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

Lily M Reid,James G Gehling,Diego C García-Bellido

doi:10.1017/jpa.2017.142

Abstract

AbstractDespite 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 protuberance 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.

Highlights

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
This study focuses on a population of Dickinsonia costata Sprigg, 1947 from Crisp Gorge, South Australia (Fig. 1)
All specimens are preserved in negative hyporelief and occupy a narrow size range, well below typical maximum size for D. costata

Summary

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. 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 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). The identification of resting traces associated with several individual specimens indicates that it may have fed on the underlying microbial mat substrate via basal surface absorption (Gehling et al, 2005; Sperling and Vinther, 2010; Ivantsov, 2011)

Methods

Results

Discussion

Conclusion