Matbased 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

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. 1A-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).

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. Photo courtesy of M. L. 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).

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. Photo courtesy of M. L. 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).

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 Lagerst├Ątten 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 organism-specific organic molecules, or biomarkers. Molecular biology and the molecular clock can be used to estimate when the ancestors of extant higher taxa first appeared in geological time.

3.1 Geology and Paleoenvironments

The Ediacaran Period encompasses the time from the Marinoan Snowball Earth glaciation to the base of the Cambrian (Knoll et al, 2004). Thus, this interval represents a transitional time from typical Proterozoic conditions, typified by low oxygen levels in the deep ocean, abundant microbialites, and an absence of metazoans, to the fully oxygenated Phanerozoic oceans containing widespread metazoans and restricted microbialites. The most intense Snowball Earth glaciations (Hoffman and Schrag, 2002), which occurred primarily between 720 Ma and 635 Ma and immediately preceded the Ediacaran Period, were the most significant paleoenvironmental events of the Neoproterozoic and may have inhibited metazoan evolution during their duration (Runnegar, 2000). The final Neoproterozoic glacial episode, during the Ediacaran at 580 Ma, predates the appearance of large metazoan fossils by less than 5 million years (Bowring et al., 2003; Narbonne and Gehling, 2003).

In addition to the biotic restrictions from the extreme climatic fluctuations, oxygen levels would have exerted a fundamental control on the evolution of metazoans, especially megascopic animals, in the Neoproterozoic. There is evidence that the deep oceans were largely anoxic during the Mesoproterozoic (Arnold et al., 2004) and that the levels of dissolved oxygen increased significantly only during the Neoproterozoic (Shields et al., 1997; Canfield, 1998). The presence of large Ediacara fossils in deep slope settings confirms that oxygen levels had increased by the late Neoproterozoic, at least in basins where substantial contour currents were present (Dalrymple and Narbonne, 1996; Wood et al., 2003).

Neoproterozoic metazoans lived in a wide range of sedimentary environments, from deep slope and basinal settings to shallow shelf and deltaic environments. Most Ediacaran fossils, such as those found in the classic localities of Ediacara (Australia), the White Sea in Russia, and Namibia, occur in shallow-water depositional environments, including storm-influenced shelf settings, prodeltaic environments, and distributary mouth bars (Saylor et al, 1995; Grazhdankin and Ivantsov, 1996; Gehling, 2000; Grazhdankin, 2004). Phosphorite facies of the Doushantuo Formation (Guizhou Province, China), which contains animal eggs and embryos, probable sponges and stem-group cnidarians, and the oldest known bilaterian fossils, were also deposited in shallow, nearshore marine environments (Dornbos et al, 2006). In contrast, diverse and abundant Ediacarans lived on a deep-water slope, well below the photic zone, in the Avalonian localities of Mistaken Point and Charnwood Forest, England (Wood et al., 2003). Other deep-water Ediacaran fossil localities include northwest Canada, where the fossils are preserved on the base of siliciclastic turbidites deposited on the continental slope (Dalrymple and Narbonne, 1996), and the Olenek Uplift in Siberia, where the fossils occur in slope and basinal carbonaceous limestones of the Khatyspyt Formation (Knoll et al., 1995).

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