Abstract

We describe a Pennsylvanian (Late Carboniferous) Parapostibulla edrioasteroid that competed for living space with an encrusting cryptostome bryozoan on the brachiopod Composita. The colonial bryozoan grew faster than the solitary edrioasteroid, and the bryozoan increased its colony size by slowly encircling the edrioasteroid. Neither organism was able to overgrow the other, even where the margins became thick, and each had to accommodate the presence and possible defenses of the other. Evidence for defensive measures by edrioasteroids has not previously been reported, but this interaction does emphasize that edrioasteroids could not move away from severe crowding or overgrowth by other organisms.

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