Trace fossils in the Ediacaran–Cambrian transition: Behavioral diversification, ecological turnover and environmental shift

Palaeogeography, Palaeoclimatology, Palaeoecology 227 (2005) 323 – 356 www.elsevier.com/locate/palaeo

Trace fossils in the Ediacaran–Cambrian transition: Behavioral diversification, ecological turnover and environmental shift Adolf Seilacher a, Luis A. Buatois b,*, M. Gabriela Ma´ngano b a

Geologisches Institut, Sigwartstasse 10, D 72076 Tu¨bingen, Germany, and Department of Geology, Yale University, P.O. Box 208109, New Haven, CT 06520, USA b Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Canada SK S7N 5E2 Received 12 November 2004; received in revised form 30 May 2005; accepted 3 June 2005

Abstract After taxonomic revision, trace fossils show a similarly explosive diversification in the Ediacaran–Cambrian transition as metazoan body fossils. In shallow-marine deposits of Ediacaran age, trace fossils are horizontal, simple and rare, and display feeding strategies related to exploitation of microbial matgrounds. Equally notable is the absence of arthropod tracks and sinusoidal nematode trails. This situation changed in the Early Cambrian, when a dramatic increase in the diversity of distinct ichnotaxa is associated was followed by the onset of vertical bioturbation and the disappearance of a matground-based ecology (ddagronomic revolutionTT). On deep sea bottoms, animals have been present already in the Ediacaran, but ichnofaunas were poorly diverse and dominated by the horizontal burrows of undermat miners. As shown by the ichnogenus Oldhamia, this life style continued to be predominant into the Early, and to a lesser extent, Middle Cambrian. Nevertheless, there was an explosive radiation of behavioral programs during the Early Cambrian. When exactly the bioturbational revolution arrived in the deep sea is uncertain. In any case, the Nereites ichnofacies was firmly established in the Early Ordovician. The rich ichnofauna in the Early Cambrian Guachos Formation of northwest Argentina probably marks a first step in this ecological onshore–offshore shift. D 2005 Elsevier B.V. All rights reserved. Keywords: Ediacaran–Cambrian; Ichnology; Microbial mats; Diversification

1. Introduction Difficulties to analyze the ichnologic record of the Ediacaran–Cambrian transition result from the tapho* Corresponding author. Tel.: +1 306 966 5730. E-mail addresses: [email protected] (A. Seilacher), [email protected] (L.A. Buatois), [email protected] (M. Gabriela Ma´ngano). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.06.003

nomic filter biogenic structures passed through, but also from taxonomic idiosyncrasies contained in published data. The latter bias applies particularly to Proterozoic trace fossils, whose rarity and antiquity raise the tendency to describe and name specimens that would otherwise pass as non-descript or be simply referred to as Planolites or Palaeophycus-like structures. In the present paper we critically review the ichnofauna of the Ediacaran shallow-marine biota

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and then discuss the development of deep-water ichnocoenoses in the Ediacaran and Early Cambrian, with new data from North Carolina (USA) and northwest Argentina. Subsequently, we raise the question of when the Cambrian agronomic revolution (Seilacher and Pflu¨ger, 1994) reached the deep sea. Finally, the taxonomy of some Ediacaran–Cambrian trace fossils is addressed in the light of new discoveries and reanalysis of selected specimens.

Skolithos by Crimes and Germs (1982) have been subsequently considered as body fossils (Crimes and Fedonkin, 1996). Specimens from the Carolina slate belt, assigned by Gibson (1989) to Monocraterion? isp. are in all probability inorganic, most likely softsediment deformation structures. The origin of Skolithos declinatus Fedonkin from the White Sea (Fedonkin, 1985) is still uncertain. In short, no undoubted examples of vertical burrows have been documented from the Ediacaran.

2. Ediacaran shallow-marine trace fossils

2.2. Xenophyophorean protozoa

While an earlier compilation (Crimes, 1994) lists 35 ichnogenera for the Ediacaran period, this number shrinks considerably in view of recent revisions (Gehling et al., 2000; Jensen, 2003; Seilacher et al., 2003). In addition, some members of the Ediacaran ichnofauna (Yelovichnus, Palaeopascichnus, Intrites and Harlaniella) are no longer considered trace fossils.

Xenophyophorea is a group of giant rhizopods having flexible, agglutinated chambers. Today they are restricted to abyssal depths (Tendal, 1972). It has been suggested that Ediacaran representatives still inhabited shallow seas and were embedded in biomats (Seilacher et al., 2003). This life style highly increased their fossilization potential. On the other hand, these structures may be easily mistaken for trace fossils, because in producing their chamber walls, these protists actively moved sand grains into the underlying mud layer just as a tracemaker would do (Fig. 1). Thus, the tightly packed chambers of Palaeopascichnus delicatus Palij, P. sinuosus Fedonkin and Yelovichnus gracilis Fedonkin were originally interpreted as meandering traces (Glaessner, 1969; Fedonkin, 1985; Crimes and Fedonkin, 1994), while chains of globular chambers (Neonereites renarius Fedonkin, N. biserialis Seilacher and Intrites punctatus Fedonkin) have been compared to backstuffed burrows. More recently, however, they have been regarded as body fossils (Haines, 2000; Gehling et al., 2000; Seilacher et al., 2003; Jensen, 2003), because both kinds do branch, which would be impossible in trace fossils of seemingly similar morphologies. Reexamination of some of the supposedly meandering trails (e.g., Palaeopascichnus) also fails to reveal the presence of actual meanders. In the same vein, Jensen (2003) questioned the trace fossil interpretation of Harlaniella podolica Sokolov (see also Palij, 1976). Harlaniella confusa Signor described by Signor (1994) is most likely also a body fossil. A third morphotype of Ediacaran xenophyophoreans consists of an agglomerate of chambers, from which agglutinated tubules radiate into the surrounding sediment. Forms corresponding to this group

