Interaction between dendritic radix structures of pelmatozoan echinoderms on a hardground in the middle Silurian (Wenlock) of southeastern Indiana, USA

A hardground surface in the middle Silurian (Wenlock: Sheinwoodian) Massie Formation is well exposed at the New Point stone quarry in Napoleon, southeastern Indiana, USA, where it is densely encrusted by pelmatozoan echinoderm attachment structures. Among the most common attachment structures encrusting the hardground are terminal dendritic radix structures attributable to the monobathrid camerate crinoid Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites. Radicles belonging to both of these genera are often modified by secretion of secondary stereom, resulting in swollen, irregular calcitic masses that extend onto the substrate and can engulf nearby bioclasts. Herein, we describe a specimen that resembles a portion of a swollen radicular attachment structure; however, close inspection reveals that is actually an interaction between radicles of Caryocrinites and Eucalyptocrinites, as indicated by the presence of both a trilobate lumen, characteristic of Caryocrinites, as well as a pentalobate lumen, characteristic of Eucalyptocrinites. Hence, what appears to be part of a single structure is actually parts of two interwoven structures belonging to entirely different taxa. Increased attention to echinoderm attachment structures consisting of overgrowths and secondarily mineralized radicles may result in recognition of additional specimens that represent analogous biotic interactions rather than simple holdfast occurrences.