Discometra luberonensis sp. nov. (Crinoidea, Himerometridae), a new feather star from the Late Burdigalian

Most fossil feather stars are known only from the centrodorsal often connected to the radial circlet. This is the case for Discometra rhodanica (Fontannes, 1877), the type species of the genus Discometra, collected from the Late Burdigalian of the Miocene Rhône-Provence basin (southeastern France). The quarries operating in this area have exposed layers from the Late Burdigalian on the northern fl ank of the Lubéron anticline near Ménerbes (basin of Apt, Vaucluse, southeastern France). These layers contain exceptionally well-preserved echinoderms, among which are three specimens of a feather star with cirri and arms still connected to the centrodorsal. They are attributed to a new species: Discometra luberonensis sp. nov. (Himerometridae). The number of arms can reach 60, as in extant species of the genus Himerometra, but the pattern of arm divisions is closer to that of the genus Heterometra, which has no more than 45 arms in extant species. Discometra luberonensis sp. nov. diff ers from D. rhodanica by the characters of its centrodorsal. Here we redescribe the centrodorsal and radial circlets of D. rhodanica based on previously and newly collected specimens. We designate a neotype for D. rhodanica, because the holotype is considered lost. Affi nities between Discometra, Himerometra and Heterometra are discussed.

On the northern fl ank of the Lubéron anticline (Apt basin, Vaucluse, SE France; Fig. 1), bioclastic limestones with the pectinid bivalve Gigantopecten restitutensis (Fontannes, 1884) dated to the Late Burdigalian contain accumulations of spectacular fossiliferous slabs exposed during the exploitation of quarries located on the Ménerbes-Lacoste Plateau (Bongrain 2013). The shells of G. restitutensis, most often with connected valves, and the tests of the echinoid Tripneustes planus (Agassiz, 1840) preserved with their spines and dental apparatus are the most abundant taxa and are associated with complete large sea stars and brittle stars. These accumulations correspond to mass sediment fl ows on the unstable slopes of the northern fl ank of the Lubéron anticline. They led to the almost instantaneous burial of benthic communities, with the result that some feather stars were exceptionally preserved. Centrodorsals, often with their radial circlet, were previously collected in the Late Burdigalian of the Rhône-Provence basin (Fontannes 1880;de Loriol 1897;Pellat 1897;Nicolas 1898;Joleaud 1907;Valette 1928;Philippe 1974). Discometra rhodanica (Fontannes, 1877) is the species that reaches the largest size.
Here we describe three specimens of a new species of Miocene feather star with arms connected to the calyx and with crowns well enough preserved to allow description of arm organization. The specimens were collected from Ménerbes in deposits that correspond to a thermal maximum at the Late Burdigalian-Langhian boundary (Demarcq 1984). In the Rhône-Provence Gulf, the Miocene Sea transgressed over a deformed and eroded substratum, leaving numerous submarine or emerging shoals separated by strongly incised valleys (Demarcq 1970;Besson 2005) (Fig. 1). Such a setting agrees with the distribution of extant species of Himerometridae, which are mainly found in shallow-water environments in the tropical western Pacifi c and northern Indian Ocean (Hess & Messing 2011). To determine whether or not our three specimens belong to D. rhodanica, we re-examined reference specimens in the collections of the museums in Lyon, Avignon and Saint-Rémy de Provence, and well-preserved specimens recently collected in Caumont-sur-Durance (Picabrier site).