2.1. Pseudofossils The ubiquity of biomats on Precambrian sea bottoms accounts for certain sedimentary structures that are rare in later deposits (Seilacher and Pflu¨ger, 1994; Seilacher, 1997). Sinusoidal shrinkage cracks (ddmanchuriophycusTT) and small-scale load casts (belephant-skin structuresQ), as well as various wrinkle patterns (e.g., bchloephycusQ, bkinneyiaQ), are now generally recognized as pseudofossils (Pflu¨ger, 1995; Hagadorn and Bottjer, 1999; Chakrabarti, 2001). In Neoproterozoic and Cambrian rocks, some of these structures have been repeatedly interpreted as trace fossils. For example, sinusoidal synaeresis cracks were referred to as Cochlichnus (e.g., Kulkarni and Borkar, 1996), while elephant skin structures and wrinkle marks have been confused with Protopaleodictyon (e.g., Durand and Acen˜olaza, 1990) and Squamodictyon (Durand et al., 1994) (see Chakrabarti, 2001 and Buatois and Ma´ngano, 2003a, for reinterpretations). Vertical burrows in shallow-marine Ediacaran deposits, typically assigned to Skolithos or, less commonly, to Arenicolites and Monocraterion, are doubtful (cf. Jensen, 2003). Supposedly vertical burrows described by Banks (1970) from Finnmark were subsequently reinterpreted as dewatering pillars (Farmer et al., 1992). Structures from Namibia assigned to

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Fig. 1. Xenophyophoreans on modern deep-sea bottoms and their Ediacaran shallow-marine counterparts. Palaeopascichnus, previously regarded as a meandering trace fossil, is now regarded as a protist body fossil. In the same vein, chains of globular chambers currently ascribed to Neonereites in Ediacaran rocks, are also considered protist body fossils (after Seilacher et al., 2003).

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(Eoporpita, Hiemalora) have been described as chondrophore colonies; so their re-interpretation does not affect the number of Ediacaran trace fossils. 2.3. Poorly defined ichnotaxa As trace fossils are more variable and poorer in distinctive characters than most body fossils, their taxonomy is also more subjective. Names appearing in the literature should therefore be properly checked before they can be treated as reliable data. In many cases, it turns out that identification was based on superficial similarities that fall into the preservational and behavioral variability of a few general ichnogenera. Ecologically Ediacaran ichnofaunas are dominated by poorly specialized grazing trails that tend to develop indistinct patterns, such as Gordia, Helminthoidichnites and Helminthopsis. As noted by Jensen (2003), all these ichnotaxa represent horizontal movement through the sediment in search for food and there are no convincing examples of true branching. Simple grazing trails that are occasionally parallel to each other have been mistaken for the bilobate trace Didymaulichnus (Bordonaro et al., 1992). A slab from the Flinders Ranges (Seilacher et al., 2003, Fig. 4) shows various grooves that may be

referred to as Helminthoidichnites. Although they fail to show a distinct behavioral program, they nevertheless contain relevant ecological information. As indicated by an associated trilobozoan body fossil (Tribrachidium), the trails are preserved on a sole face that probably corresponds to a matground. Being preserved as hypichnial grooves, they must either result from undermat mining (below the level of Tribrachidium) or have been produced after deposition of the overlying sand (probably a thin storm bed) by an animal that avoided the sponge-like Tribrachidium. As turns may be relatively sharp, one may also conclude that the tracemaker was either a very flexible worm or a shorter animal with a small turning radius. Yet, none of this information would be contained in the ichnogenus name and may be overlooked in an analysis based solely on trace fossil lists. 2.4. Nenoxites and Torrowangea These ichnogenera show distinctive features in addition to their winding course. Nenoxites curvus Fedonkin (Fig. 2) is based on a single specimen from the Winter Coast of the White Sea (Fedonkin, 1985). In schematic drawings (Crimes, 1987), it is commonly depicted as a segmented angular meander.

Fig. 2. Nenoxites curvus. Segmentation reflects a wall lining made of elongate fecal pellets oriented perpendicularly to the burrow axis. Ediacaran, White Sea, Russia.

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Re-study of the original specimen suggests, however, that the turns are not as sharp and that the segmentation reflects a wall lining made of elongate fecal pellets oriented perpendicularly to the burrow axis (Fig. 2) (see also Fedonkin, 1985). A somewhat similar form is Torrowangea rosei Webby described by Webby (1970) from New South Wales and subsequently documented from other Ediacaran to Early Cambrian shallow-to deep-marine successions (Hofmann, 1981; Narbonne and Aitken, 1990; Paczes´na, 1986). This ichnotaxon consists of horizontal, sinuous to irregularly meandering traces characterized by distinct, but irregularly spaced constrictions. It is most likely a feeding trace of deposit feeding annelids that burrowed peristaltically (Narbonne and Aitken, 1990). 2.5. Bilobate trails Bilobate trails have been commonly recorded from Lower Cambrian strata, but very rarely from the Ediacaran (Seilacher, 1956; Palij et al., 1979; Fedonkin, 1985; Crimes, 1987; Paczes´na, 1996; Jensen et al., 2002). Those displaying a regularly to irregularly meandering or spiral course and preserved as positive epireliefs have been variously referred to as Aulichnites, Taphrhelminthopsis, Taphrhelminthoida, Scolicia and, more rarely, Sellaulichnus, Jinningichnus and Multilaqueichnus (Crimes et al., 1977; Fedonkin, 1985; Crimes and Jiang, 1986; Crimes, 1987; Hofmann and Patel, 1989; Jenkins, 1995; Zhu, 1997; Li et al., 1997; Hagadorn et al., 2000; Acen˜olaza and Alonso, 2001). Zhu (1997) reanalyzed the type specimens of Sellaulichnus meishucunensis Jiang in Jiang et al. and noted that the burrows show branching and that the bilobed morphology may have resulted from collapse. Further study is necessary to evaluate if true branching occurs in this ichnotaxon. Re-examination of the type specimen of Aulichnites parkerensis Fenton and Fenton indicates that this ichnogenus is a junior synonym of Psammichnites (D’Alessandro and Bromley, 1987; Ma´ngano et al., 2002). An alternative name for some of these bilobate traces (which are much simpler than the younger Psammichnites) is Archaeonassa (Jensen, 2003). Archaeonassa is a monospecific ichnogenus that includes straight to sinuous or gently meandering traces having a median

