EPIZOANS ON FLEXICALYMENE (TRILOBITA) AND IMPLICATIONS FOR TRILOBITE PALEOECOLOGY

Epibionts on specimens of Flexicalymene (Late Ordovician, Cincinnati A rch region, U.S.A.) that were encrusted while alive show an apparent p reference for attachment along the relatively elevated sagittal axis o f the exoskeleton (glabella, axial rings of thorax and pygidium) or th e medial portion of the cephalic doublure. This site-specificity suppo rts 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 s ubstrate and perhaps benefitting from feeding currents generated by th e trilobite). Epizoans are found only on larger individuals of this tr ilobite, suggesting a terminal molt stage for Flexicalymene. Although the number of previously reported encrusted trilobites is small, a sur vey of the literature suggests the following hypotheses concerning the relationships between trilobite life habit and susceptibility to epiz oan colonization: 1) infaunal (burrow dwelling) trilobites were genera lly free of epizoans; 2) a small proportion of shallow-burrowing (semi -infaunal or ''ploughing'') trilobites were suitable hosts for epizoan s, but the epizoans preferentially colonized the elevated or anterior portions of the exoskeleton; 3) epifaunal (and some pelagic?) trilobit es were more susceptible to encrustation.