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. In contrast, the candidate traces have both positive and nega- Narbonne and Hofmann, 1987). B. sucta has been suggested to represent tive expressions with a well-defi ned, regular internal structure, and do not lateral movement similar to that of modern actinians (Seilacher, 1990), but continue through sedimentary laminae. Importantly, they can also display unlike the Mistaken Point trails, this evidence is limited to a few closely a circular impression at one end. Although crescentic features can be spaced arcs adjacent to the circular pit (Fedonkin, 1981). formed by a circular object passively dragging along a sediment-water Although no convincing Ediacaran macrofossils occur alongside these interface, the variety of orientations cannot easily be explained as tool trails, well-preserved Aspidella and Hiemalora occur 1.5 m stratigraphically

124 GEOLOGY, February 2010 Downloaded from geology.gsapubs.org on February 10, 2010 below, while a poorly preserved Charniodiscus occurs 1 m above. The cir- cular pits on the trace-bearing bed (Figs. 3A and 3B) are insuffi ciently well preserved to be classifi ed as Aspidella, though they could be an end mem- ber of its morphological spectrum (Gehling et al., 2000). Tubular body fossils, mostly known from younger Ediacaran assem- blages, have often been mistaken for trace fossils (Jensen et al., 2006). These body fossils can show features such as a basal circular disc, trans- verse segmentation, and dimensions like those seen here (e.g., Funisia; Droser and Gehling, 2008). We draw attention here to our evidence for marginal sediment displacement in the form of lateral ridges; arcuate rather than straight transverse markings; truncations rather than super- impositions where two structures meet; widening diameters at points of curvature; and a lack of branching in our material (Figs. 2 and 3; GSA Data Repository1). These observations are inconsistent with formation by tubular body fossils (cf. Jensen et al., 2005) Trace fossils with comparable crescentic backfi ll material are known from the Phanerozoic. Cross sections of backfi lled cylindrical burrows, such as Taenidium, are superfi cially similar. Our material, however, shows no evidence for cross sections through a cylindrical feature, nor does Figure 4. Modern actinian (Urticina) trails in mud, produced in our it extend into the sediment, appearing to be an entirely surfi cial trace. marine aquaria. Note concave-forward hemispherical structures (at Plagiogmus and Psammichnites are backfi lled burrows with segmented left) and positive marginal ridges (right). Scale bar = 3 cm. fl oors, but both have a signifi cant vertical component, and internal ridges that are either straight or convex forward (McIlroy and Heys, 1997) rather than concave forward as here. Although these traces are preserved beneath tuffs (just like body fos- SUGGESTED TRAIL MAKERS sils in the same formation; Conception-style preservation of Narbonne, We here make comparison with younger Phanerozoic interface trace 2005), only one trace-bearing bed has yet been found. Their scarcity may fossils. Modern pennatulacean cnidarian resting traces can appear similar be a function of water depth, with the trace makers being transported to the circular impressions at the ends of our trails (Miller, 1999). It could downslope by currents from shallower depths; or special conditions were be argued that frondose organisms, morphologically similar to Charnio- necessary for their preservation; or simply the trace makers were a rare discus, were responsible for forming these traces. We question whether element of the fauna. Mucus-bound sedimentary traces such as our anem- Charniodiscus was pennatulacean (see Antcliffe and Brasier, 2007), and one trails have low preservation potential (Jensen et al., 2005). no such traces have yet been found associated with those fronds. Mod- ern organisms, such as echinoids and sea anemones, commonly produce IMPLICATIONS FOR PALEOBIOLOGY somewhat comparable interface trails, especially in cohesive or mucous- The discovery of these large, distinctive trace fossils raises the ques- bound sediment (Fig. 4). Actinian cnidarians (sea anemones) may provide tion as to whether motile metazoans were present in the Avalon biota. This a possible biomechanical analogue for these trails (cf. Parker, 1916), and question has signifi cant implications for the paleoecology of the oldest for discoidal Ediacaran impressions in general (Gehling, 1988; Grazh- Ediacaran ecosystems, previously thought to consist entirely of nonmotile dankin, 2000). Anemones are capable of crawling across sediment and suspension feeders (Clapham et al., 2003; Narbonne, 2005). Locomotion can exhibit swimming and burrowing behavior (e.g., McClendon, 