Journal of Paleontology, 92(3), 2018, p. 323–335 Copyright © 2018, The Paleontological Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (
http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.128
Deconstructing an Ediacaran frond: three-dimensional preservation of Arborea from Ediacara, South Australia
Marc Laflamme,1 James G. Gehling,2 and Mary L. Droser3
1Department of Chemical and Physical Sciences, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada ⟨
marc.laflamme@
utoronto.ca⟩ 2South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia ⟨
gehling.jim@saugov.sa.gov.au⟩ 3Department of Earth Sciences, University of California, Riverside, California 92521, USA ⟨
mary.droser@ucr.edu⟩
Abstract.—Exquisitely preserved three-dimensional examples of the classic Ediacaran (late Neoproterozoic; 570–541 Ma) frond Charniodiscus arboreus Jenkins and Gehling, 1978 (herein referred to as Arborea arborea Glaessner in Glaessner and Daily, 1959) are reported from the Ediacara Member, Rawnsley Quartzite of South Australia, and allow for a detailed reinterpretation of its functional morphology and taxonomy. New specimens cast in three dimensions within sandy event beds showcase detailed branching morphology that highlights possible internal features that are strikingly different from rangeomorph and erniettomorph fronds. Combined with dozens of well-preserved two-dimensional impressions from the Flinders Ranges of South Australia, morphological variations within the traditional Arborea morphotype are interpreted as representing various stages of external molding. In rare cases, taphomorphs (morphological variants attributable to preservation) represent composite molding of internal features consisting of structural supports or anchoring sites for branching structures. Each primary branch consists of a central primary branching stalk from which emerge several oval secondary branches, which likely correspond to similar structures found in rare two-dimensional specimens. Considering this new evidence, previous synonymies within the Arboreomorpha are no longer justified, and we suggest that the taxonomy of the group be revised.
Introduction
The Ediacara biota represents an assemblage of large, soft-bodied, and structurally complex organisms of uncertain affinities (Laflamme et al., 2013; Droser and Gehling, 2015). Earliest attempts to classify these organisms, on the basis of morphological comparisons with modern taxa, resulted in their interpretation as primitive examples of crown-group animals (see Glaessner, 1979); however, recent studies instead suggest that the Ediacara biota consists of a number of distinct groups rather than a single clade, thus shifting the debate to discussions of stem animals and crown animals and clades outside of Metazoa (Xiao and Laflamme, 2009; Erwin et al., 2011; Laflamme et al., 2013; Dececchi et al., 2017; although see Budd and Jensen, 2015). Their disappearance prior to the diversification of complex metazoans has been proposed to represent the first biologically driven mass extinction event (Darroch et al., 2015, 2016; Schiffbauer et al., 2016). The term ‘Ediacara biota’ is herein applied to the cast and mold impres- sions of soft-bodied organisms of uncertain affinity from the Ediacaran Period (MacGabhann, 2014). Erwin et al. (2011) proposed a new classification for
Ediacaran macrofossils, which utilizes features such as branch- ing or segment architecture, body symmetry, associated trace fossils, and growth parameters, to erect distinct higher-level groupings based on shared (presumed derived) morphological, behavioral, and ontogenetic characters, while eliminating traditional groupings based on implied ecological similarities
(see discussions in Laflamme and Narbonne, 2008a, b). This classification restricts direct comparisons with modern taxa unless synapomorphies are shared. One such proposed clade, the Arboreomorpha, consists of fronds with typically bifoliate (possibly multifoliate) petaloids with parallel primary branches that are attached to a sheet and end at an outer rim (Erwin et al., 2011; Laflamme et al., 2013; Dececchi et al., 2017). The Arboreomorpha are distinct from Rangeomorpha in lacking a self-repeating and modular branching pattern (Laflamme and Narbonne, 2008a, b; Erwin et al., 2011; Brasier et al., 2012); instead, the secondary branches are globular and typically lack further subdivisions. A similar classification scheme proposed the “Frondomorpha” for Charniodiscus (in addition to other fronds and an assortment of likely holdfasts), defined as Ediacaran fossils “composed of three distinct parts: a large, relatively flattened foliate section, a central stem, and a holdfast or rooting anchor” (Grazhdankin, 2014, p. 271–272). However, this definition would include Rangeomorpha such as Charnia, Beothukis, and Culmofrons and Erniettomorpha such as Swartpuntia (Dececchi et al., 2017); instead, it is most likely that the frond morphology is a shared morphological adaptation to elevating a feeding structure into the water column (Laflamme and Narbonne, 2008a). The Arboreomorpha are known from the Avalon and White
Sea assemblages (Waggoner, 2003; Boag et al., 2016), with specimens described from Newfoundland (Canada), Charnwood Forest (England), White Sea (Russia), Ukraine, Siberia, Wernecke and Mackenzie Mountains (northwestern
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