Epibionts on specimens of Flexicalymene (Late Ordovician, Cincinnati Arch region, U.S.A.) that were encrusted while alive show an apparent preference for attachment along the relatively elevated sagittal axis of the exoskeleton (glabella, axial rings of thorax and pygidium) or the medial portion of the cephalic doublure. This site-specificity supports an interpretation of a semi-infaunal habit for this trilobite and may indicate a commensal relationship between the epizoan and host (e.g., epizoan bryozoans exploiting the trilobite exoskeleton as a hard substrate and perhaps benefitting from feeding currents generated by the trilobite). Epizoans are found only on larger individuals of this trilobite, suggesting a terminal molt stage for Flexicalymene. Although the number of previously reported encrusted trilobites is small, a survey of the literature suggests the following hypotheses concerning the relationships between trilobite life habit and susceptibility to epizoan colonization: 1) infaunal (burrow dwelling) trilobites were generally free of epizoans; 2) a small proportion of shallow-burrowing (semi-infaunal or ''ploughing'') trilobites were suitable hosts for epizoans, but the epizoans preferentially colonized the elevated or anterior portions of the exoskeleton; 3) epifaunal (and some pelagic?) trilobites were more susceptible to encrustation.
