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First Evidence for Locomotion in the Ediacara Biota from the 565 Ma Mistaken Point Formation, Newfoundland

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First Evidence for Locomotion in the Ediacara Biota from the 565 Ma Mistaken Point Formation, Newfoundland Downloaded from geology.gsapubs.org on February 10, 2010 First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland Alexander G. Liu1, Duncan McIlroy2, and Martin D. Brasier1,2 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK 2Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X5, Canada ABSTRACT Evidence for locomotion in the Precambrian fossil record is scant. Reliable Ediacaran trace fossils are all younger than 560 Ma, and consist of relatively simple horizontal burrows and trails from shallow-water deposits. Here we describe an assemblage of macro- scopic locomotory traces from deep-water environments at Mistaken Point, southeastern Newfoundland, Canada, dated to ca. 565 Ma. These trails extend the record of complex trace fossils back into the earliest Avalonian biota. Our new evidence for large motile organisms on the seafl oor at this time suggests that at least some of these early Ediacaran organisms, whose biological affi nities are widely debated, could have been muscular and of metazoan grade. INTRODUCTION Traces of animal activity are critical for our understanding of organ- ism-sediment interactions in the geological record (McIlroy and Logan, 1999). Although it is rarely possible to determine the trace-making organ- ism directly, trace fossils remain invaluable as a means of documenting the evolution of behavior (Fedonkin, 2003). The trace fossil record is Figure 1. Locality map and stratigraphic column showing position of mainly confi ned to the Phanerozoic, largely because complex organisms trace-bearing beds (stars). A: Map of Newfoundland. Mistaken Point are usually required to create them. Even so, Proterozoic trace fossils do Ecological Reserve (MPER) is in box. B, C: Map of MPER showing exist. Recent reviews of presumed Ediacaran (latest Proterozoic) traces location of Mistaken Point and fossil-bearing bed. have determined that only a few simple burrow makers were present (Sei- lacher et al., 2005; Jensen et al., 2006). These burrows generally occur in shallow-marine environments of ca. 560 Ma or younger (Seilacher et DESCRIPTION al., 2005; Droser et al., 2006), and are thought to have been produced by The new fossil horizon is within the Mistaken Point Formation on early bilaterians (Jensen et al., 2006). The White Sea assemblage of Rus- the Avalon Peninsula of Newfoundland, Canada (Fig. 1), and crops out as sia and South Australia (ca. 555 Ma; Martin et al., 2000) reveals question- a narrow ledge. It is ~50 m stratigraphically above the famous E Surface able markings related to the body fossils Yorgia and Dickinsonia (McIlroy (565 ± 3 Ma; Benus, 1988), within massive turbidites close to the base of et al., 2009; Ivantsov and Malakhovskaya, 2002; Fedonkin, 2003), along the overlying Trepassey Formation. The fossils occur on top of a 3-mm- with the mollusk-like feeding trace Radulichnus (Seilacher et al., 2003, thick homogeneous green mudstone, which overlies sharply an 18-cm- 2005). Hitherto, reliable evidence for trace fossils from the oldest Edia- thick, upward-fi ning unit of siltstone to mudstone. The latter was likely caran macrofossil-bearing successions (the Avalon), ca. 575–560 Ma, has deposited by a waning turbidity current. The upper surface of the bed is been lacking (Gehling et al., 2000; Jensen et al., 2006). capped by coarse-grained tuff, 1 mm thick, thought to be either a pri- The scarcity of trace fossils in the Avalon assemblage has been used mary or reworked water-lain tuff, which is overlain by 12 cm of turbidite. to suggest that the Avalonian organisms were sessile, and incapable of Modern weathering of the relatively soft tuff has revealed the fossilifer- escaping smothering ash falls and turbidity currents (Narbonne, 1998). ous bedding plane. We fi nd no sedimentary structures, obvious reworked Consequently, these soft-bodied organisms have been interpreted to pos- intrabasinal clasts, or evidence for current reworking and modifi cation of sess a variety of different biological affi nities, including microbes (Gra- the trace fossils in these layers. Water depth at the time of deposition is zhdankin and Gerdes, 2007), giant protists (Seilacher et al., 2003), and estimated as ~1 km, with turbidity currents sourced from an adjacent vol- sessile animals (Narbonne, 2005; see Brasier and Antcliffe, 2009, for ref- canic island arc (Wood et al., 2003). erences of further interpretations). We observe over 70 traces, ranging from 1.5 to 17.2 cm in length and The Mistaken Point locality, within the Conception Group of south- up to 13 mm in width. These traces show neither branching nor any sys- eastern Newfoundland (Fig. 1), includes remarkably preserved soft- tematic increase in width (the width of the largest example varies along its bodied Ediacaran macrobiotic communities (Narbonne et al., 2007). The length from 9 to 13 mm; Fig. 2A). The surfaces of the