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groove flanked by rounded ridges (Fenton and Fenton, 1937; Yochelson and Fedonkin, 1997). However, the ichnotaxonomic status of Archaeonassa is still uncertain (Ma´ngano and Buatois, 2003). These trails were probably made by a mollusk-like bilateral animal that bulldozed along bedding planes or buried biomats by displacing sediment along the sides of its body from the front into a terminal backfill. Sellaulichnus and Multilaqueichnus show similarities to what is usually called Taphrhelminthopsis. Taphrhelminthoida and Taphrhelminthopsis have been recurrently mentioned in Lower Cambrian rocks. In particular, the ichnospecies Taphrhelminthopsis circularis Crimes et al. and Taphrhelminthoida dailyi Hofmann and Patel seem to be typical of Lower Cambrian strata (Crimes et al., 1977; Crimes, 1987; Hofmann and Patel, 1989). However, Seilacher (1986) and Uchman (1995) demonstrated that the type specimens of Taphrhelminthopsis and Taphrhelminthoida are preservational variants of Scolicia, which is produced by spatangoid echinoids. Spatangoid traces are complex endichnial structures characterized by a meniscate backfill, a double ventral fecal string or drain, and mucus-lined vertical shafts (Bromley and Asgaard, 1975; Plaziat and Mahmoudi, 1988; Bromley, 1996). Scolicia occurs in Mesozoic and Cenozoic strata, while Paleozoic recordings should be transferred to other ichnogenera (Smith and Crimes, 1983; Uchman, 1995; SeilacherDrexler and Seilacher, 1999; Ma´ngano et al., 2002). Additionally, it should be noted that post-Paleozoic Taphrhelminthopsis and Taphrhelminthoida are preserved as positive hyporeliefs (not positive epireliefs; see Schlirf, 2002, for discussion), recording a search strategy that is similar to that of Psammichnites rather than Scolicia. Ediacaran trails consisting of two parallel furrows preserved as negative epireliefs have been described as Bilinichnus simplex Fedonkin and Palij (Palij et al., 1979). As noted by Keighley and Pickerill (1996) and Buatois et al. (1998a), this is a problematic form. Its mode of formation is difficult to explain, assuming that no metazoans bearing hard parts (i.e. arthropods) were available (Keighley and Pickerill, 1996). Bilinichnus was also regarded as a gastropod trail produced by peristaltic crawling (Fedonkin, 1985). Although it has been tentatively synonymized with Diplopodichnus (Buatois et al., 1998a), a mode of

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origin similar to that of Diplopodichnus from continental settings is hard to envisage. Furthermore, specimens illustrated as Bilinichnus simplex by Fedonkin (1994) closely resemble Archaeonassa. Though common in the Cambrian, bilobate traces preserved as hypichnial ridges are very rare, if present at all, in Ediacaran strata. Bilobate hyporeliefs have been commonly referred to as Didymaulichnus, among other names. Didymaulichnus includes smooth bilobate hypichnial ridges. Its presence in Ediacaran strata is dubious; in fact one of its ichnospecies, D. miettensis Young, has been regarded as indicative of the Early Cambrian (Young, 1972; Crimes, 1987; Walter et al., 1989).

tion was maintained in a subsequent study in which an additional species, M. randellensis, was proposed based on three fragmentary specimens (Sun, 1986). More specifically, Runnegar (1992a) regarded Mawsonites as a partly decayed medusa. An alternative interpretation was proposed by Seilacher (1989), who considered Mawsonites as a trace fossil consisting of a relatively large system of actively backfilled probings, and a vertical shaft with lateral seleniform backfill. Most probably, however, Mawsonites is a pseudofossil like ddAstropolithonTT, i.e. a sand-volcano interacting with biomats (Fig. 3). Alignment of the ddprobesTT along radial cracks is the main argument for this interpretation, which also allows for the lateral back fill.

2.6. Mawsonites 2.7. Aulozoon Mawsonites is a problematic Ediacaran taxon that was originally described as a jellyfish (M. spriggi) by Glaessner and Wade (1966). The jellyfish interpreta-

bAulozoonQ is an informal name for large fossils that look like flattened sand-filled sausages about 2

Fig. 3. Drawing of Mawsonites, which is reinterpreted as a sand-volcano interacting with biomats.

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cm wide and only 1–2 mm thick. They were originally referred to as large sinuous trails (Glaessner, 1969, Fig. 5E). As such flattening could hardly be compactional, Seilacher et al. (2003) assumed that they are trace fossils of a large flatworm. In this interpretation, the structures were first lined with mucus and then backfilled with the sand removed in front of the animal and transported along the body by ciliary waves as on a conveyor belt. Living cnidarians and platyhelminths have been suggested as modern analogues of Neoproterozoic tracemakers (Collins et al., 2000). This view is supported by a large slab (Fig. 4) (see also Runnegar, 1992b, Fig. 3.10). Elephantskin structures suggest that we deal with the sole of a sandy biomat. Uniformly sized vendobionts (Phyllozoon) were living below the mat and are therefore perfectly preserved in their original bhuggingQ positions. Dickinsonia, in contrast, lived solitarily on top, where it could use its limited mobility (Fedonkin, 1992) to digest new areas of

