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Vol. 8, No. 2 February 1998 INSIDE • G. K. Gilbert, p. 16 GSA TODAY • Forests as Carbon Sinks, p. 22 • Rocky Mountain A Publication of the Geological Society of America Section Meeting, p. 27 The Ediacara Biota: A Terminal Neoproterozoic Experiment in the Evolution of Life Guy M. Narbonne Department of Geological Sciences, A B Queen’s University, Kingston, ON K7L 3N6, Canada, [email protected] ABSTRACT The Ediacara biota is a distinctive assemblage of large, soft-bodied organisms that characterizes terminal Neoproterozoic (latest Precambrian) strata worldwide. Some Ediacaran organisms apparently were the root- stock for the Phanerozoic evolution of animals; other bizarre forms may rep- resent a failed experiment in Precam- brian evolution. The Ediacara biota and its nonactualistic preservation and ecosystem characterized the final 20 m.y. of the Proterozoic, and disap- peared near the beginning of the Cambrian “explosion” of shelly and burrowing animals. Figure 1. Photograph and artist’s reconstruction of the holotype of Swart- puntia germsi from Namibia. For simplic- INTRODUCTION ity, only three petaloids are shown. Met- The terminal Neoproterozoic was a ric scale. After Narbonne et al. (1997, Figs. 6 and 8). period of fundamental change in Earth history. Major changes included the breakup of the supercontinent Rodinia and the subsequent collision of some of the fragments that were to form Gond- wana (Hoffman, 1992; Unrug, 1997), a stone beds (Fig. 1). The Ediacara biota con- Phanerozoic systems (cf. Sepkoski, 1981). succession of at least four global glacia- tains a few forms that might be familiar to Prior to the appearance of the Ediacara tions, and some of the largest known a modern beachcomber, alongside bizarre biota, Mesoproterozoic to mid-Neopro- changes in the oceans and atmosphere taxa whose place in the evolution of mod- terozoic (1600–600 Ma) benthic communi- (Knoll and Walter, 1992). It is against this ern life is uncertain. A few “Ediacaran sur- ties had been dominated by prokaryotic backdrop that we see the appearance of vivors” have been reported from the Cam- microbes along with some sheetlike and abundant, large organisms, including the brian, but most of the archetypical forms ribbonlike algae (Knoll, 1992). Evidence first definite fossil animals in Earth his- disappeared abruptly near the Cambrian from molecular phylogeny suggests that tory, the Ediacara biota. “explosion” (Grotzinger et al., 1995). microscopic animals could have evolved The Ediacara biota typifies terminal The unique biology, ecology, and preser- sometime prior to the appearance of the Neoproterozoic rocks worldwide (Glaess- vational mode of the Ediacara biota effec- Ediacara biota, but the exact timing ner, 1984; Fedonkin, 1992; Jenkins, 1992; tively marks the end of the Proterozoic remains debatable (Wray et al., 1996; Runnegar and Fedonkin, 1992). It was Eon, and may herald the beginning of Conway Morris, 1996). The oldest known first described from Newfoundland and the Phanerozoic. megascopic Ediacara-type remains occur Namibia, but the name is derived from the in the Twitya Formation of northwestern superb assemblages of these fossils discov- EVOLUTIONARY HISTORY Canada (see Fig. 3, locality 22) immedi- ered at Ediacara in the Flinders Ranges of ately below tillites correlated with the The Ediacara biota occupies a pivotal South Australia by Sprigg (1947). In con- Marinoan-Varanger glaciation and position in the evolution of life on Earth, trast with the shelly fossils that character- believed to be about 600 m.y. old (Hof- between the largely microbial (especially ize the Phanerozoic, the Ediacara biota mann et al., 1990). The structures consist stromatolitic) communities that charac- consists almost exclusively of the remains of centimeter-scale rings and discs that are terize the classic “Precambrian” and the of soft-bodied organisms, which are typi- shelly biotas of the Cambrian and younger cally preserved as impressions on sand- Ediacara Biota continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 8, No. 2 1998 The Ediacara Biota: A Terminal Rock Stars—G. K. Gilbert ............... 16 Neoproterozoic Experiment in the GSAF Update .......................... 18 GSA TODAY (ISSN 1052-5173) is published monthly Evolution of Life ..................... 1 Washington Report .................... 20 by The Geological Society of America, Inc., with offices at 3300 In Memoriam .......................... 2 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Forests as Carbon Sinks ................ 22 9140, Boulder, CO 80301-9140, U.S.A. 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  • Rangeomorph Classification Schemes and Intra-Specific Variation: Are All

    Rangeomorph Classification Schemes and Intra-Specific Variation: Are All

    Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 Rangeomorph classification schemes and intra-specific variation: are all characters created equal? CHARLOTTE G. KENCHINGTON1,2* & PHILIP R. WILBY3 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK 2Present address: Department of Earth Sciences, Memorial University of Newfoundland, Prince Philip Drive, St John’s, NL, A1B 3X5, Canada 3British Geological Survey, Environmental Science Centre, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK *Correspondence: [email protected] Abstract: Rangeomorphs from the Ediacaran of Avalonia are among the oldest known complex macrofossils and our understanding of their ecology, ontogeny and phylogenetic relationships relies on accurate and consistent classification. There are a number of disparate classification schemes for this group, which dominantly rely on a combination of their branching characters and shape metrics. Using multivariate statistical analyses and the diverse stemmed, multifoliate rangeomorphs in Charnwood Forest (UK), we assess the taxonomic usefulness of the suite of char- acters currently in use. These techniques allow us to successfully discriminate taxonomic groupings without a priori assumptions or weighting of characters and to document a hitherto unrecognized level of variation within single taxonomic groups. Variation within the currently defined genus Pri- mocandelabrum is too great to be realistically assigned to different species and may instead reflect primary character diversity, ontogenetic changes in character state or ecophenotypic variability. Its recognition cautions against generic-level diagnoses based on single differences in character state and will be crucial in understanding the mode of growth of these enigmatic organisms. Supplementary material: Data tables, definition of the characters used in the analyses, and detailed descriptions and breakdowns of methods and results are available at https://doi.org/10.