The specimen described herein was recovered from a well-exposed, laterally continuous hardground surface at the northern end of the New Point Stone quarry in Napoleon, Ripley County, southeastern Indiana, USA (N39°12′31.39′′, W85°18′53.74′′; Fig. 1). This surface represents the contact between the basal carbonate lithofacies and the overlying mudstone lithofacies of the middle Silurian (Wenlock: Sheinwoodian) Massie Formation (Brett et al. 2012), a contact that previous workers (e.g., Foerste 1897; Frest et al. 2011) had recognized as the boundary between the ‘middle Osgood limestone’ and the ‘upper Osgood shale’ prior to regional lithostratigraphic revisions. Hardground formation reflects sediment starvation associated with the most rapid rate of relative base-level rise during a third-order stratigraphic sequence (McLaughlin et al. 2008; Brett et al. 2012; Thomka and Brett 2015a).
This hardground has been the subject of several recent studies focusing on the abundant and diverse assemblage of encrusting attachment structures belonging to pelmatozoan echinoderms (Thomka and Brett 2014a, b, 2015a, b, 2017; Thomka and Motz 2014). To date, 13 holdfast morphologies have been identified, including those attributable to holocystitid and sphaeronitid diploporitans, hemicosmitid rhombiferans, and camerate (as well as potential flexible, cladid, and disparid) crinoids (Thomka and Brett 2015a, 2017). Some of the most common attachment structures encrusting the Napoleon quarry hardground are dendritic radix structures belonging to the monobathrid camerate crinoid Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites (Thomka and Brett 2015a, b).
Although the attachment structures of these two taxa are morphologically similar, they can be distinguished from one another on the basis of lumen size and shape, and radicular structure. Lumina are trilobate and comparatively large in Caryocrinites, and pentalobate and comparatively small in Eucalyptocrinites (Halleck 1973; Brett 1981; Thomka and Brett 2015a). Radicles consist of solid rods of undifferentiated stereom with no lumina in Caryocrinites, in contrast to the recognizable lumen-bearing ossicles, often coated in a cortex of secondary stereom, in Eucalyptocrinites (Brett 1978, 1981, 1984; Thomka and Brett 2015a, b; Plotnick et al. 2016).
Description of specimen
The material described herein is reposited in the Cincinnati Museum Center (Cincinnati, Ohio, USA), under specimen number CMC IP82656 (Fig. 2). It consists of a white-coloured mass of echinodermal skeletal calcite embedded in a light-brown biomicrite matrix. Echinoderm material is oriented parallel to the bedding plane and is strongly cemented to the surrounding sedimentary rock, indicating encrustation of a hardground surface. The upper surface has the appearance of a wide, irregularly rod-like mass—a radicle—with numerous thin, thread-like extensions of calcite extending from both sides of the radicle into immediately adjacent sedimentary rock (Fig. 2A). This structure seemingly represents a portion of one primary branch of a distal radicular attachment structure, with the smaller tendril-like extensions serving to increase surface area of encrustation (Brett 1981, 1984; Thomka and Brett 2015a). The radicle is knobby and uneven in thickness throughout the visible surface, suggesting development of a coating of secondary stereom, as is commonly observed in attachment structures in both softground and hardground settings (e.g., Brett 1978, 1981, 1984; Thomka and Brett 2015a).
When viewed in cross-section (Fig. 2b), it becomes apparent that the specimen does not represent a single radicle, but, instead and exceptionally, represents two radicles that were interwoven into one composite structure. This is indicated by the presence of two discrete lumen structures, one with a trilobate lumen (representing Caryocrinites) and the other with a pentalobate lumen (representing Eucalyptocrinites). Both radicles have relatively large diameters, reflecting the overgrowth by secondary stereom (Fig. 2b). The Caryocrinites radicle is uniformly circular in outline, in contrast to the somewhat crescent-shaped outline of the Eucalyptocrinites radicle (Fig. 2b); the more irregular shape of the Eucalyptocrinites radicle appears to reflect overgrowth of the Caryocrinites radicle.
Investigation of the smaller branches extending off of the main radicles confirms the identity of this specimen as a composite Caryocrinites–Eucalyptocrinites structure. The branches emanating from the pentalobate lumen-bearing radicle contain minute pentalobate lumina of their own (Fig. 2c), as has been documented for Eucalyptocrinites in other studies (Brett 1981, 1984; Thomka and Brett 2015a; Plotnick et al. 2016). The branches extending from the trilobate lumen-bearing radicle contain no lumina and do not appear to be divisible into distinct component ossicles (Fig. 2c), as is consistent with the radix structure of Caryocrinites (Brett 1978, 1981; Thomka and Brett 2015a). The boundary between these two radicles is not visible on the upper surface of the specimen, testifying to the degree of interpenetration between the stereomic overgrowths produced by these echinoderms.