Holopus fabianii, Martinez-Soares & Roux & Giusberti & Gatto & Eléaume & Améziane, 2024
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publication ID |
https://doi.org/ 10.11646/zootaxa.5541.4.1 |
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publication LSID |
lsid:zoobank.org:pub:4F812F60-9242-4F44-8E25-99381FD7E8B3 |
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persistent identifier |
https://treatment.plazi.org/id/2F1AC921-FFDD-FFD8-F2F0-FC18FBC25D28 |
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treatment provided by |
Plazi |
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scientific name |
Holopus fabianii |
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sp. nov. |
Holopus fabianii sp. nov.
Figs. 7 View FIGURE 7 , 8H–J View FIGURE 8 , 9–11 View FIGURE 9 View FIGURE 10 View FIGURE 11 ; Tables 7–10
Synonymy. Holopus spileccense Manni & Pacioni, 2021: 69–73 , figs. 4–6; Frisone et al., 2020: 306 (non Schlüter 1878).
Type material. Eight aboral cups including the holotype ( Fig. 8H–I View FIGURE 8 ), one figured paratype (paratype 1, Fig. 8J View FIGURE 8 ), six non-figured paratypes (paratypes 2–7); 17 figured brachials including 12 IBrax (paratypes 8–19, Fig. 10A–L View FIGURE 10 ) and five IIBr (paratypes 20–24, Fig. 11A–H View FIGURE 11 ); 14 non-figured IBrax (paratypes 46–59), 21 non-figured IIBr (paratypes 25–45) including three keystone plates and three IIBr from distal series.
Other material examined. 17 IBrax, 58 IIBr of Wp<2.8 mm, 21 IIBr of Wp> 2.8 mm.
Etymology. Species dedicated to Ramiro Fabiani (1879–1954) for his contributions to stratigraphy and palaeontology of the Paleogene from north-eastern Italy.
Diagnosis. Cup with conspicuous constriction in the lower half; upper part of cup with rounded pentagonal cross-section; pronounced radial ribs and marked interradial ribs; adoral face without external interradial process; calycinal cavity unknown; estimated maximum cup size: H 21.0 mm, D 14.8 mm; IBrax about as high as wide, with Y-shaped ridge, upper part of ridge (V-shape) irregularly doubled, usually with outer branches reaching angles of maximum brachial width (Wd) and inner branches corresponding to radial axis of brachial ramifications; small IBrax with only a few tubercles or none at all; large IBrax with several tubercles especially in the proximal half; external ornamentation of proximal IIBr consisting of X-shaped positioned tubercles, muscular synarthries with fulcral ridge shorter than maximum brachial width axis, adoral border forming an obtuse angle giving relatively narrow V-shape section; keystone plates with distal muscular synarthry roughly triangular and rounded, proximal synarthry symmetrical with fulcral ridge as long as Wp, external face smooth, shaped like an isosceles triangle, four conspicuous parallel symplexial crenulations on each side; distal IIBr series higher than broad, muscular synarthries with inconspicuous fulcral ridge, marked aboral ligament pit and axial nervous canal, conspicuous adoral muscle pits.
Type stratum. Lower Lutetian , tuffite horizon .
Type locality. Cengio dell’Orbo quarry in the Chiampo Valley, north-eastern Italy .
Description of type series. Among the 8 aboral cups of the type series (quantitative characters listed in Table 7), the holotype was selected as the best preserved aboral cup ( Fig. 8H, I View FIGURE 8 ). Holotype medium sized (H 3.83 mm, D 2.75 mm), with a strong constriction (Db/Dcs 1.68) located in the aboral half and separating adoral two-thirds of cup from a broad basal part with flat attachment surface with a greater diameter than the adoral side (D/Db 0.88); adoral part with rounded pentagonal cross-section demarcated by five conspicuous radial ribs with intermediate ribs more pronounced on one side ( Fig. 8H View FIGURE 8 ) than on the other ( Fig. 8I View FIGURE 8 ); adoral face without external interradial process, calycinal cavity filled with recrystallized sediment making it impossible to identify any possible internal interradial processes, if present. Other aboral cups of type series are of variable sizes (H 1.72 to 6.55 mm, D 1.76 to 4.62 mm), usually height greater than adoral diameter (H/D 1.21 to 1.48) except in small paratype 7 (H/D 0.98), H/D increasing from nearly 1 to more than 1.4 with size ( Fig. 9 View FIGURE 9 ), similar characters as the holotype, more or less conspicuous, attachment surface often smaller than adoral diameter and very irregular, sometimes appearing as a mold of a subcylindrical substrate ( Fig. 8J View FIGURE 8 ), cup with moderate curvature (H/h~1.3) only in paratype 3.
