Holopus spileccense ( Schlüter, 1878 )

Martinez-Soares, Pablo, Roux, Michel, Giusberti, Luca, Gatto, Roberto, Eléaume, Marc & Améziane, Nadia, 2024, New Eocene species of the crinoid genera Holopus and Cyathidium (Cyrtocrinida: Holopodidae) from north-eastern Italy, Zootaxa 5541 (4), pp. 401-437 : 424-428

Holopus spileccense ( Schlüter, 1878)

Figs. 7 , 9 , 12–14 ; Tables 11–13

Synonymy. Cyathidium spileccense Schlüter, 1878: 54 , 65, pl. 3, figs. 11–15; Carpenter, 1884: 212; Jaekel, 1891: 614–616, fig. 14; Jaekel, 1907: 293; Holopus (Cyathidium) spileccense Jaekel, 1891: 567 , Holopus spileccense Jaekel, 1891: 607 , 615–616; Holopus (Cyathidium) spileccensis Jaekel, 1891: 592 ; Holopus spileccensis Jaekel, 1891: 591 , 619–620; Cyathidium spileccense Nielsen, 1913: 17 , 40, 58; Wanner, 1929: 319; Holopus spileccensis Sieverts, 1931: 174–175 ; Holopus spileccense Biese & Sieverts-Doreck, 1939: 117 ; Manni, 2005: 216–217, pl. 1 figs. 9–10, text-fig. 5a–b.

Non Holopus spileccense Manni & Pacioni, 2021: 69–73 , figs. 4–6.

Material examined. Nine aboral cups, 12 IBr, 4 IIBr

Diagnosis. Cup with strong constriction in the aboral half; adoral part with markedly pentagonal cross-section; marked radial ridge without other ornamentation; frequently slight interradial depression; calycinal cavity without inner interradial process; measured maximum cup size: H 8.1 mm, D 6 mm; IBrax wider than high; Y-shaped ridge coarse, usually simple (not doubled) with branches corresponding to radial axis of brachial ramifications; no other ornamentation; small IBrax with more marked and doubled ridge; external ornamentation of proximal IIBr with coarse X-shaped and discrete mid-radial ridge; articular facets with fulcral ridge slightly shorter than maximum brachial width; no prominent muscular or ligament pits; more distal IIBr and keystone plates unknown.

Type stratum. Lower Ypresian .

Type locality. Spilecco Hill near Bolca, north-eastern Italy.

Description of material examined. Quantitative characters of aboral cup relatively variable ( Table 11), H/D from 1.02 to 1.63 independent of size (D used as growth index; Fig. 9 ). Constriction at mid-height conspicuous (D/Dcs up to 1.73), upper adoral cup and adoral face markedly pentagonal with each angle forming a radial ridge ( Fig. 12E ), no additional ridges or granulation on external surface, no trace of interradial process and inner process masked by recrystallized sediment ( Fig. 12D ), attachment surface flat to slightly concave ( Fig. 12F ), cup curvature usually marked, especially in the two largest specimens (H/h 1.5 to 1.6) ( Fig. 12G ).

Primibrachials pentagonal except for a single anomalous quadrangular specimen MGP-PD 33245 Br7 ( Fig. 12C ). Pentagonal IBrax broader than high (0.75<H/Wd<0.86), in smallest (youngest) specimens nearly as wide as high ( Fig. 13A–B ), external face showing coarse Y-shaped ribs, no other ornamentation ( Fig. 13A–B ), Y-pattern more conspicuous in small specimen ( Fig. 13A ) than in larger ones ( Fig. 13E–F ) (quantitative characters listed in Table 12). The coarse Y-shaped rib identifies the quadrangular specimen MGP-PD 33245 Br7 as a primibrachial (probably corresponding to a bivium). Articulations poorly preserved.

IIBr belonging to proximal series slightly wider than high with H/h 1.12 to 1.42 ( Table 13), no keystone plate identified possibly due to the small number of IIBr found; external face showing a coarse X-shaped rib with a discrete mid-radial ridge ( Fig. 12A ). Distal IIBr articulations rather symmetrical ( Fig. 12B ). The articular facets appear to present a fulcral ridge that is slightly shorter than the maximum brachial width and there are no prominent muscular or ligament pits.

Remarks. The maximum sizes of the aboral cup and IBrax of H. spileccense have been estimated using the same method as for H. fabianii sp. nov. (see above) and are nearly the same as the maximum sizes measured. However, this estimation might be biased due to the small number of brachials available.