1905; requires substantial levels of energy and metabolism, suggesting that the Parker, 1916). Our laboratory studies of the modern sea anemone Urticina ecology of the Avalonian seafl oor was already quite complex, perhaps indicate that it can produce surface trails with comparable positive mar- approaching that of later marine ecosystems. Motile organisms would ginal ridges, and crescentic internal ridges with concave edges facing the have been capable of escaping areas of environmental stress, would have direction of locomotion (Fig. 4). The anterior impressions made by these the capacity to search for nutrients, and could potentially free themselves trace makers are usually obliterated during subsequent movement, except from the (often surprisingly thin) ash layers that regularly smothered where the trail ends as a circular disc. The trace varies in width along them. We consider it possible that an organism comparable with an actin- its length because the organism moves by a muscular, nonuniform infl a- ian-grade cnidarian was responsible for the traces, perhaps transported tion of its base. These actinian trails also contain regions where ridges are from shallower regions. Comparison with actinian locomotion by infl at- absent and the sediment is smooth, potentially explained by locomotion able pedal disc implies that muscular tissue may well have been present in across a more consolidated substrate. While capable of crawling, most some Ediacaran organisms, to allow control of hydrostatic infl ation (e.g., modern actinians prefer to reside on hard substrates, explaining the pau- Batham and Pantin, 1950). In terms of the history of life, the presence of city of such traces in ancient sediments. Therefore, many of the features macroscopic horizontal traces at 565 Ma suggests that geologists should of the Newfoundland traces appear at least consistant with an actinian-like look for further evidence of bioturbation in the late Ediacaran Period, prior mode of locomotion. Creation of the trails by giant single-celled protists is to widespread deep burrowing at the Precambrian-Cambrian boundary an interesting possiblility that cannot yet be rejected (McIlroy et al., 2001; (McIlroy and Logan 1999). Seilacher et al., 2003; Matz et al., 2008), although there is currently no evidence that modern foraminifera can produce surface trails with internal CONCLUSION features such as crescents (Matz et al., 2008). The rocks of Mistaken Point in Newfoundland have yielded the earliest macroscopic surface locomotory trails in the geological record, 1GSA Data Repository item 2010025, supplementary trail images, is avail- able online at www.geosociety.org/pubs/ft2010.htm, or on request from editing@ extending the occurrence of such traces back by ~5 m.y., to ca. 565 Ma. geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO We consider that these impressions are consistent with locomotion traces 80301, USA. produced by a cnidarian-like organism. The fact that these traces are

GEOLOGY, February 2010 125 Downloaded from geology.gsapubs.org on February 10, 2010 directly associated with an Ediacara biota lends support to the hypoth- zoic geobiology and paleobiology: Topics in geobiology Volume 27: Hei- esis that at least some elements of the Avalon-type biota could have been delberg, Springer, p. 115–157. motile animals. It is likely that the circular impressions at the ends of the Martin, M.W., Grazhdankin, D.V., Bowring, S.A., Evans, D.A.D., Fedonkin, M.A., and Kirschvink, J.L., 2000, Age of Neoproterozoic bilaterian body trails represent molds of the lower surface of the trace maker, and it is and trace fossils, White Sea, Russia: Implications for metazoan evolution: noted that these molds superfi cially resemble characteristics of the Ediac- Science, v. 288, p. 841–845, doi: 10.1126/science.288.5467.841. aran forms Aspidella, Hiemalora, and Charniodiscus. Matz, M.V., Frank, T.M., Marshall, N.J., Widder, E.A., and Johnsen, S., 2008, Gi- ant deep-sea protist produces bilaterian-like traces: Current Biology, v. 18, ACKNOWLEDGMENTS p. 1–6, doi: 10.1016/j.cub.2008.10.028. We thank the Parks and Natural Areas Division, Department of Environ- McClendon, J.F., 1905, On the locomotion of a sea anemone (Metridium margin- ment and Conservation, Government of Newfoundland and Labrador for granting atum): Biological Bulletin, v. 10, p. 66–67, doi: 10.2307/1535667. a permit to conduct research. Field assistance from J. Matthews and advice from McIlroy, D., Brasier, M.D., and Lang, A.S., 2009, Smothering of microbial mats A. King, along with constructive reviews from R. Callow, M. Lafl amme, and an by macrobiota: Implications for the Ediacara biota: Journal of the Geologi- anonymous reviewer were invaluable. 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