Charniodiscus and Arborea Are Separate Genera Within the Arboreomorpha: Using the Holotype of C. concentricus to Resolve a Taphonomic/Taxonomic Tangle

Abstract

Charniodiscus is one of the most iconic and first described of the Ediacaran frondose taxa. Since the diagnosis of the holotype of C. concentricus in 1958, the scarcity and poor preservation of unequivocal specimens has resulted in genus-level taxonomic uncertainty. Since the recent reinterpretation of C. concentricus as a multifoliate frond, other Charniodiscus species—all of which are bifoliate—have been left in taxonomic limbo, with most authors comparing them to the clade Arboreomorpha and also the Rangeomorpha. Reconsideration of the taphonomy of the holotype of C. concentricus has revealed that the frond is bifoliate as first described, and also that the frondose portion was broadly conical rather than planar as previously inferred. The conical frond of Charniodiscus is thus morphologically quite different from all other frondose taxa within the Arboreomorpha. Our emendation of the generic diagnosis of Charniodiscus to encompass bifoliate arboreomorphs with conical fronds without a backing sheet distinguishes Charniodiscus concentricus and C. procerus from more planar leaf-like arboreomorphs such as Arborea arborea, A. longa and A. spinosa, all of which have a distinctive backing sheet. Additionally, we find no evidence of rangeomorph-type fractal branching in Charniodiscus.

Rheotropic Epifaunal Growth, Not Felling by Density Currents, Is Responsible for Many Ediacaran Fossil Orientations at Mistaken Point

“…The orientation data of the erect-stemmed taxa do show remarkably consistent orientations among C. spinosus (Pérez-Pinedo et al, 2022), as do the closely related taxa Plumeropriscum hofmanni and Primodandelabrum sp., which meet the criteria for erect mode of life identified by Pérez-Pindeo et al (2022). The erect taxa and some recliners (Bradgatia sp.…”

Frond orientations with independent current indicators demonstrate the reclining rheotropic mode of life of several Ediacaran rangeomorph taxa

“…The most significant divergence from this paradigm of ubiquitously erect Ediacaran macroorganisms was the work of Seilacher (1992Seilacher ( , 1999, which suggested a more protistan-like mode of life for some taxa and inferred a reclining mode of life for others, such as Beothukis. This alternative hypothesis-in which life position should be the same as that in which fronds are preserved (McIlroy et al 2020(McIlroy et al , 2021 unless there is evidence to the contrary-has been successfully applied to demonstrate that Charniodiscus concentricus (and possibly the frondose portion of Charniodiscus procerus), Arborea arborea, Arborea spinosa, and Arborea longa were likely held erect or recumbent in the water column (Laflamme et al 2004(Laflamme et al , 2018Pérez-Pinedo et al 2022).…”

“…Charniodiscus has also been occasionally found in association with F. misrai, P. abyssalis, and Bradgatia (cf. "D" and "E" Surfaces at MPER; Clapham and Narbonne 2002;Pérez-Pinedo et al 2022; but see Eden et al 2022).…”

“…The Ediacaran biota has been much explored in recent years, with research debating aspects of their phylogenetic affinity (Seilacher 1989(Seilacher , 1992Budd and Jensen 2017;Dunn and Donoghue 2018), ecology (Clapham et al 2003;Darroch et al 2018;Mitchell et al 2020), mode of life (Glaessner 1985;Laflamme et al 2009;Dufour and McIlroy 2017;McIlroy et al 2021), and taphonomy (Gehling 1999;Narbonne 2005;Liu et al 2011;Bobrovskiy et al 2019). Inferences that most of the Ediacaran biota-particularly the fractal-branching frondose Rangeomorpha-lived erect within the water column during life (i.e., Glaessner 1985;Laflamme et al 2007) have recently been contested (McIlroy et al 2020(McIlroy et al , 2021(McIlroy et al , 2022Pérez-Pinedo et al 2022). The high fidelity of preservation of originally soft tissues such as fractal-like fronds (Fig.…”

“…1) and the lack of taphonomic evidence for an erect mode of life (e.g., swing marks imparted by a current, see Jensen et al [2018]) challenge a priori assumptions about erect body postures. There is therefore a need to consider the alternative hypothesis that these organisms were reclined on the seafloor, as they are preserved on bedding planes (McIlroy et al 2020, 2021, 2022; Pérez-Pinedo et al 2022).

Figure 1.Ediacaran fossils from the “Melrose Surface.” A, Fractofusus misrai ; B, Pectinifrons abyssalis ; C, Bradgatia sp.

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