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Journal of Paleontology 92(3):313–322
ripples and several bed partings revealing a silt layer up to 3mm 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 ~6.5m2.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. The Crisp Gorge fossil site is located within the Ediacara Member of the Pound Subgroup.
The Rawnsley Quartzite is the terminal package of Ediacaran- aged sediments within the Pound Subgroup of the Adelaide Rift Complex (Gehling and Droser, 2012). The Ediacara Member incorporates a number of fossiliferous facies, ranging from shore-face settings, to deeper water delta-front or pro-delta environments (Gehling and Droser, 2013). By far the richest, in situ, fossil assemblages are found within the Oscillation Rippled Sandstone facies (Tarhan et al., 2017). These comprise medium- to coarse-grained feldspathic sands with distinctive rippled bed-tops, representative of a depositional environment between fair-weather and storm wave bases.
Locality information.—The Crisp Gorge fossil locality (31°11'41.5"S, 138°31'46.5"E, Fig. 1) is primarily composed of repeating units of the Oscillation Rippled Sandstone facies (ORSF) and the deeper water Flat Laminated to Linguoid Rip- pled Sandstone facies, the latter indicative of a delta-front to pro-delta setting (Tarhan et al., 2017). Both facies are present at this locality as interbedded dark red to maroon sands and silts, with the ORSF exhibiting characteristic bed-top combined-flow
length, as the longest distance between A- and B-ends of the specimen, maximum width, taken as the widest measure per- pendicular to the midline, and the number of units. Unit counts were not taken for individuals with damaged or indistinct units, defined here as those individuals with any form of mechanical damage to the rock or sand grains that mold them, and/or those that are indistinct from neighboring units. Measurements and unit counts were made with the aid of calipers and a dissecting microscope. Observations also were made of three body fea- tures, which have been termed the A-end lip, A-end protuber- ance, and shrinkage rim (see Results for details). Statistical analysis of the D. costata population was
performed using the open access statistical software program PAST (Hammer et al., 2001). A Model II (major axis) regres- sion was used in the analysis of body size (Legendre and Legendre, 1998). A rose diagram of axial orientation was pro- duced using the open access statistical software programs R (RCoreTeam, 2015) and RStudio (RStudioTeam, 2015), and the package ‘circular’ (Lund and Agostinelli, 2015). Pairwise cor- relations among three observed features (A-end lip, shrinkage rim, and A-end protuberance) were calculated using the Pearson method for the presence-absence data collected. Significance of relationships was calculated using Chi-squared tests with Yates’ continuity correction. To test if the D. costata
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