Evolutionary Paleoecology of Ediacaran Benthic Marine Animals
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Chapter 4 Evolutionary Paleoecology of Ediacaran Benthic Marine Animals DAVID J. BOTTJER and MATTHEW E. CLAPHAM Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089- 0740, USA. 1. Introduction .................................................... 91 2. A Mat-Based World.............................................. 92 3. Nature of the Data............................................... 95 3.1. Geology and Paleoenvironments................................. 95 3.2. Lagerstätten................................................ 96 3.3. Biomarkers................................................ 97 3.4. Molecular Clock Analyses..................................... 97 4. Evolutionary Paleoecology......................................... 97 4.1. Doushantuo Fauna (?600-570 Mya) .............................. 99 4.2. Ediacara Avalon Assemblage (575-560 Mya) ....................... 101 4.3. Ediacara White Sea Assemblage (560-550 Mya) ..................... 102 4.4. Ediacara Nama Assemblage (549-542 Mya) ........................ 105 5. Discussion..................................................... 108 Acknowledgements ................................................. 110 References....................................................... 110 1. INTRODUCTION Paleobiological study of the Phanerozoic and the Precambrian largely developed as separate cultures during the 20th century. This was primarily because the presence of common body fossils with mineralized skeletons Neoproterozoic Geobiology and Paleobiology, edited by Shuhai Xiao and Alan Jay Kaufman, © 2006 Springer. Printed in The Netherlands. 91 92 D. J. BOTTJER and M. E. CLAPHAM typical of the Phanerozoic allowed certain types of science, such as biostratigraphy and benthic paleoecology, to be done that was not feasible in the Precambrian. However, as fossils have increasingly been documented from the late Neoproterozoic, approaches more typical of the Phanerozoic have become possible, leading recently to the definition of the Ediacaran Period as a formal name for the terminal Neoproterozoic interval (Knoll et al., 2004). This time period is bounded below by the Marinoan Snowball Earth glaciation, and above by the Cambrian, and is characterized by diverse suites of largely soft-bodied fossils, exemplified by the Doushantuo and Ediacara biotas. This push back from the Phanerozoic has also allowed for the first time studies on the evolutionary paleoecology of Ediacaran benthic assemblages (e.g., Clapham and Narbonne, 2002; Droser et al., 2006). In particular this is because we are beginning to understand the biological affinities of these fossils better (e.g., Fedonkin and Waggoner, 1997; Seilacher, 1999; Narbonne, 2004), although differences of opinion remain (Xiao et al., 2000; Peterson et al., 2003; Seilacher et al., 2003; Bengtson and Budd, 2004; Chen et al., 2004; Grazhdankin, 2004). This promises to be a growing field of research in the future, as we begin to apply some of the approaches developed in the Phanerozoic (e.g., Brenchley and Harper, 1998; Allmon and Bottjer, 2001) to the record of animal life in the Ediacaran. This contribution, which focuses on ecological aspects of Ediacaran benthic marine animal life, synthesizes the early stages of a research program that will eventually integrate the two worlds of Precambrian and Phanerozoic evolutionary paleoecology. 2. A MAT-BASED WORLD Much Phanerozoic benthic paleoecology has focused on the effects of bioturbation and the nature of substrates. One of the biggest differences between Phanerozoic and Proterozoic benthic ecosystems is that while vertical bioturbation and all its effects upon seafloor characteristics are nearly ubiquitous in the Phanerozoic, only limited horizontal bioturbation was present in the Ediacaran (Seilacher, 1999; Bottjer et al., 2000). Thus, a distinguishing feature of the Ediacaran is that it was a time where seafloors were commonly dominated by microbial mats (Gehling, 1999; Hagadorn and Bottjer, 1999; Wood et al., 2003; Droser et al., 2005; Gehling et al., 2005; Dornbos et al., 2006; Droser et al., 2006). The most typical expression of this transition is the widespread occurrence of stromatolites in carbonates during the Proterozoic and their decline at the end of the Precambrian, likely due to the emergence of animals (Awramik, 1971). A recent reanalysis of Evolutionary Paleoecology of Ediacaran Benthic Marine Animals 93 Awramik’s database confirms that the greatest decline in stromatolite form diversity is indeed in the Ediacaran, culminating in the Early Cambrian (Olcott et al., 2002). Although other factors were also involved (Grotzinger and Knoll, 1999), since this was a time of appearance and radiation of