Abstract

Abundant columns of Isselicrinus ariakensis occur in a limited interval of the Upper Eocene Sakasegawa Formation in western Japan. Most of these are several centimeters in length, but curiously they usually stand upright, perpendicular to the bedding plane of the mudstone, and occur as bundles of 2 to 10 columns, with their original distal parts oriented downward. Based on detailed observation of the modes of occurrence as well as on CT scanning of the specimens, a reconstruction of the presumed mode of life is presented. The individuals of Isselicrinus adopted a relay strategy in the muddy substrate, using pre-existing upright columns as anchorage. Similar upright columns of Isselicrinus are also found, or have been reported, from other Eocene and Miocene strata in Japan, Argentina, and Denmark, suggesting that these Isselicrinus adopted this specialized mode of life as an adaptation to muddy bottoms.

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