Material and methods
The specimens of D. luberonensis sp. nov. ( Fig. 2A-F) are preserved on three limestone slabs rich in echinoderms and Gigantopecten extracted from the Soubeyran quarry in Ménerbes and conserved at the Centre "Louis Lortet" de conservation et d'étude des collections (Musée des Confl uences, Lyon). The slab MHNL 20.056151 (collected by R. Lacombe, now added to M. Philippe's collection) was illustrated by Bongrain (2013: fi g. 9b). Following the expansion of urbanization in the last decades, most of the ancient outcrops where D. rhodanica had been collected are no longer accessible (Bollène, Les Angles, Caumont-sur-Durance) with the exception of Notre-Dame du Château between Saint-Etienne du Grès and Saint-Rémy de Provence (northern slope of the Alpilles). It was therefore important to locate the specimens used for the fi rst descriptions of the species. Those collected by E. Pellat and described by him (Pellat 1897) and de Loriol (1897) are housed in the collection of the Musée des Alpilles in Saint-Rémy de Provence. Those coming from ancient quarries of the Les Angles Plateau (west of Avignon) and described by Nicolas (1898) belonged to his own collection deposited in the palaeontological collection of the University Claude Bernard, Lyon I, and in private collections including the Châtelet's collection, which has been partly deposited in the Musée Requien, Fondation Calvet (Musée d'Histoire naturelle) in Avignon. However, we were unable to fi nd the calyx fi gured by Nicolas (1898: 403, fi g. 1), or those fi gured by Fontannes (1880: pl. 2, fi gs 10-11). As a consequence, we consider them as lost. Measurements of these lost specimens (Table 2) are deduced from the original publications. The three calices from the Picabrier outcrops exposed in Caumont-sur-Durance were collected by one of us (MP). This site was studied by Ulysse (1968). The centrodorsal comes from the ʻFerme Pieʼ outcrops exposed near Entrechaux (Philippe 1974). The studied specimen of Himerometra robustipinna (Carpenter, 1881) ( Fig. 2G) was collected during the Salomon 1 cruise (DW 1822, 9°51.08′ S, 160°51.8′ E, depth 51-54 m) and the studied specimen of Heterometra savignii (Müller, 1841)( Fig. 2H) during the R.P. Dollfus expedition to the Gulf of Suez. They are housed in the zoological collection of the Muséum national d'histoire naturelle in Paris.
The pictures in Figs 3, 5-6 were taken using a scanning electron microscope (SEM) type JEOL-840A at 15 kV (Electron Microscopy Platform of the Muséum national d'histoire naturelle in Paris). Ossicles from extant species were dissociated after a 12-hour bath in sodium hypochlorite, washed with distilled water and dried. All ossicles observed using SEM were coated with a colloidal platinum solution.

Results
Class Crinoidea Miller, 1821 Subclass Articulata Zittel, 1879 Order Comatulida A.H. Clark, 1908Superfamily Himerometroidea A.H. Clark, 1908 Family Himerometridae A.H. Clark, 1908 Remarks Hess & Messing (2011) placed Himerometridae within the superfamily Mariametroidea A.H. Clark, 1909 (later changed to Himerometroidea by Taylor et al. 2017) in which rod-shaped basals are absent. All genera in Himerometridae (Heterometra A.H. Clark, 1909, Himerometra, Craspedometra A.H. Clark, 1909, Amphimetra A.H. Clark, 1909, except Discometra, show coelomic grooves on the adoral side of centrodorsal. Amphimetra has multiple radiate coelomic grooves while Heterometra, Himerometra and Craspedometra show Y-shaped grooves with more or less wide branches. In Discometra, such Y-shaped features are present as canals included in the stereom and located just beneath the surface of the proximal facet of radials (Fontannes 1880;Sieverts-Doreck 1961). As a consequence, coelomic grooves are absent from the adoral surface of the centrodorsal. In Himerometra, the aboral facet of each radial displays a pair of wide grooves running in parallel in the inner part and diverging in a Y shape in the outer part; this feature is also visible on the oral side of the centrodorsal (Fig. 3A). Coelomic groove patterns are very similar in Himerometra (Fig. 3A) and Discometra (Fig. 4) but diff er markedly in each of the other genera (Rasmussen 1978: fi gs 593-594). The numerous radiating coelomic grooves (A.H. Clark 1915) distinguish Amphimetra the most from the other genera, and Hemery (2011), Summers & Rouse (2014) and Taylor et al. (2017) questioned its attribution to Himerometridae. In Himerometra, the general shape of the centrodorsal varies from hemispherical to almost discoidal. The insertions of cirri on the centrodorsal have a very variable aspect on the same individual, with or without a slight rectangular transverse relief (Fig. 3B). The radial circlet delimits a more or less wide adoral cavity whose fl anks corresponding to the inner face of the radials are sculpted by strong vermiculate fi gures (Fig. 3C). The distal articular facet of the radials is sub-trapezoidal, the internal (adoral) ligamentary areas, with no distinguishable limit adorally, are separated by a large depression, and the muscular areas are reduced and nearly inconspicuous (Fig. 3D).

