Lee et al.—Rankenella zhangxianensis
holdfast and attached substrate (Fig. 5.3). In longitudinal sections, trabs parallel/subparallel to gastral surface, diverging outward and meet dermal surface perpendicularly or subangularly (Fig. 5.4, 5.5). Thicknesses of trabs ~0.1mm(Fig. 5.6–5.8). Individual spicules not identified within trab. Trabs linked by 3–7 dendroclones, forming ladderlike series. Dendroclones measured from center of a trab to other center of a trab are ~0.02mm wide and 0.3–0.35mm long. A few Y-shaped dendroclones identified (Fig.5.9,5.10). Approximately 7–12 trabs occur between dermal and gastral surfaces (14–24mmthick) (Figs. 4.1, 4.2, 5.4).Differentiated canal systems generally absent, although two specimens contain well-differentiated canals (Fig. 5.6).
Etymology.—From Zhangxia Formation, referring to the occurrence of the species.
Type material and repository.—All figured specimens are deposited in Nanjing Institute of Geology and Palaeontology (NIGPAS159373–159392). Holotype: NIGPAS159373, Para- types: NIGPAS159374–159392. All samples are thin sections perpendicular to the bedding except for NIGPAS159387, which is a thin section parallel to the bedding. Late Cambrian Stage 5–early Guzhangian, Zhangxia Formation, Shandong Province, China.
Occurrence.—Two outcrops (Beiquanzi and Jiulongshan sec- tions) of the Zhangxia Formation in Shandong Province, China (Fig. 2). Most specimens occur within bioherms, some within surrounding packstone to grainstone.
Remarks.—Rankenella zhangxianensis shows structures gen- erally similar to the other two species of Rankenella: the type species, R. mors found in wackestone (late Cambrian Stage 4–early Cambrian Stage 5) and an Angulocellularia-Taninia- Rankenella reef (late Drumian) of Australia (Kruse, 1983, 1996; Kruse and Reitner, 2014), and R. hamdii reported from wack- estone (late Cambrian Series 3) and a Rankenella-Girvanella reef (early Furongian) of Iran (Hamdi et al., 1995; Kruse and Zhuravlev, 2008). Rankenella mors is characterized by diverse shapes including conicocylindrical, digitate or explanate struc- tures, and a relatively thin wall (3 to 5 trabs between dermal and gastral surfaces). On the other hand, R. hamdii has a thicker wall (4 to 10 trabs between dermal and gastral surfaces) and similar overall morphology with that of R. mors, plus notable occur- rence of bowl shape. The number of dendroclones that connect two nearby trabs are 3–4in R. mors and 3–10 in R. hamdii. Compared to these two other species, R. zhangxianensis is
characterized by less diverse shape (mostly obconical/ cylindrical) and a thicker wall (7–12 trabs between dermal and gastral surfaces). Spongocoel diameter of R. zhangxianensis
7
generally overlaps with that of R. mors (~13mm) and R. hamdii (~11mm in digitate shape; ~31mm in bowl shape), although some specimens exceed these range (~21mm in obconical shape). The number of dendroclones connecting each trab of R. zhangxianensis overlaps with the other species (3–7), although the number is generally larger than in R. mors and smaller than in R. hamdii. Secondary thickening of the spicule net adjacent to the dermal surface, which notably occurs in the other two species, is generally absent in R. zhangxianensis. On the other hand, angles between dermal surfaces and trabs intersecting the surfaces are also different; the angles are up to 90° in R. zhangxianensis (Fig. 5.4, 5.5), which is notably larger than ~60° in the other species. All these features collectively indicate that R. zhangxianensis is a new species that can be separated from R. mors and R. hamdii. Minor occurrence of canals within R. zhangxianensis is
noteworthy, because both Australian and Iranian species lack differentiated canals. The occurrence of canals within R. zhangxianensis suggests that the species may not belong to Rankenella because the genus has been characterized by absence of differentiated canal systems (Kruse, 1983, 1996). Rankenella zhangxianensis may be similar to young individuals of Gallatinospongia, which only develop distinct canals during the final stage of growth (R.S. Shapiro, personal communica- tion, 2014). However, the absence of canals in the largest specimen of R. zhangxianensis (Fig. 4.3) and rare occurrence of canals within the species indicate that differentiated canal systems are most likely features that seldom developed in the species. Therefore, R. zhangxianensis is closer to Rankenella than to other genera, although the species may represent a transitional form between Rankenella and other Furongian genera with canals (Gallatinospongia and Wilbernicyathus). It is necessary to have more examples of Cambrian anthaspidellids in order to determine their evolutionary history.
Rankenella zhangxianensis and its implication to other early Paleozoic sponge-microbial reefs
Preliminary sedimentological results suggest that Rankenella zhangxianensis is the oldest known anthaspidellid sponge that constructed reefs. Although many specimens only show a transverse section, some specimens with a longitudinal section suggest that R. zhangxianensis encrusted on microbialite, Cambroctoconus orientalis, or other individuals of R. zhangxianensis (Figs. 4.1, 4.2, 5.1, 5.2).Both R. zhangxianensis and C. orientalis are commonly covered by microstromatolites, and interstitial spaces between these organisms are occupied by micrite (Figs. 4.2, 5.1). Epiphyton comprises a significant volume of the reef, mainly growing upward. These data collectively suggest that Epiphyton, R. zhangxianensis,and
Figure 5. Photomicrographs of Rankenella zhangxianensis in Zhangxia Formation, Beiquanzi section. All photomicrographs are taken from bedding-parallel thin sections except for (7)and (10). Forlocation, seeFigure2.(1) NIGPAS519384. Transverse and longitudinal sections. Note that one individual is attached to the other. (2) NIGPAS519390. R. zhangxianensis encrusting on Cambroctoconus orientalis (arrow). (3) Holotype NIGPAS159373. Transverse section of the holdfast. For location, see Figure 4.1. (4) Holotype NIGPAS159373. Transverse section. Trabs subparallel to gastral surface (lower left), diverge outward and almost perpendicularly meet dermal surface (upper right). For location, see Figure 4.1. (5) NIGPAS159392. Trabs parallel to gastral surface (upper left) and subvertically meet dermal surface (lower right). (6–8) Tangential views showing longitudinal trabs and dendroclones, forming ladderlike spicule networks. (6) NIGPAS159384. Note occurrence of canal-like structures. For location, see Figure 5.1. (7) NIGPAS159387. (8) NIGPAS159384. For location, see Figure 5.1. (9, 10) Transverse sections showing dendroclones between trabs with some Y-shaped dendroclones. (9) NIGPAS159382. For location, see Figure 4.4. (10) NIGPAS159387. Scale bars: (1)5mm, (2)3mm, (3–10) 1mm.
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