Crinoid encrustation of holocystitid diploporitan echinoderms: strongly asymmetrical Silurian dendritic attachment structures with palaeobiological implications

Articulated thecae of the holocystitid diploporitan echinoderm Holocystites scutellatus from the middle Silurian (Wenlock: Sheinwoodian) Massie Formation of southeastern Indiana, USA, are encrusted by distinctive structures belonging to another echinoderm. A dendritic attachment structure consisting of multiple slender, branching radices, attributable to the camerate crinoid Eucalyptocrinites, is present on one side of each of the diploporitan thecae. However, the development of radices is remarkably asymmetrical, with all radices—including one more than 25 mm in length—being present exclusively on one side of the attachment structure. This reflects initial settlement by the encrusting crinoids near the oral or marginal regions rather than the central portion of the diploporitan thecae, which were on their sides; this essentially prohibited further outward growth of radices toward the oral area or edges, but allowed radices oriented in the opposite direction to extend over nearly the entire length of the lateral surface of the theca. Although crinoid encrustation of holocystitid diploporitan thecae is moderately common in the Massie Formation, no previously described specimens display such pronounced asymmetry with respect to radice development. More importantly, these specimens convincingly illustrate the degree to which Eucalyptocrinites attachment structure morphologies could be modified in response to local substrate variations; such skeletal modules were, indeed, highly dynamic, probably contributing to the success of taxa bearing such adaptable attachment structures.