Type series of IBrax (paratypes 8 to 19) illustrate changes in form and ornamentation with size ( Fig. 10 View FIGURE 10 , Table 8). Smallest IBrax nearly as high as wide; external surface ( Fig. 10A–E View FIGURE 10 ) with a more or less thick Y-shaped ridge, distal part of ridge (V-shape) irregularly doubled with usually outer branches reaching angles of maximum brachial width (Wp) and inner branches corresponding to radial axis of brachial ramifications; proximal external surface without tubercles on either side of axial ridge. Larger IBrax about as wide as high except paratypes 10 and 18 which show H/ Wd 0.82 and 0.84, respectively; external ornamentation ( Fig. 10I–L View FIGURE 10 ) showing Y-shaped ridge composed of a short lower segment and predominantly longer V-shaped segments corresponding to radial axis of brachial ramifications, 10 to 20 large tubercles developed irregularly over the entire external surface, sometimes transforming former ridges into rows of tubercles ( Fig. 10K–L View FIGURE 10 ). Asymmetric paratype 17 possibly belongs to a lateral trivium plate ( Fig. 10J View FIGURE 10 ). Aboral interradial process moderately developed.
Type series of IIBr, proximal series to keystone plate (paratypes 20 to23 and 25 to 39) with external ornamentation predominantly X-shaped, always with a more or less marked axial ridge and few tubercles ( Fig. 11D–E View FIGURE 11 ). Ratios H/h, H/Wp and Wp/Wd independent of size ( Table 9). Height asymmetry mostly due to oblique articulation with IBrax, 12 cases with H/h<1.3, 4 cases with 1.5<H/h<1.7 ( Fig. 11E View FIGURE 11 ) and 3 cases with 1.95<H/h<2.05 ( Fig. 11D View FIGURE 11 ). Lateral faces with 4 to 6 parallel symplexial crenulations, sometimes restricted to the highest one. Muscular synarthries with fulcral ridge shorter than maximum brachial width (W). Two types of muscular synarthries: one symmetrical with equal fulcral ridge segments parallel to W ( Fig. 11A View FIGURE 11 ), another asymmetrical with unequal fulcral ridge segments oblique to W ( Fig. 11B View FIGURE 11 ); most IIBr with one synarthry asymmetrical and the other symmetrical (IIBr1 or IIBr2?), other IIBr with two articular facets asymmetrical (IIBr1) or symmetrical (IIBr2 or 3?). Four IIBr nearly as high as wide and asymmetrical, assigned to IIBr1 of a bivium ( Fig. 11E View FIGURE 11 ). Articulations with adoral border forming an obtuse angle, ambulacral groove shallow, giving the whole ossicle a relatively flat V-shaped section.
Keystone plates of type series (paratypes 24 and 40–42, Table 10) with distal muscular synarthry roughly triangular and rounded ( Fig. 11F View FIGURE 11 ); proximal synarthry symmetrical, with fulcral ridge as long as Wp ( Fig. 11H View FIGURE 11 ); external face smooth, shaped as an isosceles triangle ( Fig. 11G View FIGURE 11 ), four conspicuous parallel symplexial crenulations on each side ( Fig. 11F View FIGURE 11 ).
A few brachials from the distal IIBr series (paratypes 43–45, Table 10) higher than broad, muscular synarthries similar to the distal articulation of the keystone plates with inconspicuous fulcral ridge, marked aboral ligament pit and axial nervous canal, conspicuous adoral muscle pits.
Complementary data from material not included into the type series. The additional material allows us to investigate in more detail the range of variation of quantitative characters (Supplementary Tables 1–2) and ratios in the main types of brachials. All material examined including the types-series: 8 aboral cups, 43 IBrax, 46 IIBr of the proximal series (including 14 with Wp> 2.8 mm) and 60 IIBr with Wp<2.8 mm (distal series plus a few keystone plates poorly preserved). Ratio of IBrax to aboral cups 5.4, very close to expected ratio of 5; ratio of IBrax to proximal IIBr 1.07, far from the expected ratio of 4 or 6 indicating a high IIBr deficit. Height asymmetry (H/h) of the proximal IIBr is independent of their size. In 50% of the proximal IIBr 1.1<H/h<1.3 (possibly IIBr2 or IIBr3), in 34.6% 1.4<H/ h<1.75 (possibly IIBr1 or IIBr2) and in 15.4% 1.75<H/h<2.3 (possibly IIBr1). Quantitative characters of IBrax attaining maximum values with H 4.78 mm, Wd 5.30 mm, Wp 4.15 mm and Wm 3.25 mm (Appendix Table 1).
Remarks. The high number of isolated ossicles of Holopus fabianii sp. nov. from the type locality provides a unique insight into intraspecific character variation in the genus Holopus . To date, no comparable data have been published for extant species. However, the absence of a crown preserved with connected brachials makes it impossible to establish the precise number of IIBr in the arm’s proximal series. The combination of H/h asymmetry, which appears to increase proximally in extant species, and the fact that 50% of the IIBr have a H/h close to 1 suggest that there might have been at least 2–3 IIBr preceding the keystone plate. Like in the extant species H. alidis , the position of the keystone plate could have varied depending on the arm and the developmental stage ( Bourseau et al. 1991).
