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Journal of Paleontology 91(5):871–882
Apart from the eight comb rows, the symmetry of ctenophores is clearly demonstrated by the plated apical organ of a sclerotized
species from the Chengjiang Lagerstätten (Ou et al., 2015, figs. 1K, 1L). Eight spiraling structures characterize the pro- blematic Eoandromeda Tang et al., 2008 from the Ediacaran of China and Australia (Tang et al., 2008; Zhu et al., 2008). Haootia quadriformis Liu et al., 2014 from the Ediacaran of Newfoundland, an impression of a soft-bodied organism with a holdfast, shows four bifurcating branches and was compared to staurozoans and interpreted as a stem group medusozoan (Liu et al., 2014; Miranda et al., 2014). In detail, the eight-fold symmetry of octocorals has a bilat-
eral component imposed upon it by the development of a ciliated organ (siphonoglyph) at the entrance to the pharynx. Almost all living octocorals are colonial with coenenchymal tissue embra- cing polyps and stolons that form ribbons or sheets, although a solitary deep-sea taxon was described by Bayer and Muzik (1976). Polyp walls in the dominantly soft body are supported by a collar of calcite spicules in adjacent areas of the coenenchyme and in the axial area of branches. However, domed colonies of Tubipora musica Linnaeus, 1758 are supported by a compact skeleton composed of porous radial tubes (1.3–1.8mm in dia- meter) and concentric stolonal platforms built of tightly con- joined spicules (Spiro, 1971). In contrast, the blue octocoral Heliopora,and therelated Epiphaxum and Nanipora have rigid internal aragonitic skeletons as well as calcite spicules (Bayer, 1992; Miyazaki and Reimer, 2015). Octocorals have a poor geological record, but reports of supposed gorgonians exist from the Ordovician (Lindström, 1978; Cope, 2005). The problematic middle Cambrian Echmatocrinus Sprinkle, 1973 has been inter- preted both as a crinoid and as an octocoral (Sprinkle, 1973; Conway Morris, 1993; Sprinkle and Collins, 1998; Ausich and Babcock, 1998, 2000), but it is quite unlike Cambroctoconus. Isolated octocoral spicules have been recognized from the Silurian (Bengtson, 1981) and possibly from the Cambrian (Microcoryne Bengtson in Bengtson et al., 1990). In Epiphaxum septifer Bayer, 1992 a cylindrical calice
~500 µm in diameter illustrated by Bayer (1992, fig. 30) has eight robust radial septa that alternate with eight internal rows of large pores through which solenia pass; the mesenteries are believed to be attached to the septa. In shape and size, the septa are similar to the septal notches in one illustrated internal mold of C. koori n. sp. (Fig. 3.9, 3.11), although this represents only one preservational variety in the Greenland material. Similar septa are also present in Heliopora coerulea, but may vary in number from eight in the deepest part of the calice up to 16 at higher levels (Bayer, 1992). Epiphaxum and Heliopora do not develop the delicate paired septa visible in C. orientalis (Park et al., 2011, fig. 2) and inferred in C. koori n. sp. (Fig. 3.7, 3.8, arrows). Among other cnidarians, Dzik et al. (2017) noted biradially
disposed sets of eight septa-like structures in non-calcified Ordovician specimens of Sphenothallus Hall, 1847, a genus known from the lower Cambrian (Zhu et al., 2000; Peng et al., 2005; Muscente and Xiao, 2015) and widely reported in younger Paleozoic strata (Van Iten et al., 1992, 2002, 2014, 2016a, 2016b). Dzik et al. (2017) noted the similarity of Sphenothallus to the polyps of present day coronate scyphozoans (Medusozoa).
Pore structure of Cambroctoconus
The pore structure characteristic of Cambroctoconus is highly unusual within calcified Cnidaria, although pores occur in the septa of Ordovician–Devonian calostyline rugose corals (Lindström, 1870; Kaljo and Reiman, 1958; Weyer, 1973; Hill, 1981; Elias, 1986), the septa and walls of tabulate corals (Hill, 1981), in some scleractinian corals (Stolarski, 2000; Stolarski and Roniewicz, 2001), and in rare calcified octocorals such as Epiphaxum and Nanipora (Bayer, 1992; Miyazaki and Reimer, 2015). In the widespread Silurian–Devonian rugosan genus Calostylus Lindström, 1868, the combination of pores, synapticulae, and irregular, retiform septa may produce a peri- pheral porous structure that partially replaces the regular pattern of radial septa, and is reminiscent of the pore structure in
Cambrocotoconus.In Helminthidium Lindström, 1882 from the Silurian of Gotland, this porous zone occupies the entire area within a prominent epitheca, but the pores may be visible externally in Calostylis due to the frequent distal lack of an epitheca (Kaljo and Reiman, 1958). In this respect, Calostylis resembles Cambroctoconus (Fig. 1.4), but the abundant septa and domed tabulae of the former find no equivalence in Cambroctoconus. In its most clearly expressed form (Fig. 4.1–4.4), there is
superficial resemblance of the pore structure of C. koori n. sp. to the stereom of echinoderms, but the latter is much more regular, better ordered, and with greater connectivity of the stromal tis- sue, as demonstrated by Clausen and Peel (2012) in their study of well-preserved echinoderm plates occurring together with C. koori in GGU sample 271718. Smith and Jell (1990) described a plate with poorly ordered stereom from the Beetle Creek Formation (Cambrian Series 3) of Queensland, but it has no morphological resemblance to coralla of Cambroctoconus. Echinoderm stromal tissue consists of muscles and ligaments connecting adjacent skeletal elements, a function that is readily discounted in Cambroctoconus. Archaeocyathan sponges are abundant in the early Cam-
brian and combine radial septa and pores in their skeletons; they are usually studied as thin sections of their calcareous cups. A small assemblage of archaeocyaths is known from the Parallel- dal Formation (Cambrian Series 2, Stage 4) in Peary Land, in strata equivalent to those yielding the problematic zoantharian Cothonion (Debrenne and Peel, 1986; Peel, 2011), but there are no co-occurrences with Cambroctoconus koori n. sp. Septa in the intervallum of Archaeocyathus pearylandensis Debrenne and Peel, 1986 are porous and undulating, especially abaxially, but the septa are many times more numerous than in Cambroctoconus and there is no suggestion of an octagonal form (Debrenne and Peel, 1986, fig. 8a). Internal molds of archaeocyaths are rarely illustrated, but a
juvenile specimen figured by Wrona (2004, fig. 25I) shows a superficial resemblance to some specimens of C. koori n. sp. (Fig. 3.6), with narrow septa and coarse tubercles reflecting pores through the outer wall. Coincidently, eight septa are pre- sent in this specimen at this size (diameter 3.5 mm), but these culminate adaxially at an inner wall, which delimits an octago- nal central cavity; much larger numbers of septa are present in larger specimens and in other taxa. The pores in Wrona’s (2004) specimen are much larger and simpler in form than the irregular
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