Paleobiological and Taphonomic Implications of Atypical Diploporitan Echinoderm Thecal Attachment Structures—Two Illustrations from the Middle Silurian (Wenlock: Sheinwoodian) of Southern Indiana

The holocystitid diploporitan Paulicystis occurs within the middle Silurian (Wenlock: Sheinwoodian) Massie Formation of southeastern Indiana, where it utilized its large thecal base as an attachment structure. In softgrounds, Paulicystis typically employed the strategy of permanently encrusting small bioclastic fragments that were eventually overgrown by, and incorporated into, the basal theca. We describe two specimens of Paulicystis from the mudstone lithofacies of the Massie Formation that display unusual and/or noteworthy aboral attachment surfaces that provide information on the paleoautecology and taphonomy of this genus. One specimen displays a secondarily infilled basal depression corresponding to the original, elongate bioclastic fragment that was encrusted; rather than a broad, flat base, this specimen terminates in a blunt, rounded aboral lobe. This individual must have survived separation from its substratum and responded by infilling its cavity. The other specimen is characterized by a fairly typical, slightly concave basal surface that is encrusted by bryozoans and microconchids. This skeletobiontic association either reflects (a) earlier encrusters of the substratum that was subsequently encrusted by the echinoderm, having been removed along with the thecal base or, more likely, (b) fauna that encrusted the basal surface very shortly after the death, detachment, and reorientation of the diploporitan but prior to its ensuing rapid burial.