PATTERNS OF EVOLUTION OF THE EDIACARAN SOFT-BODIED BIOTA DMITRIY GRAZHDANKIN Trofimuk Institute of Petroleum Geology and Geophysics, prospekt Akademika Koptyuga 3, Novosibirsk, 630090, Russia; and Novosibirsk State University, ulitsa Pirogova 2, Novosibirsk, 630090, Russia, ,dima.grazhdankin@gmail.com. ABSTRACT—When each of the Avalon-, Ediacara-, and Nama-type fossil assemblages are tracked through geological time, there appear to be changes in species composition and diversity, almost synchronized between different sedimentary environments, allowing a subdivision of the late Ediacaran into the Redkinian, Belomorian and Kotlinian geological time intervals. The Redkinian (580–559 Ma) is characterized by first appearance of both eumetazoan traces and macroscopic organisms (frondomorphs and vendobionts) in a form of Avalon-type communities in the inner shelf environment, whereas coeval Ediacara-type communities remained depauperate. The Belomorian (559–550 Ma) is marked by the advent of eumetazoan burrowing activity in the inner shelf, diversification of frondomorphs, migration of vendobionts from the inner shelf into higher energy environments, and appearance of tribrachiomorphs and bilateralomorphs. Ediacaran organisms formed distinctive ecological associations that coexisted in the low-energy inner shelf (Avalon-type communities), in the wave- and current-agitated shoreface (Ediacara-type communities), and in the high-energy distributary systems (Nama- type communities). The Kotlinian (550–540 Ma) witnessed an expansion of the burrowing activity into wave- and current- agitated shoreface, disappearance of vendobionts, tribrachiomorphs and bilateralomorphs in wave- and current-agitated shoreface, together with a drop in frondomorph diversity. High-energy distributary channel systems of prodeltas served as refugia for Nama-type communities that survived until the end of the Ediacaran and disappeared when the burrowing activity reached high-energy environments. This pattern is interpreted as an expression of ecosystem engineering by eumetazoans, with the Ediacaran organisms being progressively outcompeted by bilaterians. INTRODUCTION T HE EDIACARAN soft-bodied biota, a highly distinctive assemblage of macroscopic organisms preserved as casts and molds in siliciclastic, volcaniclastic and carbonate sedi- ments, has been documented globally within a discrete and well- constrained interval of geological time between 580 and 540 Ma. Within this range there is a clustering of three taxonom- ically distinct fossil assemblages that have come to be known as Avalon-, Ediacara-, and Nama-type biotas and have been interpreted as biogeographic provinces, as biofacies, or as preservational artifacts (Waggoner, 1999, 2003; Grazhdankin, 2004; Narbonne, 2005; Gehling and Droser, 2013). The Avalon-, Ediacara-, and Nama-type assemblages also can be viewed as successive stages in the evolution of complex multicellularity, segmentation, motility, and biomineralization (Gehling, 2007; Shen et al., 2008; Xiao and Laflamme, 2009; Erwin et al., 2011; Laflamme et al., 2013) thereby providing the basis for chronostratigraphic subdivision of the upper series of the Ediacaran System (Narbonne et al., 2012). In this study the Avalon-, Ediacara-, and Nama-type assemblages are shown to be a manifestation of a larger scale macroevolutionary/macro- ecological process, the shift in biodiversity of Ediacaran soft- bodied biota from low-energy shelf to high-energy shoreface ecosystems. HIGH-LEVEL GROUPINGS OF EDIACARAN ORGANISMS A little over 280 species of Ediacaran soft-bodied organisms have been described, of which half represent valid taxa and the rest either pseudofossils or synonyms reflecting life stage and preservational variations. At least eight high-level taxonomic groups can be recognized that constitute three major clades, the vendobionts, frondomorphs and stem group eumetazoans (Grazhdankin, 2011; Erwin et al., 2011) (Fig. 1). Vendobionta.—The vendobionts constitute a clade of foliate organisms defined by a synapomorphic character, ‘‘the serial or fractal quilting of the flexible body wall, which stabilized shape, maximized external surface and compartmentalized the living content’’ (Seilacher, 1992, p. 607). The clade comprises three groups: the rangeomorphs, dickinsoniomorphs, and petalonamae. In addition, palaeopascichnids can be regarded as another group of vendobionts. The body of rangeomorphs was compartmentalized into a fractal network of branched tubular structures (Narbonne, 2004; Grazhdankin and Seilacher, 2005; Narbonne et al., 2009) (Fig. 1.7). Dickinsoniomorpha are flat and thin, at first glance bilaterally symmetric forms that consist of a large number of segments, with one segment having a distinctive crescent or rhomboidal shape and superficially resembling an arthropodal head-shield; the segments of the two sides do not line up perfectly along the midline, however, and instead show offset or alternate glide reflection symmetry (as opposed to mirror reflection symmetry) (Ivantsov, 1999, 2001, 2004, 2007) (Fig. 1.3). Petalonamae, an imperfectly defined group dominated by serially quilted body plans, is most exotic to mainstream biology; their moldic preservation within sandstone is intriguing in that the specimens appear in variously curved and oriented shapes, approximately resembling internal moulds of pots and troughs (Ivantsov and Grazhdankin, 1997; Grazhdankin and Seilacher, 2002) (Fig. 1.4). The rhomboidal segment in dickinsoniomorphs could be homologous to the rhomboidal segment in petalonamae and rangeomorphs, thus strengthening the phylogenetic relation- ship among these three groups (Fig. 2). Palaeopascichnida consist of series (sometimes multiple series and even foam-like aggregations) of sausage-shaped or globular chambers that are occasionally branching (Seilacher et al., 2003; Jensen, 2003) (Fig. 1.1). The eponymous genus Palaeopascichnus Palij, 1976 was originally described from the Studenitsa Formation of Podolia, Ukraine as the meandering trace of a sediment feeder. Other members of the group were subsequently described from the Vendian of the Southeast White Sea area, but 269 Journal of Paleontology, 88(2), 2014, p. 269–283 Copyright Ó 2014, The Paleontological Society 0022-3360/14/0088-0269$03.00 DOI: 10.1666/13-072