A single specimen (Fig. 2) was photographed prior to destruction of the hardground surface. Based on comparison with published descriptions of echinoderms from the Napoleon quarry (Frest et al. 1999, 2011), it is apparent that this represents the holdfast of the sphaeronitid diploporitan Finitiporus boardmani (cf. Frest et al. 2011, pp. 67–68, text-Fig. 41, pl. 1.14–1.17). The holdfast was tightly embedded in the surrounding carbonate rock, preventing collection at the time the photograph was taken. Due to subsequent quarrying, the portion of the hardground surface was damaged, making photographs such as Fig. 2 the only remaining record of this fossil occurrence.
The proximal end of the attachment (labeled ‘p’ Fig. 2) is detached from the theca along an irregular, somewhat jagged surface; it is unclear whether catastrophic breakage or decay-induced disarticulation followed by more gradual breakage was responsible for separation between the holdfast and theca. The distal end of the holdfast (labeled ‘d’ in Fig. 2) is completely separated from the primary substrate, having become dislodged and reoriented prior to burial. Frest et al. (2011) noted that F. boardmani exclusively encrusted relatively large, flat surfaces such as hardgrounds, microbioherms and cephalopods; however, the dimensions of the basal attachment area do not allow identification of the specific bioclast or lithoclast, if any, that was initially encrusted.
Slightly more than 40 mm in length, the holdfast is relatively wide at the proximal end, tapering slightly to a minimum at the base of the elongate, presumably hollow, stem-like attachment before expanding into a basal disc of approximate width to the proximal end of the holdfast (Fig. 2). Embedment in matrix prevents determination of the total number of plates comprising the structure. Frest et al. (2011) reported distal portions of thecae of F. boardmani that consisted of both six and seven plates, both of which are feasible given the width of exposed plates (Fig. 2). Portions of three elongate (~35 mm long) plates are exposed, with one plate completely exposed (labeled ‘1’ in Fig. 2), approximately half of another plate visible (labeled ‘2’ in Fig. 2) and only a small portion of the third plate visible (labeled ‘3’ in Fig. 2). Sutures between adjacent holdfast plates are regular and straight with no evidence of ankylosis or crenulation. All plates are devoid of macroscopic pores, a state that is relatively rare among the diploporitan assemblage of the Massie Formation, but a distinctive characteristic of F. boardmani (Frest et al. 2011; Thomka and Brett 2014a). The basal attachment disc (labeled ‘d’ in Fig. 2) is relatively solid, though it appears that one of the sutures between elongate plates of the stem-like area extends into the flared disc, so it may be composed of multiple plates, possibly six or seven corresponding to the plates of the rest of the distal theca.