Stratigraphical range
Miocene-Recent (Western Pacifi c), possibly since the Eocene.

Emended diagnosis
Centrodorsal hemispherical to low hemispherical or discoidal; cirrus-free aboral apex concave or fl attened; adoral side with inter-radial ridges; cirrus sockets with or without low transverse rectangular ridge and closely placed, often in 3 (rarely 5) irregular rows. Cirrals without aboral spines. Radials with internal, Y-shaped coelomic canals just beneath proximal surface; radial circlet nearly equal to or lower and narrower than centrodorsal; interarticular ligament fossae large, separated by wide midradial area. Rod-shaped basals absent. Numerous arms, up to about 60; brachitaxes with 2 or 4 ossicles (2 frequent beyond tertibrachitaxis); ligamentary synarthry at br1-2; syzygy with radiating crenularium at br3+4; ligamentary articulations irregularly placed beyond br5. Gislén (1924) designated D. rhodanica as the type species of the genus, so it is important to clarify and complete the description with the new data provided here. Discometra is mainly distinguished from all other genera of Himerometridae by its internal coelomic canals located near the aboral surface of the radials.

Etymology
The epithet refers to the Massif du Lubéron, a major mountain range in the Provence area where the specimens on which the description is based were collected. Lubéron is part of the greater administrative area (Région) called Provence-Alpes-Côte d'Azur in the southeast of France.

Paratypes
Specimen MHNL 20.056148 (Fig. 2C-D) with largest centrodorsal diameter, aboral cirrus-free depression fl at and very slightly granular; about 30 cirri preserved around centrodorsal hiding proximal part of crown; complete cirrus with 25 cirrals, length 26 mm, longest cirrus fragment 27 mm with 28 cirrals. Specimen MHNL 20.056151 (Fig. 2E-F) with most fl attened centrodorsal, aboral surface fl at and smooth with two small pits; only ⅔ of crown preserved, one brachitaxis visible, with same pattern as holotype; visible arms 38, allowing an estimated total number close to 60. (Fontannes,

Emended diagnosis
Species known only from its calyx (centrodorsal + radial circlet). Centrodorsal hemispherical, often more fl attened 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 ⅓ maximum centrodorsal diameter) and bordered by smaller apical cirrus sockets arranged in two irregular rows; apical depression becoming wider (to ⅔ maximum diameter) and deeper with increasing centrodorsal diameter and with numerous radial grooves.

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: fi g. 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. 5A-B). Although smaller, its good preservation and its characters are similar to those of the specimen illustrated by Fontannes (1880: pl. 2, fi g. 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; fl attened socket fl oors 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 fi lled with sediment. They correspond to the vermiculate grooves reported above in the extant species of Himerometridae (Fig. 3C).  (Fontannes, 1877). B. Discometra eggenburgensis (Schaff er, 1912). Modifi ed from Sieverts-Doreck (1961) in which A was modifi ed from Fontannes (1880).

