Laflamme et al.—Three-dimensional preservation of Arborea


revealing hypothesized internal, soft-bodied organic skeletons serving as primary structural supports for epifaunal fronds. The combination of a variety of preservational types of


the same taxon within a single, temporally restrictive locality provides a unique opportunity to dissect the internal and exter- nal morphology of an Ediacaran organism and refines the interpretation of characters preserved in two-dimensional casts.


Frond morphology and the history of Charniodiscus and Arborea


Ediacaran fronds (Fig. 1) represent a convergent evolution toward a shape that elevates a foliate feeding structure higher into the water column (Laflamme and Narbonne, 2008a, b) to effectively reduce competition with crowded lower tiers (Clapham and Narbonne, 2002; Ghisalberti et al., 2014). Arboreomorpha frond morphology consists of a basal anchoring structure typically circular to bulbous in shape, which was either buried beneath/within (Tarhan et al., 2010; Laflamme et al., 2011; Burzynski and Narbonne, 2015) or stuck to the upper surface (Seilacher, 1992) of benthic microbial mats. The anchoring disc was attached to a cylindrical stem from which emerged a bifoliate petalodium (possibly multifoliate; Brasier and Antcliffe, 2009). Petaloids are composed of several primary branches that branch off directly from the central stalk. The poor preservation of the holotype of Charniodiscus


325


concentricus Ford, 1958 makes it difficult to evaluate the nature of the petalodium. Brasier and Antcliffe (2009, fig. 12) sug- gested that C. concentricus could represent a multifoliate frond, consisting of several independent petaloids all circling a central stalk (“Laser scanning also suggests that Charniodiscus concentricus had three or more rows of segments [Dzik 2002], much like Rangea [Gurich 1929, 1930; Germs 1973; Jenkins 1985] and Swartpuntia [Narbonne et al. 1997], rather than the two rows…”; Brasier and Antcliffe, 2009, p. 372), and that much of the disorganization of the petaloidium could be explained by overfolded petaloids (“Interpretative drawing showing inferred presence of three or more rows of 1° branches” [note ‘1° branches’=primary branches, and ‘rows’=petaloids]; Brasier and Antcliffe, 2009, p. 375). The multifoliate nature of the holotype was also proposed by Dzik (2002), who suggested that the poor preservation may have resulted from the presence of “more than two vanes” (vanes=petaloids; Dzik, 2002, p. 322) and that “available evidence from Charniodiscus is compatible with the four-vane model of Rangea anatomy” (Dzik, 2002, p. 322). If true, the multifoliate petalodium in C. concentricus is strikingly different from Australian speci- mens presently referred to as C. arborea and C. oppositus Jenkins and Gehling, 1978, which display clear bifoliate sym- metry with only two petaloids.We propose that this represents a fundamental difference in construction, and as a result we follow the recommendations of Brasier and Antcliffe (2009) and Dzik (2002) in referring to C. arborea and C. oppositus as Arborea Glaessner and Daily, 1959. Another contentious aspect concerns the architecture of


the primary branches in Charniodiscus and Arborea. Brasier and Antcliffe (2009) interpreted the primary branches in the holotype of C. concentricus as repeatedly branched elements


that resemble the fractal branching of rangeomorphs such as Charnia and Rangea (Narbonne, 2004; Dececchi et al., 2017), albeit not as well preserved. When investigating the secondary branching in the holotype, Brasier and Antcliffe (2009) concluded that “Laser profiles confirm that these arcuate 1° branches are further subdivided into numerous 2° branches. Their rangeomorph structure is not displayed. No examples of 3° or 4° branches have yet been seen in the holotype” (Brasier and Antcliffe, 2009, p. 372). This condition was further explored by Brasier et al. (2012), who applied the term ‘furling’ to account for the difference in structure between typical rangeomorphs and Charniodiscus: “rangeomorph units furled but undivided at first-order level, so that second-order subdivi- sions cannot be seen (cf. some examples of Charniodiscus)” (Brasier et al., 2012, p. 1111). By contrast, Laflamme et al. (2004) and Laflamme and Narbonne (2008a) argued that Arborea from Australia were composed of a series of parallel, peapod-like primary branches that housed several oval second- ary branches that lacked modular subdivisions; this branching was termed Arborea-type branching (Laflamme and Narbonne, 2008a). The crux of this distinction in branching, either a furled rangeomorph branch (Brasier et al., 2012) or a morphologically (and evolutionarily) distinct arboreomorph branch (Laflamme and Narbonne, 2008a), will play heavily in our continued attempts to decipher the natural history of these Ediacaran clades. The debate can be summarized as the explanation of the simplified branching morphology seen in Charniodiscus (holotype) and Arborea—does it represent a modified (or poorly preserved) rangeomorph branch, thus allying them with Rangea and Charnia, or does it represent an independent ancestry, thus justifying Arboreomorpha as a clade (Dececchi et al., 2017)? In summary, assuming that: (1) the petaloid arrangement


in Arborea from Australia is distinct from the structure in Charniodiscus from England (multifoliate), and (2) the primary branching architecture (rangeomorph vs. arboreomorph) may also be distinct, we recommend the use of the traditional Australian designation of Arborea to represent bifoliate fronds with Arborea-type primary branching from Australia, while Charniodiscus is kept for multifoliate frondose forms from Charnwood Forest England. Considering these new diagnostic structures, a revision of Newfoundland Charniodiscus material is warranted but beyond the scope of this study. Ultimately, the present study of exceptionally preserved Arborea allows for a refined description of Arboreomorpha primary branches (Dececchi et al., 2017) as presented in the Systematic Paleontology.


Materials and methods


With the discovery of multiple, three-dimensionally preserved Arborea fronds from Nilpena in the Flinders Ranges of South Australia, the architecture of the primary branches can be thoroughly evaluated. Furthermore, by comparing various tapho- morphs of Arborea from South Australia, it is possible to differ- entiate between dorsal and ventral sides of the petalodium and isolate internal structures that are preserved as composite molding.


Repository and institutional abbreviation.–In the following section, a new description of Arborea is based on 84 specimens from the South Australian Museum (SAM).


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