Allaire et al.—Early Ordovician eocrinoids from Morocco
five successive rows). The anal cone (and associated periproct) is located within the CD interradial area (Fig. 2.2). The collection of abundant material from several new sites
discovered by J. Destombes in Tremadocian levels around Agdz and Zagora led Chauvel (1971) to revise the genus Rhopalo- cystis and to identify four new species: R. fraga Chauvel, 1971, R. grandis Chauvel, 1971, R. zagoraensis Chauvel, 1971, and R. sp. A. The first one, R. fraga (locality 1773; Fig. 1), has a small strawberry-shaped theca, with its aboral-most elements (basal, infralaterals) characterized by a particularly stocky appearance. The second species, R. grandis (locality 1725 = 1157; Fig. 1) possesses a very large theca, composed of numerous irregular plates, which are covered with small granu- lations. The third species, R. zagoraensis (locality 809 = 1725 = 1157; Fig. 1), is comparable in size to R. destombesi but differs from it by the presence of numerous plates intercalated both between the infralateral and lateral circlets and between the lateral and radial circlets, and a strong ornamentation (vermiculate radiating ridges). Finally, several specimens from locality 1738 (=1737 = 1750; Fig. 1) were left in open nomenclature (R.sp. A). Although their morphology appeared to be distinct from that of other species of Rhopalocystis, their poor preservation pre- vented Chauvel (1971) fromdescribing themaccurately and from assigning them to a new taxon. Additional new material collected by J. Destombes in the central Anti-Atlas prompted the revision of R. zagoraensis and
the description of three additional taxa by Chauvel (1978). The first one, R. havliceki Chauvel, 1978 (localities 1725 = 1157, 2082; Fig. 1) corresponds to large individuals with numerous lateral plates. This species was mostly defined by its orna- mentation: radials are spine-shaped and infralaterals bear a rounded protuberance (umbo). The two other taxa were left in open nomenclature (R. sp. B and R. sp. C) due to the poor preservation of the material. Rhopalocystis sp. B (locality 1738 = 1737 = 1750; Fig. 1) was defined on the basis of several internal molds of poorly preserved thecae showing similarities with both R. fraga and R. destombesi. Rhopalocystis sp. C (locality 2082; Fig. 1) was described from a very incom- plete portion of theca (with both its aboral and adoral extremities missing). Several plates of this fragmentary theca show a strong ornamentation consisting of particularly robust, spine-shaped protuberances. Finally, additional material collected by J. Destombes
(locality 2367; Fig. 1) led Chauvel and Régnault (1986) to revise the genus Rhopalocystis and to identify two new species, which they considered as morphologically intermediate between R. destombesi and R. zagoraensis. In the scheme defined by Chauvel and Régnault (1986), specimens with one or several intercalary plates (forming an incomplete circlet) inser- ted between infralaterals and laterals, and/or between laterals and radials, were attributed to R. destombesi. Individuals with a complete circlet of intercalary plates between infralaterals and laterals were assigned to R. dehirensis Chauvel and Régnault, 1986. Those with more than one circlet of intercalary plates inserted between infralaterals and laterals were identified as R. lehmani Chauvel and Régnault, 1986. Both R. dehirensis and R. lehmani could also have additional plates between laterals and radials. Finally, Chauvel and Régnault (1986) suggested that all specimens with three regular circlets of intercalary plates
687
between infralaterals and laterals and one circlet between laterals and radials should be attributed to R. zagoraensis. The taxonomy and morphological disparity of Rhopalo-
cystis have not been reinvestigated since Chauvel and Régnault (1986). As a result of the successive systematic descriptions produced by Ubaghs (1963), Chauvel (1971, 1978), and Chauvel and Régnault (1986), the genus Rhopalocystis now includes ten species: R. dehirensis, R. destombesi, R. fraga, R. grandis, R. havliceki, R. lehmani, R. zagoraensis, R. sp. A, R. sp. B, and R. sp. C. However, as already pointed out by several authors (e.g., Smith, 1988; Lefebvre et al., 2016b), it is very likely that this high intrageneric taxonomic diversity is largely overestimated and at least partly reflects ecophenotypy and/or wide morphological disparity. In recent years, abundant new material of Rhopalocystis was collected from three new local- ities in Zagora area (Z-F1, Z-F12c, Z-F25; Fig. 1). Therefore, the aim of this study is to analyze the morphological disparity of Rhopalocystis and to propose a critical reassessment of its systematics based on abundant material available in the histori- cal Destombes collections and the newly collected specimens.
Geological setting
In the Agdz-Zagora area, the Lower Ordovician succession corresponds to a thick (~1,000 m), relatively monotonous series of claystones and fine siltstones (Lefebvre et al., 2016a; Martin et al., 2016; Vaucher et al., 2016), which is traditionally sub- divided into three successive stratigraphic units: the Lower Fezouata Formation, the Upper Fezouata Formation, and the Zini Sandstone Formation (Destombes, 1962, 1971; Destombes et al., 1985; Vidal, 1996). However, in the central part of the Anti-Atlas, the two lower units cannot be distinguished, and they form together the Fezouata Shale (Tremadocian to Floian) (Lefebvre et al., 2016a; Martin et al., 2016). In both areas, the different sedimentary facies observed in this succession range from proximal offshore to distal lower shoreface and illustrate a sedimentary system dominated by the swell, strongly influenced by storms but modulated by tides (Vaucher et al., 2017). In the past twelve years, successive field excursions in the
Ternata plain (north of Zagora) resulted in the detailed logging of the whole Lower Ordovician succession and in the definition of a precise biostratigraphic framework based on abundant new collections of acritarchs and chitinozoans (Nowak et al., 2016), conodonts (Lehnert et al., 2016), and graptolites (Gutiérrez-
Marco and Martin, 2016; Martin et al., 2016). During this recent field work, three new sites yielding abundant remains of Rhopalocystis (localities Z-F1, Z-F12c, Z-F25; Fig. 1) were discovered (Allaire et al., 2015). All Rhopalocystis-bearing strata are located in the Fezouata Shale of the Agdz-Zagora area: most of them occur within the Ternata plain (Destombes localities 1157 = 1725, 1737 = 1738 = 1750, 2367; new localities Z-F1, Z-F12c, Z-F25; Fig. 1); two sites are located around Agdz (Destombes localities 1773, 2082; Fig. 1). The resulting biostratigraphic reassignment of Rhopalocystis- bearing localities suggests a wider stratigraphic range for this genus (Fig. 3), from the middle Tremadocian (A. victoriae Zone) to the middle Floian (?B. jacksoni Zone) (Lefebvre et al., 2016b). In the Agdz-Zagora area, the first 240 m of the Fezouata Shale (early to middle Tremadocian, A. matanensis to
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