Allaire et al.—Early Ordovician eocrinoids from Morocco
A. victoriae graptolite Zones) yielded extremely few remains of marine invertebrates (rare specimens of inarticulate brachio- pods, trilobites, and graptolites), with the remarkable exception of locally abundant accumulations of fully articulated thecae of Rhopalocystis (Fig. 3, locality 1737 = 1738 = 1750, A. victoriae Zone, middle Tremadocian; Destombes, 2006; Allaire et al., 2015; Lefebvre et al., 2016b). In the Fezouata Shale, the most abundant and diverse echinoderm faunas, including several Rhopalocystis assemblages (Fig. 3, localities 809 = 1157 = 1725, Z-F1, Z-F12c), were recorded within the A. murrayi Zone (late Tremadocian; Allaire et al., 2015; Lefebvre et al., 2016b). The last two localities (Fig. 3; 2367, Z-F25) yielding Rhopalocystis assemblages are respectively attributed to the H. copiosus Zone (late Tremadocian) and the ?B. jacksoni Zone (middle Floian), (Lefebvre et al., 2016b).
Taphonomy and paleoecology
Two main types of preservation are described for the different Rhopalocystis-dominated assemblages (Ubaghs, 1963; Allaire et al., 2015). The first kind of preservation (e.g., localities 2367, Z-F12c, Z-F25; Fig. 4.1, 4.2, 4.5) corresponds to extre- mely dense accumulations of Rhopalocystis (fully articulated thecae) in storm-generated, massive sandstone lenses (Allaire et al., 2015; Lefebvre et al., 2016b). Fragmented stems are sometimes preserved, either isolated or articulated to the thecae. Brachioles are usually missing. This first taphofacies can be interpreted as storm-generated Konzentrat-Lagerstätten (Vaucher et al., 2016), resulting from the downslope transport and accumulation of allochthonous Rhopalocystis-dominated communities. These Konzentrat-Lagerstätten suggest the per- sistence of Rhopalocystis-dominated eocrinoid meadows in shallower and high-energy environmental conditions, from the middle Tremadocian (A. victoriae Zone) to the middle Floian (?B. jacksoni Zone), in the Ternata plain (Lefebvre et al., 2016b; Vaucher et al., 2016). The second type of Rhopalocystis-dominated assemblage
(e.g., localities 1773, Z-F1, Fig. 4.3, 4.4, 4.6) is restricted to the lower part of the A. murrayi Zone. It corresponds to low-density accumulations of exquisitely preserved specimens of Rhopalo- cystis conserved in fine laminated siltstones (Allaire et al., 2015). Most of them are moderately disarticulated, and some thecal plates are frequently slightly displaced or collapsed. Large portions of stems are often preserved articulated to the theca. Brachioles are seldompresent (Ubaghs, 1963;Allaire et al., 2015). These assemblages can be interpreted as Konservat-Lagerstätten
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(Vaucher et al., 2016), resulting from the in situ burial of Rhopalocystis communities by distal storm events. The Rhopalocystis-dominated communities could have
occupied both proximal environments with high-energy condi- tions (e.g., localities Z-F12c, Z-F25) and more distal environ- ments associated with lower energy conditions (e.g., locality Z-F1) (Allaire et al., 2015; Lefebvre et al., 2016b). The stem of Rhopalocystis seems to be long and flexible with no attachment structure (Fig. 2.4). Therefore, individuals could possibly attach the distal extremity of their stem by winding it around other organisms (Fig. 2.5), allowing them to maintain their position in relatively high-energy environmental conditions.
Materials and methods
Sample preparation.—Because all individuals are preserved as imprints of external molds, latex casts were made and whitened with ammonium chloride (NH4Cl), to be observed and drawn under a stereomicroscope with a camera lucida. The specimens were photographed with a Nikon D3X camera (objective Macronikkor 60mm) under raking light and with the camera (Leica DCI) of the stereomicroscope. The best-preserved thecae were selected (58 specimens) to perform the cladistic and morphometric analyses.
Cladistic analysis.—Nine thecal multistate characters (e.g., ornamentation, plate shape, number of plate circlets, supple- mentary plates) were investigated through a cladistic analysis to characterize maximally parsimonious clusters, that is, groups of specimens that minimize within-group interspecimen variability with respect to between-group interspecimen morphological variability (Table 1; Fig. 5). The different states of these characters were determined for the 58 selected individuals. The thecal length
was measured and discretized into two classes determined by a finite Gaussian mixturemodel and itsAkaike information criterion (Akaike, 1974; Dempster et al., 1977). The inferred cutoff value obtained was 4.74cm. Consequently, the character state ‘small’ was assigned to the specimens with a thecal length less than 4.74 cm; the character state ‘large’ was assigned to those with a thecal length equal to or greater than 4.74cm. Rather than a distance-based phenetic (cluster) analysis, a
cladistic analysis was performed here using Wagner parsimony on unordered multistate characters as implemented in the PARS software from the PHYLIP 3.67α3 package (Felsenstein et al., 2005). Among other considerations, this choice was strongly motivated by the presence of missing values in the
Figure 2. Rhopalocystis destombesi Ubaghs, 1963. (1, 2) Morphological features of R. destombesi, holotype (A29134 and A29120, part and counterpart); drawings modified from Ubaghs (1963, text-fig. 2.2, 2.3); locality 809 (=1157 = 1725), late Tremadocian (A. murrayi Zone). (1) Theca showing the posterior interradial area (between the radius C and D), which contains the anal protuberances and the periproct. (2) Oral surface of the theca showing the five ambulacra (each consisted of four brachioliferous plates), the oral plates (six), and the peristomial area covered with very small plates. p. = periproct; an. p. = anal protuberance; br. = brachioliferous plates; o. = oral plates. (3) Presentation of the six variables used in the morphometric analysis, R. destombesi (ML 20.269.380a). L(ab) = aboral thecal length; L(ad) = adoral thecal length; W(B) = basal width; W(IL) = infralateral width; W(L) = lateral width; W(R) = radial width. (4) Reconstitution of R. destombesi, based on the holotype (A29134 and A29120) and on a specimen coming from the sample A29122; drawing modified from Ubaghs (1963, text-fig. 1); locality 809 (=1157 = 1725), late Tremadocian (A. murrayi Zone). The theca is shown in anterior aspect (radius A); the brachioles are not entirely represented; the small brachiole located on the ambulacra A is considered in development. (5) Artistic reconstruction of a Rhopalocystis destombesi meadow, with other typical organisms of the Fezouata Shale (Lower Ordovician, central Anti-Atlas, Morocco): conulariids (Eoconularia sp.), demosponges (Pirania auraeum and Hamptonia christi), stylophorans (Thoralicystis sp.), hyolithids (Elegantilites sp.), and arthropods (xiphosurid and basal chelicerate). The stem of Rhopalocystis seemed to be flexible; they could probably be attached by winding it around sufficiently solid elements. Illustration by mad meg. Scale bars = 1 cm.
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