Reid et al. –Population analysis of Dickinsonia costata from South Australia
material as normal (Shapiro-Wilk; W=0.98; p=0.07). A single- component Gaussian finite mixture model best fitted the body length data based on optimization of the BIC, suggesting that these data are best described by a single distribution (rather than multiple distributions).
Axial orientation and paleocurrent direction.—Based on pri- mary ripple shape and orientation of the bed top surfaces of the Crisp Wall, a primary paleocurrent direction of ~55° from north has been established (Reid et al., 2017). No consistent axial orientation is observed in the Crisp Wall specimens (Fig. 3.2). Hence, there appears to be no relationship between the axial orientation of D. costata and either paleocurrent or burial flow direction.
Morphological features.—A marked A-end protuberance occurs in 15% of the Crisp Gorge specimens (Figs. 2, 4). Broadly confined to the terminal A-end unit, this asymmetric bulge appears in deep negative relief relative to the surrounding organism, often developing into a point towards the peripheral margin. Larger protuberances extend for up to 5mm, in smaller specimens encompassing nearly half the sagittal length. In larger specimens where greater detail is visible, protuberances often observed to be offset from the primary axis, to either the left or the right, towards and sometimes encompassing A-end units. This feature is observed in five other small Dickinsonia within the South Australian Museum collections. The deep relief and consistent positioning of the protuberance exclude the possibility that it is the result of the organism overlying a TOS feature within the substrate. The A-end lip appears as a raised, upper-surface rim in
nearly half (45%) of observed specimens (Figs. 2, 4). It is visible as a rim that outlines the body through the outer margin of the units at a thickness of no more than 1mm. It occurs most commonly through the A-end and mid-body units, and is visible on some specimens distorting the peripheral margins of individual units. A ‘shrinkage rim’ feature occurs in 38% of Crisp Gorge
specimens, appearing as a positive relief rim extending 1–2mm from the peripheral margin and in contrast to the negative hyporelief of the body mold (Figs. 2, 4). In most specimens, it does not extend the full circumference of the body. In some instances, however, a pronounced continual peripheral rim surrounds the body and can contain a secondary outline, interpreted as representing a second stage of shrinkage (Gehling et al., 2005). In several specimens where shrinkage is most marked, a faint impression of lower-surface units is visible within the shrinkage rim. A test of correlations between these two features shows a weak, but significant, positive relationship between the occurrence of a shrinkage rim and an A-end lip (Pearson’s correlation coefficient=0.22, p-value <0.01).
Discussion
The abundance and narrow size distribution of the juvenile population on the Crisp Wall surface suggest that Dickinsonia costata may have been an opportunistic species that established on this surface shortly after deposition and stabilization of the underlying substrate (Reid et al., 2017). Dickinsonia is noted
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throughout all of the fossiliferous facies of the Ediacara Member in a broad range of sedimentary environments and at a range of localities, although more commonly in adult size ranges and usually at lower abundances (Droser and Gehling, 2015; Evans, 2015) than those observed in the Crisp Wall. At the Nilpena fossil locality on the western border of the Flinders Ranges, Dickinsonia is the third most abundant body fossil, exceeded only by Funisia and Aspidella, repectively, both of which represent anchored taxa (Gehling and Droser, 2013).
Opportunistic species display a range of identifying char-
acteristics, many of which are demonstrated by Dickinsonia. They are typically generalists, capable of survival within a broad range of environmental and physiological parameters, as demonstrated by the identification of Dickinsonia in all fossili- ferous facies of the Ediacara Member, and in situ within the majority of these paleoenvironments (Gehling and Droser, 2013). Species utilizing such a life history strategy may display a rapid increase in population numbers, often evidenced by the numerical domination of a given community or the rapid colo- nization of substrate previously devoid of organisms (Levinton, 1970). Dickinsonia comprises >50% of the Crisp Wall com- munity, a trait that is not uncommon in other Ediacara Member communities (Droser and Gehling, 2015). The Crisp Wall population displays minimal variance in
size and is normally distributed, in contrast with other observed D. costata populations. Broadly speaking, Dickinsonia records a right-skewed population and a spread of body sizes spanning juvenile to adult (Zakrevskaya, 2014; Evans, 2015). There are two potential reasons for this discrepancy: (1) this population was the result of a single, isolated reproductive event; and (2) this population was buried in the very early stages of con- tinuous or recurring, episodic recruitment, butwas too immature to record a bimodal, clustered, or skewed distribution. Regardless, the distribution of this juvenile population does not negate the likelihood of longer-term continuous recruitment within the taxon, and supports the notion that the Crisp Wall surface records an immature, pioneer-stage community (Reid et al., 2017). Based on the length to width ratio, Dickinsonia costata
displays a juvenile isometric growth pattern (Fig. 3.3). This is consistent with previous findings, which have focused on larger individuals of the taxon and suggest that overall growth was isometric, with length and width increasing at a steady ratio throughout the majority of the organism’s lifecycle (Runnegar, 1982; Evans et al., 2017). This is despite the observed decrease in unit addition, indicating that overall shape is maintained independent of unit number (Evans et al., 2017; Hoekzema et al., 2017). Although all specimens are broadly ovoid in shape,
variation in shape does occur. This may be accounted for by flexibility of the organism, and may be related to flexure resulting from movement of the organism in relation to the underlying microbial substrate. It may also be due to natural variation within the population. Likewise, a minor amount of size variation may be accounted for by the presence of the shrinkage rim in organisms that display this feature. The lack of axial orientation noted in Dickinsonia both
in this study and previously is in contrast to observations made for other Ediacaran forms (see Evans et al., 2015). Several Ediacaran taxa, including Thectardis avalonensis
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