Large diploporitan thecal attachment structures on diploporitan thecae: An uncommon encrustation relationship involving stalked echinoderms from the Silurian of the Cincinnati Arch region, eastern midcontinental USA

Holocystitid diploporitans (Echinodermata) are abundant within the middle Silurian (Wenlock: Sheinwoodian) Massie Formation of southeastern Indiana, USA. Diploporitan thecae are commonly encrusted by a variety of organisms, including the attachment structures of crinoids; however, no diploporitan aboral thecal attachments have previously been described in detail as encrusters on co-occurring diploporitan thecae. Herein, we describe thecae of that are encrusted by large discoidal attachment structures attributable to undetermined holocystitid diploporitans. The thecal attachment structures are discoidal, multi-plated, pore-bearing, and characterized by circular central depressions floored by flat platforms — a morphology that is identical to that of diploporitan attachments described from a laterally continuous hardground surface at the same locality. These encrusted specimens are significant because they demonstrate that (1) encrusting diploporitans were capable of developing identical aboral attachment morphologies when encountering both hardgrounds and sufficiently large bioclasts in softgrounds; and (2) encrusted diploporitan thecae were capable of remaining articulated long enough and in a sufficiently stable position to serve as substrata for large thecal attachments. Taken collectively, holocystitid diploporitans were characterized by greater aboral morphological plasticity and resistance to disarticulation than generally recognized.