Reid et al. –Population analysis of Dickinsonia costata from South Australia
inferred life habit in near-constant contact with the underlying microbial substrate on which it likely fed (Sperling and Vinther, 2010). This close relationship with the dense, often ropey, substrate may have made Dickinsonia more or less impervious to current action, with the exception of higher-energy events lifting a small portion of the body away from the substrate (Evans et al., 2015). The correlation between the shrinkage rim and A-end lip
suggests that these features may have represented a physical response by the organism to the burial process. The shrinkage rim is also observed in Dickinsonia specimens from a number of localities across a range of sizes and has been interpreted as representing the shrinkage of the organism, due to a loss of mass associated with desiccation during early diagenesis (Gehling et al., 2005). Its correlation with the A-end lip suggests that this feature may also be due to the burial process, and that these features combined could be the result of a muscle-like contrac- tion in response to the burial event, indicating displacement rather than loss of mass, and resulting in a thickening of the peripheral margin of the organism. The occurrence of this lip most commonly through the terminal A-end unit may be due to this unit representing the thinnest section through the fossil, and therefore making it most apparent in this location. In contrast to these features, the striking A-end protuber-
ance noted in this population is observed in only a limited number of small or juvenile Dickinsonia elsewhere in the South Australian Museumcollections. Although broadly limited to the terminal A-end unit, placement of the protuberance within this region appears to be random. The deep-relief nature of the fea- ture suggests that it was largely impervious to the burial process and was therefore likely constructed of the same material as the rest of the organism. Why it appears in such a high concentra- tion in the Crisp Gorge material, but remains relatively rare in other Dickinsonia populations such as those at the Nilpena fossil locality, remains unclear. Given the juvenile size range of this population, however, it appears most likely that this feature is an artifact of early growth and may reflect a biological structure found exclusively in juveniles.Were it to be observed in a single individual, it may be dismissed as a preservational abnormality. It is unlikely, however, that this structure is taphonomic because there is nothing to suggest that the mode of preservation varied based on the size of the organism preserved.
Conclusions
The Dickinsonia costata population from Crisp Gorge provides an excellent opportunity to examine this taxon in juvenile form, and investigate the physical characteristics of such a population. Dickinsonia was likely a hardy opportunist with a single, juvenile size mode, which grew isometrically by maintaining its overall size ratio. The presence of the morphological features observed, including the shrinkage rim and A-end lip, requires further investigation in adult specimens, with the study of larger individuals perhaps providing higher-resolution material for examination. The A-end protuberance described in this popu- lation is most likely an artifact of early growth, however, further examination of juvenile Dickinsonia from other Ediacara surfaces will be helpful to confirm this.
Acknowledgments
This research has been supported by a student research grant from the University of Adelaide 2014-007, a summer research scholarship from the University of South Australia PD134814, an Australian Government Research Training Program Scholarship, and a South Australian Museum Early Career Researcher Postgraduate Scholarship to LMR. DCGB is sup- ported by Australian Research Council Future Fellowship FT130101329. Collection and transport of the majority of material utilized in this project was funded by an NSF grant (EAR 9004601) to B.N. Runnegar. We thank C. Reschke and family for access to the Crisp Gorge locality. A. Liu and J. Holmes are thanked for field assistance and feedback. This manuscript benefited from feedback provided by M. Lee, J. Antcliffe, M. Laflamme, and J. Hoyal Cuthill.
References
Clapham, M.E., Narbonne, G.M., Gehling, J.G., Greentree, C., and Anderson, M.M., 2004, Thectardis avalonensis: a new Ediacaran fossil from the Mistaken Point biota, Newfoundland: Journal of Paleontology, v. 78, p. 1030–1036.
Darroch, S.A., Rahman, I.A., Gibson, B., Racicot, R.A., and Laflamme, M., 2017, Inference of facultative mobility in the enigmatic Ediacaran organism Parvancorina: Biology Letters, v. 13, e20170033.
Droser, M.L., and Gehling, J.G., 2015, The advent of animals: the view from the Ediacaran: Proceedings of the National Academy of Sciences, v. 112, p. 4865–4870.
Evans, S.D., 2015, Ecology and Biology of Dickinsonia, an Iconic Member of the Ediacara Biota From Nilpena, South Australia [MSc Thesis]: Riverside, CA, University of California, Riverside, 43 p.
Evans, S.D., Droser, M.L., and Gehling, J.G., 2015, Dickinsonia liftoff: evidence of current derived morphologies: Palaeogeography, Palaeo- climatology, Palaeoecology, v. 434, p. 28–33.
Evans, S.D., Droser, M.L., and Gehling, J.G., 2017, Highly regulated growth and development of the Ediacara macrofossil Dickinsonia costata: PLoS One, v. 12, e0176874.
Fraley, C., Raftery, A.E., Murphy, T.B., and Scrucca, L., 2012, package “mclust”: Version 4 for R: Normal Mixture Modeling for Model-Based Clustering, Classification, and Density Estimation, v. 5.3, https://cran.
r-project.org/web/packages/mclust/index.html
Ford, T.D., 1958, Pre-Cambrian fossils from Charnwood Forest: Proceedings of the Yorkshire Geological and Polytechnic Society, Geological Society of London, v. 31, p. 211–217.
Gehling, J.G., and Droser, M.L., 2009, Textured organic surfaces associated with the Ediacara biota in South Australia: Earth-Science Reviews, v. 96, p. 196–206.
Gehling, J.G., and Droser, M.L., 2012, Ediacaran stratigraphy and the biota of the Adelaide Geosyncline, South Australia: Episodes - Newsmagazine of the International Union of Geological Sciences, v. 35, p. 236.
Gehling, J.G., and Droser, M.L., 2013, How well do fossil assemblages of the Ediacara Biota tell time?: Geology, v. 41, p. 447–450.
Gehling, J.G., Droser, M.L., Jensen, S., and Runnegar, B.N., 2005, Ediacara organisms: Relating form to function, in Briggs, D.E.G., ed., Evolving Form and Function: Fossils and Development: New Haven, CT, Yale University, p. 43–66.
Glaessner, M.F., 1958, New fossils from the base of the Cambrian in South Australia: Transactions of the Royal Society of South Australia, v. 81, p. 185–188.
Glaessner, M.F, 1959, Precambrian Coelenterata from Australia, Africa and England: Nature, v. 183, p. 1472–1473.
Glaessner, M.F., and Wade, M., 1966, The late Precambrian fossils from Ediacara, South Australia: Palaeontology, v. 9, p. 599–628.
Gold, D.A., Runnegar, B., Gehling, J.G., and Jacobs, D.K., 2015, Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia: Evolution and Development, v. 17, p. 315–324.
Gooden, B., 2014, Segmentation and oxygen diffusion in the ediacaran ‘Dickinsonia’: an applied analysis: Journal and Proceedings of the Royal Society of New South Wales, v. 147, p. 107.
Hammer, Ø., and Harper, D.A., 2006, Paleontological Data Analysis: Oxford, Wiley-Blackwell, 356 p.
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