animal groups, it implies that the stromatolite form diversity decline was strongly affected by increasing development of passive and/or active interactions between evolving animals and microbial sedimentary structures. Some part of this was likely caused by the increasing disruption due to more intense bioturbation. Recent research has shown that there is also substantial evidence for the common presence of seafloor microbial mats in siliciclastic settings during the Neoproterozoic (Hagadorn and Bottjer, 1997; Gehling, 1999; Hagadorn and Bottjer, 1999). Despite their widespread occurrence in Proterozoic carbonates, mat structures in siliciclastics are less dramatic than the stromatolites found in carbonates, so they have received less attention. Their subdued appearance is because siliciclastic settings are non-mineral- precipitating seafloor environments, so vertical dimensions of such structures are commonly on the millimeter scale. Microbially-mediated structures in siliciclastic settings have been given intriguing names, reflecting the lack of understanding, until recently, of their origin. Thus, we have Ediacaran wrinkle structures (Fig. 1 A-C) and “elephant skin” (Fig. 1D) from a variety of depositional environments. Much of what we know about the distribution of mats in Ediacaran strata is from the presence of trace fossils, such as the scratch-like trace Radulichnus that can be found associated with Kimberella (Fig. 1D), a possible stem-group mollusc that is found in Ediacaran-aged deposits from both Russia and Australia. These widespread structures are interpreted as the feeding traces produced by Kimberella as it scraped the mat-covered seafloor. Other diffuse impressions show movement of Yorgia and Dickinsonia, and possibly also their feeding activities, on mat-covered sediment (Fedonkin, 2003; Gehling et al., 2005). Interpretations of the taphonomy of the Ediacara biota by Gehling (1999) also demonstrate the typical occurrence of microbial mats on siliciclastic Ediacaran seafloors. For example, Ediacaran bedding surfaces at Mistaken Point, Newfoundland, are commonly covered by a red coating of limonitic “rust,” implying an enrichment by organic matter and hence the presence of a mat. Such features are also found in Lower Cambrian marine facies which contain bedding planes representing seafloors once extensively covered by microbial mats (Dornbos et al., 2004). Some relatively unweathered Mistaken Point outcrops of bedding planes show the presence of pyrite, indicating that the red “rust” is oxidized from pyrite, which likely formed as a decomposition product of the mat organic matter (Gehling et al., 2005). 94 D. J. BOTTJER and M. E. CLAPHAM The presence of crinkly carbonaceous laminae within siltstone intervals at Mistaken Point confirms the widespread distribution of microbial mats (Wood et al., 2003). In addition, the abundance of discoidal fossils is likely an indicator that the disc was a holdfast structure embedded in the seafloor below a mat-bound surface (Seilacher, 1999; Wood et al., 2003). Strata in which the Doushantuo biota of southwest China are found have not been studied extensively for the presence of microbial structures, but preliminary investigations indicate the presence of microbial mats on Doushantuo seafloors (Dornbos et al., 2006). Despite the increasing presence of animals through the Ediacaran, the common presence of microbial mats strongly influenced benthic organism ecology, and hence their morphology as well as taphonomy (Gehling, 1999; Seilacher, 1999). Figure 1. Evidence for mat-dominated Neoproterozoic benthic ecosystems. (A) Wrinkle structures preserved on the surface of wave ripples, Rawnsley Quartzite, Ediacara Hills, South Australia. From Selden and Nudds (2004). (B) Wrinkle structures in deep-water turbiditic siltstone, Drook Formation, Mistaken Point, Newfoundland. Coin is 19 mm diameter. From Margaret Fraiser. (C) Wrinkle structures from the late Ediacaran Wyman Formation, Silver Peak Range, Nevada. Scale bar 1 cm. From Hagadorn and Bottjer (1999). (D) Kimberella fossil and associated Radulichnus grazing trace on microbial “elephant skin” (arrow) bedding surface, White Sea, Russia. Scale bar divisions are 1 cm. From Fedonkin (2003). Evolutionary Paleoecology of Ediacaran Benthic Marine Animals 95 3. NATURE OF THE DATA A broad variety of data from both earth and biological sciences is incorporated in the analysis of Ediacaran paleobiology and evolutionary paleoecology. The habitats of Ediacaran animals can be reconstructed through geological analysis of depositional environments. The fossils themselves are almost exclusively found in Lagerstätten where soft tissues are preserved as molds and casts, although the presence of animals in ancient environments can sometimes be inferred through studies of preserved