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, ⅔ 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. 6C-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 fl attening, broadening and deepening of aboral cirrus-free depression, centrodorsal / radial circlet height ratio, and fl aring 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.  Gislén (1924) pointed out the strong affi nities 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, fi g. 11) and considered lost. It was collected from the same outcrops as the holotype of D. rhodanica. Fontannes's fi gure 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 fi eld of variation of the species. We consider it to be a junior synonym of D. rhodanica.
However, Sieverts-Doreck (1961) had pointed out that the two specimens from Notre-Dame du Château attributed to D. rhodanica by de Loriol (1897: fi gs 9-10) could not belong to family Himerometridae. From these two specimens, we were successful in locating the specimen illustrated in fi g. 9 only. This specimen consists of the radial circlet, which shows on its well-preserved aboral face traces of fi ve rod- shaped basals (Fig. 5D). A corresponding structure is present on the adoral face of an isolated centrodorsal collected near Entrechaux (Ferme Pie) (Fig. 5C). Rod-shaped basals are absent in the superfamily Himerometroidea to which Himerometridae belongs (sensu Hess & Messing 2011). In addition, the distal articular facet of the radials has small but conspicuous and clearly delimited muscular areas and triangular interarticular ligament fossae. In the specimen attributed by Vadasz (1915: pl. 7, fi gs 24-26) to D. rhodanica, the distal articular facet of radials exhibits these same characters, which are incompatible with an attribution to Himerometridae according to Taylor et al. (2017). Unfortunately, Hess & Messing (2011: fi g. 52-3a-d) used fi gures from these two specimens to illustrate the genus Discometra.
In calices from the Rhône-Provence Miocene basin, the main quantitative characters vary mainly with size within each site, but not when considering all the sites together. The ratios indicate a large variability independent of size ( Table 2). This result suggests the infl uence of local environmental conditions. However, large specimens exhibit a marked tendency towards fl attening of the centrodorsal and extension of the aboral cirrus-free apical depression. Regarding this variability, the quantitative data are too sparse to draw taxonomic conclusions. Only the qualitative characters of the centrodorsal aboral depression allow us to distinguish two species. Jagt et al. (2002) recognized fi ve species (Discometra eggenburgensis, D. meneghiniana, D. michelottii, D. rhodanica and D. speciosa) of Cenozoic feather stars attributed to the genus Discometra, among which D. meneghiniana is here considered to be a junior synonym of D. rhodanica (see below).

Discometra luberonensis sp. nov.
Two genera of extant Himerometridae have a large number of arms: Heterometra with up to 48 arms and Himerometra, which reaches about 60 arms as in D. luberonensis sp. nov. Himerometra displays well separated primibrachitaxes (Fig. 2G), and tertibrachitaxes are of 2 brachials only. Heterometra has brachitaxes of 2 or 4 brachials and primibrachitaxes in lateral contact as in D. luberonensis sp. nov. (Fig. 2B), or separated (Fig. 2H) as in Himerometra. The centrodorsals of D. luberonensis sp. nov. diff er from those of D. rhodanica and D. eggenburgensis in having a less hemispherical general shape, an aboral depression devoid of cirrus sockets, and a fl atter and smoother aboral depression devoid of both cirrus sockets and radial grooves (Fig. 2B, D, F) as is common in other extant Himerometridae ( Fig. 2G-H). Attribution of this new species to the genus Discometra needs to await confi rmation of the presence of internal coelomic canals near the proximal surface of the radials.

Discometra rhodanica (Fontannes, 1877)
By comparing calices from other sites with the growth series from Picabrier, it appears that the neotype from Notre-Dame du Château and the holotype from Bollène fi t into this sequence in size and shape. The holotype of D. meneghiniana and the large specimens from Les Angles have their aboral depression moderately developed despite their large size. Some specimens of D. eggenburgensis (Sieverts-Doreck 1961: fi gs 3a, 4) have a broad aboral depression with radiate grooves, but more discrete than those observed in the largest specimen from Picabrier. In view of the morphological variations in D. eggenburgensis described by Sieverts-Doreck (1961) and our observations on D. rhodanica, we do not have a robust discriminating character to distinguish the two species based on the calyx alone. Pending additional data on the crown and cirri in the two species, we suggest that the distinction between them should be temporarily maintained.
(Musée Requien -Musée d'Histoire naturelle -Avignon), and Virginie Olier (Musée des Alpilles, Saint-Rémy de Provence). The following small MNHN funding sources helped get together all pieces of this work: Actions Transversales du Muséum "Formes possibles, Formes réalisées", "Biodiversité actuelle et fossile. Crises, stress, restaurations et panchronisme: le message systématique", "Taxonomie moléculaire: DNA Barcode et gestion durable des collections". We also wish to thank Géraldine Toutirais who greatly facilitated our work at the Plateau Technique de Microscopie Électronique et de Microanalyses du Muséum national d'histoire naturelle in Paris. We would like to warmly thank Ben Thuy and Charles Messing for their help to improve the original manuscript.