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the living mat. Accordingly, Dickinsonia specimens are preserved only as vague phantoms pressed through the mat by compaction (Gehling, personal communication 2003). In the trace fossil interpretation (Seilacher et al., 2003), while bulldozing along the base of the biomat, the Aulozoon producer reacts upon collision with an undermat Phyllozoon in specific ways. If approaching it at a low angle, the animal contours the vendobiont and turns away, while collisions at larger angles are avoided by passing either above or below the Pyllozoon. In contrast, Aulozoon does not react to the Dickinsonia phantoms. However, Aulozoon departs from the typical style of Ediacaran trace fossils being a conspicuous structure, significantly larger than unquestionable trails. For the moment, Aulozoon should be treated as a problematic form. The study of additional slabs may eventually point to an alternative interpretation as a body fossil (Gehling, written communication 2004).

Fig. 4. Drawing of a basal surface with Phyllozoon (death masks), Dickinsonia (phantom preservation) and Aulozoon (sand-filled ribbons). Ediacaran, Flinders Ranges, Australia (after Seilacher et al., 2003, Fig. 5).

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2.8. Dickinsoniid resting trails Resting and locomotion traces of Dickinsonia and the related genus Yorgia have been recorded in Ediacaran shallow-marine deposits of the White Sea (Ivantsov and Malakhovskaya, 2003; Fedonkin, 2003) and Australia (Gehling, unpublished). In the White Sea, the presence of body fossils in direct association with the trace fossils allows identification of the producers, namely Yorgia waggoneri Ivantsov and Dickinsonia tenuis Glaessner and Wade (Ivantsov and Malakhovskaya, 2003). In addition, similarities in shape and size between the resting trace and the body fossil allow linking a third type of trace with Dickinsonia costata Sprigg, although the two have not been found in direct connection. In contrast to dickinsoniid body fossils, which are preserved as negative hyporeliefs, trace fossils are preserved as positive hyporeliefs. As noted by Fedonkin (2003), the producers glided over the sea bottom during short pulses of locomotion that alternated with long resting phases associated with intense mucus production. These findings are remarkable because they documentat for the first time trace fossils produced by vendobionts. 2.9. Radulichnus This is one of the few Ediacaran trace fossils that was made from above the sediment surface. Originally considered as scratch marks of trilobite legs (Monomorphichnus; Jenkins, 1995), these are clearly scratches rasped by paired radular teeth (Gehling et al., unpublished). More exactly, their producer was the bsoft limpetQ Kimberella (see Fedonkin and Waggoner, 1997), whose ventral death masks are commonly associated (Fig. 5) and have occasionally been found in flagranti in the White Sea area (Fedonkin, 2003). In contrast to later radula traces, the paired scratches of the Ediacaran form are arranged not in meanders, but in fans whose tips are always missing. As Gehling concluded (personal communication 2001), Kimberella did not move-on while it was browsing, but used a long proboscis that swung out more widely the further it extended away from the stationary body. Equally unusual is the preservation of the radular scratches. In the Mesozoic and Cenozoic, Radulichnus is preserved only on hard surfaces, such as mollusk shells, while scratches made on soft substrates

became wiped-out as the producer bulldozed over them. Only when the surface was indurated by tough biomats could they escape such fate. This was certainly the case of Ediacaran sea bottoms and of very shallow Cambrian deposits in Saudi Arabia (Seilacher, 1977) and China (Dornbos et al., 2004). This also explains why only the raspings of adult Kimberella are preserved: the radulae of smaller individuals did not penetrate deep enough to appear as undertraces at the base of the mat. Nor did Kimberella leave a trail when it moved from one station to the next. 2.10. Treptichnus and supposed Chondrites Treptichnus is typically Phanerozoic, with the first appearance of Treptichnus pedum (Seilacher) regarded as index of the Precambrian–Cambrian boundary (Brasier et al., 1994; Narbonne et al., 1987). However, in South Australia (Jensen et al., 1998), Namibia (Jensen et al., 2000) and Newfoundland (Gehling et al., 2001), Treptichnus may occur in strata that still contain Ediacaran body fossils and it certainly persisted into the Ordovician (Fig. 6). In any case, branched, three-dimensional burrow systems indicating shallow-tier bioturbation, were already present by the end of the Neoproterozoic. Chondrites has also been mentioned in Ediacaran strata (e.g., Jenkins, 1995). However, these structures are preserved as furrows that lack the characteristic burrow fill. They have been reinterpreted as poorly preserved specimens of the body fossil Hiemalora (Narbonne, personal communication 2002). 2.11. Plug-shaped burrows Plug-shaped burrows, preserved as positive hyporeliefs, are regarded as resting or dwelling traces of cerianthid or actinarian anemones (Pemberton et al., 1988). Although Ediacaran examples, commonly included in Bergaueria, have been recorded in a few localities (e.g., Crimes and Germs, 1982; Fedonkin, 1985), distinction from body fossils, such as Intrites, Beltanellifomis and Beltanelloides, is problematic (Crimes, 1992; Crimes and Fedonkin, 1996; Jensen, 2003). However, specimens from Canada described by Narbonne and Hofmann (1987) and Seilacher et al. (2003) display the typical morphology of plugshaped burrows and includes a series of overlapping

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Fig. 5. The primitive mollusk Kimberella as the tracemaker of the radular scratches Radulichnus. Originally considered as scratch marks of trilobite appendages, these are regarded here as scratches rasped by paired radular teeth. Note that the scratches of smaller individuals did not penetrate deep enough to produce undertraces at the base of the biomats. Ediacaran, White Sea, Russia (after Seilacher et al., 2003).