Schlüter (1878) figured four aboral cups without describing them in the text. The current whereabouts of these are unknown to us. Three of the four are rather straight without significant curvature. The fourth is markedly curved (H/h 1.56). It also appears to be the best-preserved specimen and its adoral face and side view are figured enlarged, showing the main characters of the species ( Fig. 14 ; Schlüter 1878, pl. 3, figs 11, 12). Its cup is pentagonal, higher than wide (H/D 1.22), with a conspicuous constriction (D/Dsc 2.09) in the lower part (H/ht 1.33), marked radial ribs, slight interradial depressions with an outer interradial process, and symmetric, rounded muscular synarthries. While not explicitly shown in the drawing, the calycinal cavity is described in the text as being partly filled with sediment. We here designate this drawn aboral cup as the lectotype of H. spileccense . The side views of the three paratypes show variable general shape (1.37<H/D<1.78, 1.4<D/Dcs<2.0). Jaekel (1891: 616, fig. 14) figured an aboral cup (sent to him by Schlüter) that is very similar to the lectotype except that it is not curved (H/D 1.26, H/h 1.03, H/ht 1.45, D/Dcs 1.98). As Schlüter (1878) and Jaekel (1891) do not give precise measurements, we can only deduce ratios with some confidence from the figures. Measures taken from a printed copy of Schlüter (1878) are listed in Table 11. Manni (2005) described and figured a single aboral cup from the Michelotti Collection (H/D 1.0, H/h 1.04, H/ht 1.92, D/Dcs 1.29). With the 9 additional aboral cups described here, we now have 15 cups from Spilecco allowing us to assess the variation of quantitative and qualitative characters in H. spileccense . A comparison with data for H. fabianii sp. nov., based on eight aboral cups, indicates a similar range of variation in the two species ( Table 14). Holopus spileccense differs in having the adoral part of the cup more markedly pentagonal and having five usually well-marked radial ribs as well as interradial depressions that can be more or less pronounced.

A comparison of the quantitative characters of the brachials of H. spileccense and H. fabianii sp. nov. reveals further differences ( Tables 15 and 16). The former species differs in having IBrax that are wider than high (H/Wd up to 0.93, mean 0.84) and IIBr are less asymmetrical (H/h up to 1.43, mean 1.19) tending to be almost as high as wide (mean H/Wp 0.91). The brachials of both species differ further in their ornamentation. While the number of brachials available is limited, the ornamentation patterns remain consistent: coarse ribs without granulation in H. spileccense versus finer ridges with pronounced derived granulation in H. fabianii sp. nov. Nevertheless, both species share the same Y-shaped ridge pattern in IBrax and X-shaped ridges in IIBr. The two species are likely closely related as both might lack the inner interradial processes in the aboral cup and share keystone plates, which have not yet been found in H. spileccense . The presence of a well-developed keystone plate would put both these species closer to the extant H. alidis than to the Carribean congeners. However, if the drawings of H. spileccense by Schlüter are accurate, its rounded radial articulations with an interradial depression and the outer interradial process are reminiscent of H. rangii .

The brachials from the Lutetian of the Chiampo Valley attributed to H. spileccense by Manni and Pacioni (2021) are instead here referred to H. fabianii sp. nov. (see above). Thus, to date, the stratigraphic distribution of H. spileccense appears confined to the Ypresian.

Occurrence. Lower Eocene (lower Ypresian) of Spilecco Hill near Bolca, north-eastern Italy.

Figures

FIGURE 7. Main quantitative characters of aboral cup and IBrax in fossil Holopodidae. S: Cyathidium holopus from Seymour Island. Straight lines: values of H/D and H/Wd. Other values in mm.

FIGURE 9. Main quantitative characters of Holopodidae from the Eocene of north-eastern Italy. Cy: Cyathidium, Ho: Holopus, Ho.a.: juvenile of H. alidis (MNHN-IE-2014-2185). Straight lines: values of H/D and H/Wd. Other values in mm.

FIGURE 12. Holopus spileccense, Spilecco. A–B: secundibrachials, A: external surface (MGP-PD 33247 Br8), B: distal facet (MGP-PD 33247 Br4), C: non-axillary IBr, external surface (MGP-PD 33245 Br7), D–G: aboral cups (MGP-PD 33242), D: adoral face (A7), E–F: A6, E: oblique lateral adoral view, F: oblique aboral view, G: lateral view, A2. Scale bars: 1 mm.

FIGURE 13. Holopus spileccense, Spilecco, IBrax, variation in external ornamentation (MGP-PD 33244).A–B: small specimens, A: Br12, B: Br9, C–E: medium-sized specimens, C: Br14, D: Br13, E: largest specimen, Br1, F: Br16. Scale bars: 1 mm.

FIGURE 14. Lectotype of Holopus spileccense (modified from Schlüter 1878, pl. 3, figs. 11–12). A: adoral face, B: side view. Scale bars: 1 mm.