The genus Paulicystis is an uncommon but distinctive component of the middle Silurian Holocystites Fauna of North American diploporite echinoderms. Within the Massie Formation (Wenlock: lower Sheinwoodian) of southern Indiana, USA, Paulicystis is shown to be an ecologically versatile sclerobiont capable of modifying its aboral region in order to encrust diverse substrata, both bioclastic and abiogenic. Most commonly, Paulicystis attached to bioclasts projecting from soft, fine, siliciclastic-dominated sediment. Bioimmuration preserves chemically unstable aragonitic encrusting attachment sites as inexplicit steinkerns, commonly cephalopod phragmocones. Well-preserved attachment sites frequently included other diploporite thecae. Paulicystis attached by a distinctive broad attachment disk that could be either flat-based or curved depending on the nature of the underlying surface. On laterally continuous carbonate hardgrounds, Paulicystis developed a flat-based aboral disk that was directly cemented to slightly elevated, well-winnowed hummocks. Comparison of strategies employed to occupy the range and diverse characteristics of successfully encrusted substrata (i.e., hardgrounds, cephalopod shells, echinoderm thecae, and bioclastic debris) highlights the paleoautecological significance of aboral morphological plasticity and the ability to detect and respond to substrate properties. Paulicystis is thus among the most adept of Silurian sclerobiontic organisms, being capable of sclerobiosis in both hard- and soft-substrate settings.