discs. Therefore, these structures, included in Bergaueria sucta, reveal the lateral displacement of an anemone-like animal. Plug-shaped burrows, however, are much more abundant since the Cambrian (Crimes and Anderson, 1985; Crimes and Fedonkin, 1996). 2.12. Final remarks on shallow-marine Ediacaran ichnodiversity In conclusion, Ediacaran trace fossils have a much lower numerical diversity than it appears from the literature. They are also less common than in younger rocks. Behavioral complexity is limited as well: except for the burrow systems of Treptichnus and the scratches of Radulichnus, there are no systematic search patterns. Equally notable is the absence of tracks or trails of larger arthropods, or of sinusoidal nematode trails, which could well be preserved and

easily recognized. Even small benthic arthropods appear to have been absent, because they would likely have destroyed the biomats and produced a flocculent surface layer (Waloszek, 2003). This situation changes dramatically in the Early Cambrian, when not only the number of recognizable taxa, but also their behavioral and tiering complexity reaches much higher levels. Since shallowmarine ichnocoenoses from the Lower Cambrian have been well described (e.g., Jensen, 1997), we focus here on another problem: the conquest of deep sea bottoms.

3. Trace fossils from Ediacaran and lowermost Cambrian deep sea deposits Deep-sea deposits are characterized by thick sequences of laminated mudstones and coarser-grained

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Fig. 6. Stratigraphic distribution of Treptichnus pedum in Ediacaran to Ordovician rocks.

turbidites and the absence of wave-induced structures (e.g., oscillation ripples; hummocky cross stratification). For the present study two areas have been

chosen, where thick deep-marine turbiditic successions are exposed: the Albemarle Group of the Carolina slate belt in eastern United States and the

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Puncoviscana Formation, northwest Argentina. Because of intense tectonic deformation, lithologic uniformity, and scarcity of guide fossils, these successions are difficult to subdivide. Nevertheless, a Neoproterozoic age is established for the Albemarle Group by a handful of Ediacaran body fossils (Pteridinium carolinense Gibson et al. (Gibson et al., 1984). The Puncoviscana Formation is regarded as Ediacaran to Early Cambrian. This age is based on stratigraphic relations with the overlying upper Lower to Middle Cambrian Meson Group, trace fossils and radiometric dates (see Ma´ngano and Buatois, 2004a; Buatois and Ma´ngano, 2005). The ichnofossiliferous strata are considered Nemakit– Daldynian in age (Buatois and Ma´ngano, 2003a). 3.1. Ediacaran deep-marine ichnofaunas The ichnology of the Albemarle Group was first analyzed by Gibson (1989). Re-study of this ichnofauna suggests a number of taxonomic reassessments. In particular, the Floyd Church Member of the McManus Formation contains a poorly diverse assemblage consisting of Circulichnis montanus Vyalov (=?Gordia arcuata Ksia˛ykiewicz of Gibson, 1989), Helminthoidichnites tenuis Fitch [=Planolites beverleyensis (Billings) and Planolites montanus Richter of Gibson, 1989], ?Helminthopsis isp., Oldhamia recta isp. n. (=Syringomorpha nilssoni? of Gibson, 1989) and Treptichnus? isp. (=?Neonereites isp. of Gibson, 1989). Other forms described from this unit are here considered as dubious. The ichnofauna is dominated by nondiagnostic and poorly specialized grazing trails (Helminthopsis ichnoguild of Buatois and Ma´ngano, 2003b), that also occur in shallow-marine environments of the same age. Oldhamia recta (see taxonomic Appendix) made by a small undermat miner and preserved in positive as well as negative hyporeliefs, is so far the only distinctive ichnospecies. These early pioneers had no obvious ancestry in shallow-marine realms. Ediacaran deep-marine ichnofaunas have also been recorded from the Mackenzie Mountains in Canada (Narbonne and Aitken, 1990; MacNaughton et al., 2000) and central Spain (Vidal et al., 1994). Nonspecialized grazing trails similar to those from the McManus Formation were recorded in both cases. Structures indicative of microbial mats were noted by Mac-

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Naughton et al. (2000). As shown by these trace fossils, deep-sea bottoms were colonized by benthic animals already in Ediacaran times. However, behavioral diversity, as well as the disparity of life styles, remained very low. Colonization of deep sea bottoms during the terminal Proterozoic is also supported by the body fossil record (Narbonne, 1998, 2005; Narbonne and Gehling, 2003; Clapham et al., 2003; Grazhdankin, 2004). 3.2. Early Cambrian deep-marine ichnofaunas The Puncoviscana Formation of northwest Argentina provides a glimpse into the ecology of early Phanerozoic deep-marine ecosystems. Puncoviscana trace fossils were first described in the seventies (Acen˜olaza and Durand, 1973) and have since been the focus of a series of studies (e.g., Durand and Acen˜olaza, 1990; Acen˜olaza et al., 1999a). More recently, the composition and paleoenvironmental significance of this ichnofauna and its importance for the ecology of deep sea infaunal communities in the Early Cambrian has been stressed (Buatois and Ma´ngano, 2003a,b, 2004). Deep-marine trace fossil assemblages of the Puncoviscana Formation are present in the San Antonio de los Cobres/Cuesta de Mun˜ano area (Salta Province) and, further south, in the Sierra de la Ovejerı´a region of the Catamarca Province. This association is dominated by the ichnogenus Oldhamia (O. antiqua Forbes, O. flabellata Acen˜olaza and Durand, and O. radiata Forbes; see taxonomic Appendix and Fig. 7), and grazing trails, such as Helminthoidichnites tenuis and Helminthopsis tenuis Ksia˛ykiewicz. Other components are Palaeophycus tubularis Hall, Cochlichnus anguineus Hitchcock and Diplichnites isp. (Fig. 8A) (Buatois and Ma´ngano, 2003b). Apparently branching specimens of sinusoidal traces (Cochlichnus-type) were also found (Fig. 8B). Most of these trace fossils were emplaced in the uppermost millimeters of a relatively firm substrate. Wrinkled surfaces, ripple patches and palimpsest ripples suggest the presence of microbial mats (Figs. 8A–B, 9A–B). Benthic communities developed in direct association with these organically bound surfaces. Their strategies included mat grazing and undermat mining (Buatois and Ma´ngano, 2003b). Similar Early to, more rarely, Middle Cambrian deep-marine ichnofaunas (Oldhamia association) are