traces are marked succession shows net upward shallowing, and was likely deposited in an by regular crescentic internal divisions, formed by thin ridges of siltstone extensional backarc basin (Wood et al., 2003). Fossiliferous horizons are (Fig. 2). The spacing between these individual siltstone packets is ~1 mm. known from deep-marine sediments of the Drook Formation (575 ± 1 Ma; Each trace typically bears marginal ridges (Figs. 2 and 3C–3E). Such Bowring et al., 2003) to shallow-marine sediments of the Fermeuse For- ridges provide key evidence for movement of an object along the surface mation (ca. 560 Ma; Fig. 1). We describe here the discovery of macro- of the sediment (Jensen et al., 2005, 2006), and can be used to distinguish scopic traces consistent with locomotion along the sediment-water inter- trace fossils from abiogenic structures such as synaeresis and desiccation face, within the Mistaken Point Formation. This suggests that the benthos cracks (Parizot et al., 2005). The crescents show different orientations may have included rare ancestors of early animals ca. 565 Ma. between adjacent specimens, ruling out formation by a unidirectional © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2010; 2010 v. 38; no. 2; p. 123–126; doi: 10.1130/G30368.1; 4 fi gures; Data Repository item 2010025. 123 Downloaded from geology.gsapubs.org on February 10, 2010 A C B E D Figure 2. Locomotion trace from Mistaken Point Formation, New- Figure 3. Locomotion traces and circular pits, Mistaken Point Forma- foundland. A: Largest observed trail on bedding plane. B–D are tion, Newfoundland. A, B: Isolated circular pits, unrelated to traces. close-up images of crescentic internal divisions in A. B: Distal end of C: Two traces (arrowed), one (left) strongly curving, other ending in trail. Note pyrite crystals embedded in ash surrounding trail. C: Cen- disc (top right). D: Another trace (arrowed), gently curving but with tral section of trail. D: Proximal section of trail with terminal circular clear marginal ridges. Traces can curve by as much as 60°–70°. E: impression. Scale bars = 1 cm. A cast is housed in Oxford University Three individual traces (arrowed), exhibiting positive marginal ridges Museum of Natural History (OUM ÁT.418/p). and crescentic internal structure. Cast of specimen E is housed in Oxford University Museum of Natural History (OUM ÁT.419/p). Scale bars = 1 cm. current. At the distal end of several specimens, a negative circular impres- sion can also be seen (Figs. 2D and 3C). There is no evidence to suggest any preferred orientation of trails, which can be either straight or gently or scratch marks formed by unidirectional contourite currents (the back- curved (Figs. 3C–3D). Several specimens have portions with smooth, pos- ground hydrodynamic regime; Wood et al., 2003), though turbidite infl u- itive-relief regions. In the largest specimen, the circular impression and the ence cannot be discounted. Physical sedimentological processes cannot proximal half of the trace are preserved in negative relief, while the ridges readily explain formation of these features, therefore biological processes of the crescentic features within the trace are preserved in positive relief. must be considered. More distally, the trace becomes smoother (with no ridges or troughs) Several biogenic features bear some resemblance to our material. with positive relief (Fig. 2). Small pits (1–2 cm diameter) also occur in The Ediacaran fossil Palaeopascichnus was originally described as a trace negative relief on the same surface, but these lack the radial or concentric fossil (Palij et al., 1983), but has been reinterpreted as a protistan body markings diagnostic of Aspidella (Gehling et al., 2000; Figs. 3A and 3B). fossil (Seilacher et al., 2003). Palaeopascichnus consists of a number of lunate chambers, which in branching specimens are not concave but DISCUSSION convex in the direction of growth (Jensen et al., 2006). Examples from When documenting evidence for ancient life, it is important to falsify Newfoundland also have more regular internal divisions, and lack discs at any possible abiogenic mechanisms for producing the observed features. their ends (Gehling et al., 2000). Two trace fossils reported from the Edia- Glacially induced striae and tectonically induced bedding cleavage lin- caran, Archaeonassa and Bilinichnus (Jensen et al., 2006), compare with eations are common in this region, but cannot explain the formation of our material in being horizontal and in having raised marginal ridges, but the features documented herein: the candidate traces show no consistent neither has the prominent crescentic features of our Mistaken Point mate- orientation, and exhibit directional changes (Figs. 3C and 3D). While rial. Circular trace fossils with limited lateral movement, evidenced by abiogenic conchoidal and feather fractures occur on many bedding sur- the presence of lunate structures, have been recorded from the late Edia- faces, these tend to be irregular, asymmetrical, and crosscut sedimentary caran as Beltanelliformis brunsae or Bergaueria sucta (Fedonkin, 1981; laminae.
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