The recrystallized sediment within the calycinal cavity masks possible inner interradial processes, whose presence and characters would have been useful to describe and compare to extant species. These structures might have been absent as in H. spileccense (see below).
The maximum size of the aboral cup and IBrax of H. fabianii sp. nov. can be estimated by comparing the greatest width of the facets corresponding to the same articulation joining two adjacent ossicles. The proximal articulation of the largest IIBr (paratype 20) is 1.6 times wider (Wp 5.06 mm) than the largest distal articulation of IBrax (Wm 3.25 mm). The largest proximal articulation of IBrax is 2 times wider (Wp 4.15 mm) than the largest distal articulation of the available aboral cups (paratype 2, Wr 2.03 mm). Thus, the estimated size of the individual to which paratype 20 brachial belonged was 3.2 times larger than paratype 2. Therefore, the aboral cup of H. fabianii sp. nov. could possibly be as large as H 21.0 mm and D 14.8 mm, a size intermediate between the known maximum aboral cup sizes of H. alidis (H 13 mm, D 9 mm) and H. rangii (H 30 mm, D 25 mm).
Holopus fabianii sp. nov. displays closer affinities with H.alidis than with H.rangii ,as both possess a differentiated keystone plate beyond IIBr2 separating proximal and distal arm series ( Figs. 5C–H View FIGURE 5 and Fig. 11F–H View FIGURE 11 ). Both species also have an aboral cup with rather marked radial ribs. Yet, H. alidis differs in having a distinctly granulated external cup and brachials, including the keystone plates, with a predominantly tuberculated ornamentation unlike the nearly smooth keystone plates found for H. fabianii sp. nov. ( Fig. 5C View FIGURE 5 , Fig. 5 G View FIGURE 5 , Bourseau et al. 1991, pl. 12). However, judging from our observation on Cyathidium , the relevance of the latter character might be called into question. The IBrax ornamentation with Y- or V-shaped ridges common in H. fabianii sp. nov. is restricted to juveniles of H. alidis ( Fig 4B, C View FIGURE 4 ) and tends to disappear under the tuberculate ornamentation in older specimens. Moreover, the adoral side of the fulcral ridge of the proximal-most brachials often conspicuously crenulated in H. alidis ( Fig. 5A–B View FIGURE 5 ) is not found in more distal synarthries ( Fig. 5E View FIGURE 5 ). This character appears to be absent in H. fabianii sp. nov. ( Fig. 11A–B View FIGURE 11 ). Furthermore, the IIBr fulcral ridges in H. alidis ( Fig. 5B View FIGURE 5 ) and H. rangii ( Fig. 6D, F View FIGURE 6 ) are as long as the brachial is wide, which contrasts significantly with what is observed in H. fabianii sp. nov. ( Fig. 11A–C View FIGURE 11 ). This appears to be due to a lateral aboral displacement of the adoral surfaces of the synarthries, “opening” the articulation and thus giving the brachial a rather flat V-shape. The brachials of H. alidis (and H. rangii ) are thus far thicker than those of H. fabianii sp. nov., and, in IBrax, exhibit a more pronounced distal interradial process.
Holopus plaziati , known exclusively from isolated brachials (7 IBrax + 1 proximal IIBr), comes from the lower Eocene of the Corbières in southern France ( Roux et al. 2021). Its IBrax differ from those of H. fabianii sp. nov. and H. spileccense in being generally smaller with a smaller H/Wd ( Fig. 7 View FIGURE 7 ), and in having muscular synarthries with usually conspicuous crenulation on the adoral side of the fulcral ridge and sometimes on the adoral border of the muscular area. While syntype 2 of H. plaziati appears similar to the IBrax of H. fabianii sp. nov., IBrax of syntypes 3 and 7 have adoral articulation facets larger than in H. fabianii sp. nov. In H. plaziati this increases the size of the distal interradial process and the general thickness of the brachial, resulting in an ambulacral groove that is more deeply carved into the ossicle. The conspicuous lateral symplexial crenulations in H. plaziati are variable (parallel to undulated or branched). Its aboral surface displays irregular tuberculate ornamentation without formation of interradial or Y-shaped ridges.
Manni and Pacioni (2021) attributed 30 brachials from Cava Boschetto (= Cengio dell’Orbo quarry, the type-locality of H. fabianii sp. nov.) to H. spileccense (Schlüter) , a taxon known only from cups found in the Ypresian strata of Spilecco Hill (see below). These 30 brachials are housed in the MGP-PD collections. Manni and Pacioni (2021) figured the ornamentation of 5 IBrax and 6 IIBr from the proximal series but did not identify keystone plates. The variation of X-shaped ridges on IIBr was well-illustrated and corresponds to the range observed in the material here ascribed to H. fabianii sp. nov. The quantitative characters of these brachials also fall within the variation range of those of H. fabianii sp. nov. (except IBrax MGP-PD 31442 which presents the largest Wd, 5.3 mm) and they should thus be attributed to this species.
Occurrence. Middle Eocene (Lutetian) of Chiampo Valley, north-eastern Italy.
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