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Fig. 7. Ichnospecies of Oldhamia made by unknown undermat miners. Note the explosive radiation of behavioral programs in the Early Cambrian.

known from folded rocks in Ireland, Belgium, Alaska, Canada, United States and Morocco (Forbes, 1849; Malaise, 1883; Sollas, 1900; Churkin and Brabb, 1965; Crimes and Crossley, 1968; Dhonau and Holland, 1974; Hofmann and Cecile, 1981; El Hassani and Willefert, 1990; Lindholm and Casey, 1990; Sweet and Narbonne, 1993; Hofmann et al., 1994; Holland, 2001). Contemporaneously with shallower habitats, deep-sea ichnocoenoses experienced a burst in behavioral diversity. During the first stage, however, this radiation was restricted to the ichnogenus Oldhamia. It reached its climax by the Early Cambrian, when various behavioral modifications are represented by distinct ichnospecies (Buatois and Ma´ngano, 2003b). Oldhamia has rarely been recorded in shallow-marine facies (Crimes et al., 1977; Kowalski, 1987; Goldring and Jensen, 1996; Buatois and Ma´ngano, 2004). In deep-marine environments,

Fig. 8. Trace fossils associated with Oldhamia in the San Antonio de los Cobres ichnofauna, Puncoviscana Formation, northwest Argentina. A. Poorly preserved arthropod trackway. B. Branching Cochlichnus. Field photos.

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Fig. 9. Biomat structures from the Puncoviscana Formation, San Antonio de los Cobres, Argentina. A. Chloeophycus. B. cf. Kinneyia. Field photos.

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however, Oldhamia is always the dominant ichnotaxon and occurs in relatively low-diversity assemblages, in contrast to shallow-marine settings where it is only an accessory element of otherwise diverse ichnocoenoses (Buatois and Ma´ngano, 2003b, 2004). Within the Puncoviscana Formation, the Oldhamia-dominated association is in sharp contrast with the ichnofaunas of the Guachos Formation in the Sierra de Mojotoro area of Salta Province. This facies, which can be conveniently studied in the Los Guachos quarry or on sidewalks in the city of Salta, displays a higher diversity of trace fossils and more varied ethologic patterns (see Appendix and Fig. 10). The Guachos facies consists of centimetric flagstones of very fine-grained siltstone, which are massive or laminated and continue laterally over large distances. Their sharp erosional bases, graded bedding, and rhythmic interbedding with mudstone layers suggest sedimentation from distal turbidity currents. However, some of these beds contain combined-flow ripples and ripple cross-lamination,

Fig. 10. The Guachos ichnofauna, Guachos Formation, northwest Argentina.

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suggesting deposition near to the slope break, rather than in deep basinal settings (see also Omarini et al., 1999). 3.3. Concluding remarks on Ediacaran and Early Cambrian deep-marine ichnodiversity On deep sea bottoms, benthic faunas were present since the Ediacaran (Narbonne and Aitken, 1990; MacNaughton et al., 2000; Crimes, 2001; Orr, 2001). In Ediacaran times, ichnofaunas were as yet very simple. Cambrian deep sea ichnocoenoses are richer, but they still differ from their younger equivalents by being less diverse, lacking the typical elements of the Nereites ichnofacies, and containing instead shallow-marine elements, such as arthropod trackways (Orr, 2001). Ecologically, undermat miners continue to dominate ichnocoenoses into the Early Cambrian, with Oldhamia experiencing a remarkable behavioral diversification. Regardless the taxonomic relationships of their makers, Oldhamia ichnospecies represent an ecologic guild that became rare after the Cambrian agronomic revolution, particularly in shallow-marine environments (Seilacher and Pflu¨ger, 1994). In this sense, the Oldhamia assemblage represents a Proterozoic bhangoverQ in the deep sea (Buatois and Ma´ngano, 2003b). There is little doubt that the Oldhamia ichnospecies stand for animals that differed at least at the species level. Behavioral programs are too distinct to be environmentally induced. Furthermore, different ichnospecies are only very rarely associated on the same bedding plane. In the Blow Me Down Brook Formation of western Newfoundland, different Oldhamia ichnospecies occur at different stratigraphic levels (Lindholm and Casey, 1990). They can potentially be used for biostratigraphic zonation.

4. Arrival of the Cambrian explosion at the deep sea bottom ddCambrian explosionTT means many things: (1) the sudden radiation of metazoan phyla, following the extinction (Vendobionts) or the retreat to the deep sea environment (Xenophyophoreans) of the giant protozoans that had been dominating shallow-marine benthic biota in Ediacaran times (Seilacher et al.,

