Paleobiology, 42(4), 2016, pp. 574–594 DOI: 10.1017/pab.2016.20


Ediacaran distributions in space and time: testing assemblage concepts of earliest macroscopic body fossils


Thomas H. Boag, Simon A. F. Darroch, and Marc Laflamme


Abstract.—The mid-late Ediacaran Period (~579–541Ma) is characterized by globally distributed marine soft-bodied organisms of unclear phylogenetic affinities colloquially called the “Ediacara biota.” Despite an absence of systematic agreement, previous workers have tested for underlying factors that may control the occurrence of Ediacaran macrofossils in space and time. Three taxonomically distinct “assemblages,” termed the Avalon, White Sea, and Nama, were identified and informally incorporated into Ediacaran biostratigraphy. After ~15 years of new fossil discoveries and taxonomic revision, we retest the validity of these assemblages using a comprehensive database of Ediacaran macrofossil occurrences. Using multivariate analysis, we also test the degree to which taphonomy, time, and paleoenvironment explain the taxonomic composition of these assemblages. We find that: (1) the three assemblages remain distinct taxonomic groupings; (2) there is little support for a large-scale litho-taphonomic bias present in the Ediacaran; and (3) there is significant chronostratigraphic overlap between the taxonomically and geographically distinct Avalonian and White Sea assemblages ca. 560–557Ma. Furthermore, both assemblages show narrow bathymetric ranges, reinforcing that they were paleoenvironmental–ecological biotopes and spatially restricted in marine settings. Meanwhile, the Nama assemblage appears to be a unique faunal stage, defined by a global loss of diversity, coincident with a noted expansion of bathymetrically unrestricted, long-ranging Ediacara taxa. These data reinforce that Ediacaran biodiversity and stratigraphic ranges of its representative taxa must first statistically account for varying likelihood of preservation at a local scale to ultimately aggregate the Ediacaran macrofossil record into a global biostratigraphic context.


Thomas H. Boag* and Marc Laflamme. Department of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road,Mississauga, Ontario, L5L 1C6, Canada. E-mail: tomboag@stanford.edu, marc.laflamme@utoronto.ca. *Present address: Department of Geological Sciences, Stanford University, 450 Serra Mall Building 320, Room 118, Stanford, California, 94305-2215, U.S.A.


Simon A. F. Darroch. Department of Earth and Environmental Sciences, Vanderbilt University, 5726 Stevenson Center, 7th Floor, Nashville, Tennessee, 37240, U.S.A. E-mail: simon.a.darroch@vanderbilt.edu


Accepted: 12 April 2016 Published online: 25 May 2016 Data available from the Dryad Digital Repository: http://dx.doi.org/10.5061/dryad.1mh30


Introduction The Ediacaran Period (635–541Ma) represents


a critical interval in the evolution of complex life, bridging the microscopic eukaryotic groups of the Cryogenian (Bosak et al. 2012; Riedman et al. 2014) to the eventual radiation of macro- scopic animal life that epitomizes the Cambrian explosion(Erwinetal.


2011). Cambrian


Lagerstätten such asChengjiang and the Burgess Shale are pivotal to our understanding of the diversification of phylum-level animal clades, yet they tell us comparatively little about perplexing stem- and crown-group metazoan origins, which sit protractedly in the late Neoproterozoic (Peterson et al. 2004, 2008; Erwin et al. 2011). Furthermore, Ediacaran-aged fossil assemblages represent the earliest


© 2016 The Paleontological Society. All rights reserved.


evolution of complex ecological interactions such as bioturbation and predation, often hypothe- sized to be the requisite biological drivers for the Cambrian explosion of morphological innovation (Stanley 1976; Bengston and Zhao 1992; Peterson and Butterfield 2005; Sperling et al. 2013). To address these questions, heightened


interest has focused on the intermediary Ediacaran fossil record, dominated by a globally distributed marine assemblage of multicellular organisms colloquially referred to as the “Ediacara biota” (Narbonne 2005). These macrofossils, restricted in this study to lineages of large, soft-bodied organisms predominantly preserved as casts and


molds of Ediacaran age (Laflamme et al. 2013), first appear after the Gaskiers glaciation


0094-8373/16


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