shells from Quaternary and Pliocene formations in Japan often are penetrated in cylindrical form through the inner surface to the outer one. Especially, we examined in this paper about the shells which were bored in a bottomless bowl-like shape. The borehole is morphologically similar to one which is opened by a modern gastropod. The contents of examination are a) morphological characters (Fig. 1), b) penetrated species (Table 2), c) ratio of penetrated species' numbers to the total in each formation and locality (Figs. 2, 3), d) ratio of penetrated shells' number to the total in each species (Fig. 4), e) position of boreholes on valves (Fig. 5), f) diameter of boreholes at the inner and outer surfaces, g) size of bored and nonbored shells (Figs. 7, 8). The boreholes are accepted as a evidence of predation by gastropod. Ancient carnivorous gastropods could not be decided exactly, because we have no direct evidences. But judging from modern drillers, it has been safely assumed that they might be Tectonatica and Neverita. The specific content of penetrated shells and their percentages to the total number species differ in each fossil assemblage. When the content of a fossil assemblage is similar, the species of penetrated shells are also very similar. The position of boreholes on a valve is definite in each species, but the reason could not be solved. The percentage of penetrated shells decreases with the increasing size of a shell. The number of inferred penetrated individuals attains to 60-80% in case of small size. So the penetration may be the principal cause of death for some kinds of molluscs.
- 地学団体研究会の論文
- 1971-01-25