2003); (2) the emergence of mineralized skeletons in unrelated animal phylla, and thereby (3) the beginning of the ecological arm’s race and the increasing complexity of trophic chains. For the ichnologist, however, the most relevant change was the bioturbational destruction of resistant microbial mats that affected all benthic ecosystems except the most hostile environments. Responsible for this turnover (bagronomic revolutionQ of Seilacher and Pflu¨ger, 1994; bCambrian substrate revolutionQ of Bottjer et al., 2000) were the many kinds of infaunal animals that penetrated deeper into the sediment and left distinctive trace fossils within the sediment or along lithologic interfaces. Equally, or even more important, was the activity of interstitial animals. While being too small (Waloszek, 2003) to leave behind recognizable trace fossils, they transformed the uppermost millimeters of the sediment from a resistant biomat into a soft and fluffy mixed layer. While being no more suitable for the attachment of sessile organisms, such substrates facilitated diffusional exchanges between pore and sea water. The fact that the evolutionary burst affected trace fossils as much as body fossils puts an end to the notion that the Cambrian explosion only results from the higher fossilization potential of mineralized skeletons. On the other hand, trace fossils can also be used to track the proliferation of this event into deepsea environments, where mineralized skeletons are less likely to be preserved. While the agronomic revolution transformed shallow-marine environments already in the Tommotian– Atdabanian, the precise timing of this event in the deep sea is still uncertain. As previously discussed, the Oldhamia ichnoguild was still rampant in the deep sea during the Early Cambrian. It also seems to have persisted into the Middle Cambrian, as suggested by its presence in the uppermost strata of the Grand Land Formation of arctic Canada (Hofmann et al., 1994) and in the Kerrn Nesrani Formation of Morocco (El Hassani and Willefert, 1990). More importantly, Crimes (1976) documented ichnofaunas from the Lower to Middle Cambrian Bray Group of Ireland, where shallow-marine strata that already contain abundant vertical burrows (Skolithos, Arenicolites) are followed by deep-marine deposits with Oldhamia.

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Available data suggest that increasing predator pressure and competition for ecospace and/or resources in shallow-marine ecosystems triggered emigrations into deeper settings by the end of the Cambrian (Crimes, 2001; Orr, 2001). The main lineages of deep-marine trace fossils were established in the deep sea by the Early Ordovician (Seilacher, 1963). In particular, Crimes et al. (1992) documented a moderately diverse ichnofauna in Arenigian turbidites of the Ribband Group in southeastern Ireland. It includes graphoglyptid networks, radial traces, meandering trails and back-filled structures. More recently, a deep-marine ichnofauna of Late Tremadocian age has been recorded in the Puna area of Argentina (Benedetto et al., 2002; Ma´ngano and Buatois, 2003). This suggests that a deep-marine ecosystem of modern aspect (Nereites ichnofacies) existed at least since the Early Ordovician (Orr, 2001; Ma´ngano and Droser, 2004). Intensified bioturbation at all scales put not only an end to an ecology based on microbial matgrounds. It also led to the closure of the so-called bdeep-water slope-basin taphonomic windowQ through destruction of non-biomineralized tissues, which had been commonly preserved in Cambrian deep-marine lagersta¨tten (Orr et al., 2003). Intense bioturbation by a mobile infauna resulted in a series of changes, most notably enhanced microbial degradation, chemosymbiosis, increased substrate permeability, and the disruption of geochemical gradients conducive to mineral authigenesis (McIlroy and Logan, 1999; Orr et al., 2003). Further search in Upper Cambrian turbidites will be necessary to exactly date the advent of the agronomic revolution on deep sea bottoms.

5. Conclusions 1. Ediacaran ichnodiversity is much lower than it appears from the literature. Also, trace fossils are less common in Ediacaran strata than in younger rocks and behavioral complexity is still very limited. Furthermore, arthropod tracks or trails and sinusoidal nematode trails are conspicuously absent. This situation changed dramatically in the Early Cambrian, where not only the number of recognizable ichnotaxa, but also their complexity reached much higher levels, particularly in shallow-marine environments. In other words, the shallow-marine trace

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fossil record shows a similarly explosive diversification as skeletal body fossils. Thus, the bCambrian explosionQ is not a preservational artifact. 2. On deep sea bottoms, benthic biotas were already present in the Ediacaran, but Ediacaran deep-marine ichnofaunas were as yet extremely simple. As exemplified by the behavioral diversification of Oldhamia, they also diversified in the Early Cambrian, although the dominant life style (undermat mining) was more reminiscent of Ediacaran times (Seilacher, 1999). This means that the Cambrian explosion and the agronomic revolution did not coincide (Ma´ngano and Buatois, 2004b). 3. While the agronomic revolution transformed the ecology of shallow-marine environments by the Early Cambrian, it is still uncertain when exactly this event arrived in the deep sea. By Early Ordovician times, the main lineages of deep-marine trace fossils were already present (Nereites ichnofacies). Accordingly, the ichnofauna of the Guachos Formation may represent a first step in this onshore/offshore expansion.

Acknowledgments We thank Jim Gehling, Richard Jenkins, So¨ren Jensen and Guy Narbonne for valuable discussions. Palaeo-3 reviewers Alfred Uchman and So¨ren Jensen and editor Finn Surlyk provided useful comments. Ricardo Alonso and Cristina Moya showed the authors outcrops near the city of Salta. Ignacio Sabino helped Buatois during fieldwork in Quebrada del Toro. We also thank Cope MacClintock for his assistance at the Yale Peabody Museum, New Haven, Connecticut, USA and Florencio Acen˜olaza, who provided access to the collections at the Miguel Lillo Institute, San Miguel de Tucuma´n, Argentina. Financial support was provided by the Antorchas Foundation (Buatois) and the Research Council of the University of Tucuman (Ma´ngano). During the last phase of the study, financial support was provided by the University of Saskatchewan Start-up funds (Buatois) and a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant 311726-05 awarded to Buatois. as well as by a NSERC Discovery Grant 311727-05 awarded to Ma´ngano.

