Discometra rhodanica
Creators: name: Eléaume, Marc
name: Roux, Michel
name: Philippe, Michel
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Crinoidea
Comatulida
Discometra
Discometra rhodanica

description: Discometra rhodanica (Fontannes, 1877)
Figs 3E, 4A, 5 A–B, 6
Eugeniacrinus? rhodanicus Fontannes, 1877: 669.
Antedon meneghinianus Fontannes, 1880: 412–413, pl. 2, fig. 11a–c.
Antedon rhodanicus – Fontannes 1880: 410–412, pl. 2, fig. 10a–c. — de Loriol 1897: 121–124, pl. 4, fig. 8 (non 9–10). — Nicolas 1897: 131–132, fig. 6; 1898: 398–399, 402–404, fig. 1. — Airaghi 1904: 44–45, figs 14–15. — Schütze 1904: 156–162, pl. 2, figs 4–5. — Albus 1930: 291–292, pl. 11, figs 5, 7.
Discometra rhodanica – Gislèn 1924:180–182. — Biese & Sieverts-Doreck 1939: 106–107. — SievertsDoreck 1961: 110–117, 124, text fig. 2. — Rasmussen 1978: T890, fig. 593-4a.
Antedon cf. rhodanicus – Valette 1928: 31–34, fig. 5.
Discometra meneghiniana – Biese & Sieverts-Doreck 1939: 105.
non Antedon rhodanica – Vadasz 1915: 89–90, pl. 7, figs 24–26.
non Discometra rhodanica – Rasmussen 1978: T890, fig. 593-4b–c. — Hess & Messing 2011: 107, fig. 52-3a–d.
Emended diagnosis
Species known only from its calyx (centrodorsal + radial circlet). Centrodorsal hemispherical, often more flattened in large specimens, diameter can reach 14 mm; lateral surface with large cirrus sockets arranged in up to 3 irregular rows, each at bottom of a more or less marked depression; aboral pole depressed, conical in smaller (young) specimens (to ¼ maximum centrodorsal diameter) and bordered by smaller apical cirrus sockets arranged in two irregular rows; apical depression becoming wider (to ¾ maximum diameter) and deeper with increasing centrodorsal diameter and with numerous radial grooves.
Material examined
Neotype
FRANCE • 1 calyx; Bouches du Rhône, Notre-Dame du Château near Saint-Etienne-du-Grès; MASR 2020.5011 (figured by de Loriol 1897: fig. 8).
Other material
FRANCE • 1 calyx; Gard, Les Angles; UCBL 200025a • 2 calices; same collection data as for preceding; MRA 3.000.348, 3.000.349 • 1 centrodorsal; same collection data as for preceding; UCBL 200025b • 3 calices; Vaucluse, Caumont-sur-Durance, Picabrier; MHNL 20.062686, 20.062687, 20.062688.
Locus typicus
Late Burdigalian, facies with Chlamys praescabriuscula (Fontannes, 1878), near Bollène (Vaucluse, southern France).
Description
Neotype
Extensive investigations in the collections of MHNL and MRA to retrieve the specimens used by Fontannes (1880) to describe D. rhodanicus and D. meneghinianus have been unsuccessful. They can be considered lost. Moreover, the locality from which they were collected, now located in an urbanized area, has now become inaccessible. The specimen illustrated by de Loriol (1897: fig. 8) from the Late Burdigalian of Notre-Dame du Château (east of Saint-Etienne du Grès), an outcrop still accessible, is housed in the collection of the Musée des Alpilles in Saint-Rémy de Provence (MASR 2020.5011). We designate this specimen as the neotype (Fig. 5 A–B). Although smaller, its good preservation and its characters are similar to those of the specimen illustrated by Fontannes (1880: pl. 2, fig. 10a–c). This specimen consists of a hemispherical centrodorsal and radial circlet. Its dimensions are given in Table 2. The aboral surface of the centrodorsal (Fig. 5A) shows a marked depression surrounded by small apical cirrus sockets (diameter <0.4 mm) partially worn out or biocorroded. The lateral surface is covered with larger cirrus sockets (diameter 0.5 to 0.9 mm) more or less oval and hollow; flattened socket floors sometimes displaying a roughly rectangular, low transverse ridge (Fig. 5B, arrow) like in extant Himerometridae (Fig. 3B). These larger cirrus sockets are arranged in up to three irregular rows. The maximum diameter of the radial circlet is much smaller than that of the centrodorsal (ratio 0.82). Centrodorsal height almost equals that of the radial circlet. Radial distal articular facet is almost entirely occupied by ligamentary areas, while muscular areas are comparatively inconspicuous. Its geometry is similar to that observed in Himerometra (Fig. 3D). Irregular radiating grooves can be seen on the walls of the central adoral cavity, which is partially filled with sediment. They correspond to the vermiculate grooves reported above in the extant species of Himerometridae (Fig. 3C).
Calices from the Picabrier deposit at Caumont-sur-Durance
Three calices illustrate morphological changes during growth (Fig. 6). Centrodorsal diameter of the smallest specimen (MHNL 20.062688) is hemispherical, 7.7 mm across (Fig. 6B), with an aboral face similar to that of the neotype. Centrodorsal of the largest specimen (MHNL 20.062686) is more discoidal, 12 mm across (Fig. 6A); its aboral depression deeper, ¾ of centrodorsal diameter; apex completely devoid of cirrus sockets and covered by numerous radial grooves, as in one of the specimens illustrated by Fontannes (1880) (Fig. 4A). Cirrus sockets more or less circular, slightly concave and well delineated, and each located at the bottom of a well-marked depression (Fig. 3E). These depressions become almost contiguous, separated by fragile, often worn out crests. Centrodorsal of specimen MHNL 20.062687 is 9.9 mm across; specimen shows an intermediate morphology (Fig. 6 C–D).
Specimens from other sites
Most of the other specimens examined were collected in the Rhône-Provence basin by one of us (MP) and are represented by centrodorsals only or centrodorsal and radial circlets, often worn out or biocorroded. Nevertheless, some characters of the calyx, such as degree of centrodorsal flattening, broadening and deepening of aboral cirrus-free depression, centrodorsal / radial circlet height ratio, and flaring of adoral cavity delimited by the radial circlet, are still visible and prove to be very variable (Table 2). The ancient quarries of Les Angles, now inaccessible, provided many specimens partly described by Nicolas (1898). Those that we have found in older collections, including the largest known specimens (UCBL 200025a, MRA 3.000.348, MRA 3.000.349), are worn out and do not allow detailed observation.
Remarks
Gislén (1924) pointed out the strong affinities between D. rhodanica, D. eggenburgensis and D. meneghiniana (Fontannes, 1880). Discometra meneghiniana is only known from the single calyx described by Fontannes (1880: pl. 2, fig. 11) and considered lost. It was collected from the same outcrops as the holotype of D. rhodanica. Fontannes’s figure suggests that this specimen is strongly worn out; in particular the aboral depression of the centrodorsal displays an abnormally regularly circular appearance. It nevertheless falls within the field of variation of the species. We consider it to be a junior synonym of D. rhodanica.
description: Published as part of Eléaume, Marc, Roux, Michel & Philippe, Michel, 2020, Discometra luberonensis sp. nov. (Crinoidea, Himerometridae), a new feather star from the Late Burdigalian, pp. 121-137 in European Journal of Taxonomy 729 on pages 128-133, DOI: 10.5852/ejt.2020.729.1193, http://zenodo.org/record/4419816