Thousands of aggregating Archaeocidaris echinoids, many with Crurithyris planoconvexa brachiopods and fenestellid and encrusting bryozoan epibionts on their spines, occur in a Pennsylvanian (Missourian) echinoderm Lagerstätte in the Winchell Formation of north-central Texas. This occurrence documents the earliest recorded case of epibionts on echinoids, including usually unpreserved epibionts repeatedly attached to echinoid hosts. Additionally, this occurrence provides further insight into deep-time biotic interactions between mobile hosts and their attaching organisms.

All epibionts are preferentially distributed on the test-proximal ends of the spines. In a contemporaneous sample, no significant size differences are noted between echinoids with and those without epibionts. Epibiont distribution on the spines may occur from one or a combination of causes: antifouling behavior of the echinoid, accidental mechanical removal of epibionts via abrasion, or larval preference in settling. Crurithyris planoconvexa size ranges agree with previous studies of other attached brachiopod size distributions.

The echinoid-epibiont association commensal relationship appears to have benefited the epibionts by means of associational defense, transportation benefits, increased water-current and food-gathering possibilities, open habitat for colonization, and decreased competition with other epibionts. Conversely, costs for the epibionts would have included potential mortality from echinoid predators and transport into unfavorable environments. For the echinoids in this association, costs, such as increased drag on spines and exposure to epibiont grazers, far outweighed the benefits (e.g., potential camouflage effects), although both likely were minimal.

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