Journal of Paleontology, 92(3), 2018, p. 313–322 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 re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.142


An Ediacaran opportunist? Characteristics of a juvenile Dickinsonia costata population from Crisp Gorge, South Australia


Lily M. Reid,1,2 Diego C. García-Bellido,2,3 and James G. Gehling2,3


1School of Natural and Built Environments, University of South Australia, Mawson Lakes, South Australia ⟨lily.reid@mymail.unisa.edu.au⟩ 2South Australian Museum, North Terrace, Adelaide, South Australia ⟨Jim.Gehling@samuseum.sa.gov.au⟩ 3School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, South Australia ⟨Diego.Garcia-Bellido@adelaide.edu.au⟩


Abstract.—Despite 70 years of study, Dickinsonia remains one of the Ediacara biota’s most enigmatic taxa with both morphological characters and phylogenetic affinities still debated. A large population of relatively small Dickinsonia costata present on a semi-contiguous surface from the Crisp Gorge fossil locality in the Flinders Ranges (South


Australia) provides an opportunity to investigate this taxon in its juvenile form. This population supports earlier findings that suggest D. costata’s early growth was isometric, based on the relationship between measured variables of length and width. The number of body units increases with length, but at a decreasing rate. A correlation between a previously described physical feature, present as a shrinkage rim partially surrounding some specimens and a novel, raised lip in some specimens, suggests that both features may have been the result of a physical contraction in response to the burial process, rather than due to a gradual loss of mass during early diagenesis. A marked protuber- ance in 15% of the population is also noted in limited specimens within the South Australian Museum collections and appears to be present only in juvenile D. costata. Both the abundance and narrow size range of this population support the notion that Dickinsonia was a hardy opportunist, capable of rapid establishment and growth on relatively immature textured organic-mat substrates.


Introduction


The Ediacara biota (575–541 Myr) represent the first appearance of complex, macroscopic life on Earth, and body plan diversity spans a broad array from discs and tubes to quilted and fractal forms. Taxonomic affinities and life histories of many of these organisms remain either controversial or poorly understood. The genus Dickinsonia has been the subject of near-continual debate since it was first described 70 years ago (Sprigg, 1947). Descriptive and molecular-informed studies have sought to place Dickinsonia into various metazoan groups, as well as other king- doms. Interpretations range from a pelagic cnidarian (Sprigg, 1947; Valentine, 1992) and polychaete worm (Glaessner and Wade, 1966; Wade, 1972) to benthic placozoan (Sperling and Vinther, 2010) and ctenophore (Zhang and Reitner, 2006). Taphonomic and morphological arguments have been used to place Dickinsonia in the proposed kingdom Vendobionta as a serially quilted unicellular organism (Seilacher et al., 2003), or as a ground-dwelling lichen (Retallack, 2007). Other studies have focused on Dickinsonia’s unique, serially repeating body plan to estimate oxygen requirements, most recently demonstrating that adequate oxygenation may have been achieved via surficial diffusion (see Runnegar, 1982; Gooden, 2014). This body plan has also been utilized to suggest Dickinsonia was a bilaterian-grade animal, due to interpreted growth via the terminal addition of units from a posterior growth region (Runnegar, 1982; Gold et al., 2015). Most recently, growth models have been employed to infer a eumetazoan affinity for Dickinsonia via studies suggesting both


terminal (Evans et al., 2017) and pre-terminal (Hoekzema et al., 2017) addition of body units. The identification of resting traces associated with several individual specimens indicates that it may have fed on the underlying microbial mat substrate via basal sur- face absorption (Gehling et al., 2005; Sperling and Vinther, 2010; Ivantsov, 2011). This study focuses on a population of Dickinsonia costata


Sprigg, 1947 from Crisp Gorge, South Australia (Fig. 1). The Crisp Gorge population comprises 53.5% of the total abundance on the semi-contiguous surface (Reid et al., 2017), and provides an excellent opportunity to study this taxon in its juvenile form. The remainder of the community comprises seven taxa and the biogenic sedimentary structure ‘Mop’ (Tarhan et al., 2010). Three additional taxa on the surface are also interpreted as juvenile: Parvancorina minchami Glaessner, 1958, Tribrachidium heraldicum Glaessner, 1959, and a single spe- cimen of Rugoconites enigmaticus Glaessner and Wade, 1966. The Textured Organic Surface (TOS) components of the microbial mat substrate include a range of structures, including round, deep relief bosses and regions of TOS components including ‘groove’ and ‘weave’ (Gehling and Droser, 2009). The overall fossil surface, however, is relatively smooth, and is interpreted as immature (Reid et al., 2017).


Geologic setting


In South Australia, the Ediacara Member marks the fossiliferous shallow marine, deltaic succession of the Rawnsley Quartzite.


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