Evidence for biotic interaction is not obtainable easily in the fossil record. Epizoans cemented to the skeletons of host organisms provide an opportunity to study possible host–epizoan interactions and the ecological structure of hard substratum communities. One such possibility is sponges and their associated encrusters, but few such occurrences have been documented from the fossil record. Early Ordovician limestones of the San Juan Formation in the Argentine Precordillera bear an important sponge fauna consisting mainly of demosponges. This Early Llanvirn assemblage has the highest diversity, and most sponges were subjected to epizoan encrustations. A total of nineteen species included in eleven genera of sponges were examined to record epizoans encrustations. Type and relative abundance of epizoans were compiled for each sponge species. Five kinds of epizoans were recorded: bryozoans, stemmed echinoderms, brachiopods, other sponges, and scars and borings of undetermined organisms. Crinoids and blastozoans are widespread common epizoans on sponges. Small brachiopods are attached to sponges showing no location preference; rarely, juvenile sponges appear on conspecific adults. Sinuous scars and worm-like organisms are found in almost all the sponges. Deep borings, like those found in Devonian or younger sponges, are absent. The analysis of epizoan distribution on each sponge species shows a fairly homogeneous distribution of echinoderms on sponges but host specificity of bryozoans. Here, surface texture seems to have been a significant controlling factor. Almost all the trepostome bryozoa adhere to Rhopalocoelia tenuis Carrera. This taxon is a cylindrical sponge whose smooth surface texture contrasts sharply with the rough and open textured spiculation observed among other species in the fauna. This feature seems to have been a key factor for the mutual relationship. Rhopalocoelia tenuis shows a reaction to epizoans, modifying the shape of growth and changing the direction of the spicular structure in the vicinity of the epibiont, providing evidence that encrustation occurred while the sponge was alive.

Diversity, composition and encrusting strategies of the Argentine epizoan community reflect the early evolutionary stage of this biotic relationships in the Early Paleozoic. This contrasts with the more varied and diversified relationships among organisms that were able to encrust, bore, and chemically perforate hard parts after the Devonian. The Early to Middle Ordovician was a time of rapid evolutionary diversification among hard substrate communities. The Argentinean epizoan community appears to be transitional between the first hard substrate encrusters (Late Cambrian), which were eocrinoid dominated and were not accompanied by macroborers or bryozoans, and the later Paleozoic communities, in which bryozoans were the most prolific utilizers of attachment and macroborers were common.

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