An Ediacaran Opportunist? Characteristics of a Juvenile Dickinsonia Costata Population from Crisp Gorge, South Australia
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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 〈[email protected]〉 2South Australian Museum, North Terrace, Adelaide, South Australia 〈[email protected]〉 3School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, South Australia 〈[email protected]〉 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 terminal (Evans et al., 2017) and pre-terminal (Hoekzema et al., 2017) addition of body units. The identification of resting traces The Ediacara biota (575–541 Myr) represent the first appearance of associated with several individual specimens indicates that it may complex, macroscopic life on Earth, and body plan diversity spans have fed on the underlying microbial mat substrate via basal sur- a broad array from discs and tubes to quilted and fractal face absorption (Gehling et al., 2005; Sperling and Vinther, 2010; forms. Taxonomic affinities and life histories of many of these Ivantsov, 2011). organisms remain either controversial or poorly understood. The This study focuses on a population of Dickinsonia costata genus Dickinsonia has been the subject of near-continual Sprigg, 1947 from Crisp Gorge, South Australia (Fig. 1). The debate since it was first described 70 years ago (Sprigg, 1947). Crisp Gorge population comprises 53.5% of the total abundance Descriptive and molecular-informed studies have sought to place on the semi-contiguous surface (Reid et al., 2017), and provides Dickinsonia into various metazoan groups, as well as other king- an excellent opportunity to study this taxon in its juvenile form. doms. Interpretations range from a pelagic cnidarian (Sprigg, The remainder of the community comprises seven taxa and 1947; Valentine, 1992) and polychaete worm (Glaessner and the biogenic sedimentary structure ‘Mop’ (Tarhan et al., 2010). Wade, 1966; Wade, 1972) to benthic placozoan (Sperling and Three additional taxa on the surface are also interpreted Vinther, 2010) and ctenophore (Zhang and Reitner, 2006). as juvenile: Parvancorina minchami Glaessner, 1958, Taphonomic and morphological arguments have been used to Tribrachidium heraldicum Glaessner, 1959, and a single spe- place Dickinsonia in the proposed kingdom Vendobionta as a cimen of Rugoconites enigmaticus Glaessner and Wade, 1966. serially quilted unicellular organism (Seilacher et al., 2003), or as a The Textured Organic Surface (TOS) components of the ground-dwelling lichen (Retallack, 2007). Other studies have microbial mat substrate include a range of structures, including focused on Dickinsonia’s unique, serially repeating body plan to round, deep relief bosses and regions of TOS components estimate oxygen requirements, most recently demonstrating that including ‘groove’ and ‘weave’ (Gehling and Droser, 2009). adequate oxygenation may have been achieved via surficial The overall fossil surface, however, is relatively smooth, and is diffusion (see Runnegar, 1982; Gooden, 2014). This body plan has interpreted as immature (Reid et al., 2017). also been utilized to suggest Dickinsonia was a bilaterian-grade animal, due to interpreted growth via the terminal addition of units Geologic setting from a posterior growth region (Runnegar, 1982; Gold et al., 2015). Most recently, growth models have been employed to infer In South Australia, the Ediacara Member marks the fossiliferous a eumetazoan affinity for Dickinsonia via studies suggesting both shallow marine, deltaic succession of the Rawnsley Quartzite. 313 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.42, on 28 Sep 2021 at 07:29:51, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/jpa.2017.142 314 Journal of Paleontology 92(3):313–322 ripples and several bed partings revealing a silt layer up to 3 mm thick (Reid et al., 2017). The majority of the sample material examined for this study was removed from the Crisp Gorge fossil locality and now forms a framed rock wall in the ‘First Life: Ediacara Biota Gallery’ of the South Australian Museum. This material is referred to as the ‘Crisp Wall’ throughout this study. The Crisp Wall surface is comprised of 16 semi- contiguous slabs of medium- to coarse-grained feldspathic sandstone from the ORSF and gives a surface area of ~6.5 m2.A further seven fossiliferous slabs determined to be semi- contiguous with the bedding surface of the existing Crisp Wall slabs have been identified at the Crisp Gorge fossil site and are incorporated into this study. Materials and methods The Dickinsonia costata specimens identified on the Crisp Wall, combined with the newly identified Crisp Gorge slabs, yielded a total of 150 specimens suitable for analysis. All measurements and observations were made from positive- relief, latex casts taken from the Crisp Wall specimens to aid identification of otherwise negative-relief features and allow for the use of a dissecting microscope. For the purpose of this study, phylogenetically neutral labels are used to describe a number of morphological features of the studied D. costata. This is to aid in the identification of features, while avoiding ambiguity or incidental taxonomic affinities that may be inferred from using body terms usually associated with animals. The terms ‘A-end’ and ‘B-end’ are used to refer to either end of the specimens as defined by the placement of a midline, with the A-end characterized by the terminal A-end unit (Fig. 2.1). The B-end is located at the opposite end of the body, and is defined by decreasing unit size (Fig. 2.1). Measurements for each specimen included the maximum Figure 1. Locality map of the Flinders Ranges and Crisp Gorge fossil site. length, as the longest distance between A- and B-ends of the The Crisp Gorge fossil site is located within the Ediacara Member of the specimen, maximum width, taken as the widest measure per- Pound Subgroup. pendicular to the midline, and the number of units. Unit counts were not taken for individuals with damaged or indistinct units, The Rawnsley Quartzite is the terminal package of Ediacaran- defined here as those individuals with any form of mechanical aged sediments within the Pound Subgroup of the Adelaide Rift damage to the rock or sand grains that mold them, and/or those Complex (Gehling and Droser, 2012). The Ediacara Member that are indistinct from neighboring units. Measurements and incorporates a number of fossiliferous facies, ranging from unit counts were made with the aid of calipers and a dissecting shore-face settings, to deeper water delta-front or pro-delta microscope. Observations also were made of three body fea- environments (Gehling and Droser, 2013). By far the richest, tures, which have been termed the A-end lip, A-end protuber- in situ, fossil assemblages are found within the Oscillation ance, and shrinkage rim (see Results for details). Rippled Sandstone facies (Tarhan et al., 2017). These comprise Statistical analysis of the D. costata population was medium- to coarse-grained feldspathic sands with distinctive performed using the open access statistical software program rippled bed-tops, representative of a depositional environment PAST (Hammer et al., 2001). A Model II (major axis) regres- between fair-weather and storm wave bases. sion was used in the analysis of body size (Legendre and Legendre, 1998). A rose diagram of axial orientation was pro- Locality information.—The Crisp Gorge fossil locality duced using the open access statistical software programs R (31°11'41.5"S, 138°31'46.5"E, Fig. 1) is primarily composed of (RCoreTeam, 2015) and RStudio (RStudioTeam, 2015), and the repeating units of the Oscillation Rippled Sandstone facies package ‘circular’ (Lund and Agostinelli, 2015). Pairwise cor- (ORSF) and the deeper water Flat Laminated to Linguoid Rip- relations among three observed features (A-end lip, shrinkage pled Sandstone facies, the latter indicative of a delta-front to rim, and A-end protuberance) were calculated using the pro-delta setting (Tarhan et al., 2017). Both facies are present at Pearson method for the presence-absence data collected.