Holopus plaziati, Roux & Martinez & Vizcaïno, 2021

Roux, Michel, Martinez, Alain & Vizcaïno, Daniel, 2021, A diverse crinoid fauna (Echinodermata, Crinoidea) from the Lower Eocene of the Gulf of Languedoc (Corbières, Aude, southern France), Zootaxa 4963 (2), pp. 201-242 : 233

Holopus plaziati n. sp.

Fig. 17

Type material. The type series consists of 8 brachials, all figured as syntypes ( MNHN.F. A82016 ) ( Fig. 17 ) .

Etymology. This species is dedicated to Jean-Claude Plaziat, author of many works on the Ilerdian in Corbières, who gave us the benefit of his knowledge of the field, especially concerning the site of Réqui.

Diagnosis. As description of the brachial type series; aboral cup unknown.

Type stratum. Base of blue marls of the middle Ilerdian above Solenomeris limestones, late NP10, but maybe already NP11.

Type locality. Réqui near Montlaur (Val de Dagne, Aude) .

Description of type series. Quantitative characters of brachials of type series given in Table 18. Pentagonal axillary primibrachial (IBr1ax), external surface covered with coarse granulation, straight ( Fig. 17A ) to concave ( Fig. 17F ) lateral borders; internal face with Y-shaped neural groove, junction of two distal muscular synarthries forming conspicuous process ( Fig. 17B, E ), neural groove closing only at level of distal muscular synarthries; distal muscular synarthry with narrow, protruding fulcral ridge, inner edge of fulcral ridge with regular, well-marked crenulation, aboral ligament area slightly concave with narrow fossa of same size as neural canal, adoral ligament area beveled ( Fig. 17J ), muscular area oval towards central lumen and forming series of small fossae running along adoral ligament area opposite the lumen ( Fig. 17D, G, J ); proximal muscular synarthry often poorly preserved with linear fulcral ridge sometimes narrow, bordered by a reduced aboral ligament area and a deep adoral ligament area covered with elongated crenulations ( Fig. 17K ); variable lateral faces often narrow without crenulations ( Fig. 17C ) or wide with field of long crenulations on one of faces ( Fig. 17K–L ). Some smaller, irregularly shaped IBr1ax with occasionally curved proximal face and muscular synarthry offset on one side ( Fig. 17H–I ). Proximal subrectangular secundibrachials subrectangular, wider than high, 3 to 5 strong lateral crenulations on each side ( Fig. 17M, O ), muscular synathry symmetrical with respect to axial plane ( Fig. 17M ), no synarthry observed on opposite facet, possibly a synostosis, but feature probably related to poor preservation ( Fig. 17N–O ).

Remarks. The extant species, H. rangii , often shows a crown with a trivium of three large pentagonal IBr and well-developed arms and a bivium of two smaller IBr, sometimes irregularly shaped and with shorter arms ( Carpenter 1884). The bivium is on the inside of the crown curvature and the trivium on the outside. This more or less marked arrangement is independent of the position of the anus ( Grimmer & Holland 1990). It is thought to develop during growth under the influence of a unidirectional current ( Donovan 1992). In H. plaziati n. sp., large pentagonal IBrax with few or no lateral crenulations ( Fig. 17A–F ) and those smaller, with widely developed lateral crenulations ( Fig. 17K–L ) or irregularly shaped ( Fig. 17H–I ), suggest the presence of a trivium and a bivium, respectively ( Table 18). All other fossil species of Holopus are known only by their aboral cup. The oldest specimen has been recorded from Late Campanian chalk in northern Germany ( Jagt et al. 2010). Manni (2005) and Frisone et al. (2020) confirmed the presence of the genus in the Eocene of northeast Italy. The Réqui site provided the first examples of brachials of an extinct species of Holopus . The corresponding aboral cup remains to be discovered.

Occurrence. Early Ypresian in Corbières (middle Ilerdian), species only known from Réqui near Montlaur (Val de Dagne, Aude).

Figures

FIGURE 17. Brachials of type series of Holopus plaziati n. sp. from Réqui (MNHN.F.A82016).A–L: IBr1ax, M–O: proximal IIBr, A: aboral view of syntype 1; B: adoral view of syntype 2, C: lateral adoral view of syntype 6; D–F: syntype 3, D: distal view, E: adoral view, F: aboral view; G: detail of distal muscular synarthry of syntype 4 (il: inner ligament, m: muscular area, ol: outer ligament); H–I: syntype 5, H: aboral view, I: proximal view; J: detail of distal muscular synarthry of B, adoral view; K–L: syntype 7, K: lateral-adoral view, L: detail of lateral crenulations; M–O: syntype 8; M: lateral-proximal view; N: distal-adoral view, N: lateral-proximal view, O: lateral-distal view. Scale bar equals 1 mm, except for G, J and L, where it equals 0.5 mm.