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Appendix A. Taxonomic In this Section we provide descriptions and interpretations of selected trace fossils from the Albemarle Group of North Carolina, the Grand Pitch Formation of Maine, the Besonderheid Formation of South Africa and the Puncoviscana Formation of northwest Argentina, including its coeval units, the Suncho Formation of Catamarca Province and the Guachos Formation outcropping near the city of Salta. Trace fossils are listed alphabetically. Figs. 11–23 ?Curvolithus isp. (Fig. 11A) Unit: Guachos Formation. Localities: Los Guachos quarry (Salta Province, Argentina). Remarks: The name Curvolithus stands for continuous burrows that have a flatly elliptical cross section and appear trilobed in upper surface views (Buatois et al., 1998b). They were probably made by flatworms that burrowed by removing sediment in front of the head and conveying it backwards along the body (Seilacher, 1990). This is a technique used by various kinds of infaunal bulldozers. In Curvolithus, however, there is not only a terminal backfill formed behind the body; in addition, sediment was redeposited laterally along the right and left margins of the animal; i.e. in

front of the terminal backfill (Heinberg, 1973). The combination of the two backfill structures makes the trace fossil appear trilobed. Specimens from Los Guachos (Fig. 11A) are not as well preserved as the ones from younger shallow-marine sandstones and their three-dimensional morphology is uncertain. Accordingly, they are assigned to Curvolithus with doubts. This ichnogenus is also known in Lower Cambrian shallow-marine deposits of Canada (Narbonne et al., 1987; Narbonne and Myrow, 1988). Curvolithus has also been mentioned in possibly Ediacaran rocks of Australia (Webby, 1970). cf. Heliochone isp. Seilacher and Hemleben (1966) (Fig. 11B) Unit: Guachos Formation. Localities: Los Guachos quarry, now on Salta city sidewalk (Salta Province, Argentina). Remarks: This large trace fossil consists of a continuous ring that is surrounded at some distance by a concentric ring of round dots. It can only be understood by comparison with a still larger trace fossil from the Lower Devonian Hunsru¨ck Shales of Germany (Seilacher and Hemleben, 1966). In the latter case, serial sectioning revealed that the basic structure was a ring-shaped tunnel with equidistant vertical outlets. As the inhabitant widened and lowered this tunnel system, it produced a conical backfill with radial

Fig. 11. Trace fossils from the Guachos Formation, northwest Argentina. Salta sidewalks. A. ?Curvolithus isp. B. cf. Helichone isp. C. cf. Treptichnus pedum.

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Fig. 12. Ichnospecies of Oldhamia. A. Oldhamia alata. Upper specimen is the holotype. MPEF IC-424 (Paleontological Museum Egidio Feruglio, Argentina). Puncoviscana Formation, northwest Argentina. B. Oldhamia antiqua. Grand Pitch Formation, Maine. C. Oldhamia flabellata. Puncoviscana Formation, northwest Argentina. D. Oldhamia geniculata. Holotype. Field photo. Puncoviscana Formation, northwest Argentina.

walls left by the outlets. The radial backfills are not seen in the Puncoviscana specimen. Accordingly, this specimen is only compared with Heliochone. Oldhamia alata isp. n. (Figs. 12A, 13) 2004 Oldhamia n. isp. Buatois and Ma´ngano, Fig. 2G Holotype: MPEF IC-424 (Paleontological Museum Egidio Feruglio, Argentina), El Mollar (Quebrada del Toro) (Salta Province, Argentina), Punoviscana Formation. Unit: Puncoviscana Formation. Localities: Rio Capillas and El Mollar (Quebrada del Toro) (Salta Province, Argentina).

Diagnosis: Relatively complex Oldhamia with thin, unbranched probes which contour previous ones as in a Lophoctenium-like spreite. Alternating wings are usually centrifugal and only occasionally centripetal, but successive probes always proceed to the convex side. Lobation within wings is common. Description: Complex systems oriented parallel to the bedding plane formed by closely spaced probings resembling a small Lophoctenium structure. Probings always proceed to the convex side commonly starting at the center and protruding outward. However, a centripetal pattern has been detected in the construction of some wings (Fig. 7). Opposite, 1808 symmetrical wings are occasionally observed,

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Fig. 13. Variations of Oldhamia alata. Puncoviscana Formation, northwest Argentina. Based on specimens housed at the Paleontological Museum Egidio Feruglio (Trelew, Argentina).

but more complex asymmetrical systems in which successive wings alternate along an imaginary axis are more common (Fig. 13). Successive wings do not cross each other. Some specimens consist of up to eight asymmetric wings forming complex cumulative structures. Lobation within wings reflected by abrupt changes in the length of probes is another well developed feature, with two to four lobes in each wing. Remarks: This ichnospecies was informally referred to as ba new ichnospecies of OldhamiaQ by Buatois and Ma´ngano (2004). As in all ichnospecies of Oldhamia, the probings end blindly. However, in O. alata they not only bend back on previous ones, but follow them so closely that the whole structure looks like the spreite of a minute Lophoctenium. This probably means that the backfill is not terminal, but that processed sediment was continuously pushed to one side (i.e. towards the previous probing) as the animal ate its way from the center to the periphery. The probings are usually protruding outward in a centrifugal fashion from a central point or imaginary axis. The second wing in the specimen illustrated in Fig. 7, however, protruded towards the center and direction changed by 1808 in the last wing. This means that the direction of strip mining was

determined by the curvature of the first probe rather than a fixed program. In all cases the spreite grew on the convex side. As in many feeding burrows, the development of subsequent wings was also constrained by a taboo against overcrossing previous structures. Lobation within the wings reflects yet another fabricational constraint. Normally the probes end in harmony with the previous ones, so that the tips form a smoothly curved margin. However, from time to time the length of the probes exceeded a certain limit that was possibly related to the length of the worm-like tracemaker. The response to this dilemma was always the same: start a new lobe with shorter probes, but without loosing contact with the previous ones. Contact was only given up when the available mat area had been filled. Therefore, the animal had to produce a new exploratory probe followed by the construction of a new wing-shaped spreite further ahead on the opposite side. Oldhamia alata records a highly specialized Oldhamia that efficiently explored the microbial mat. The fact that there is no unexplored sediment between successive probes reflects an improved feeding program compared to other Oldhamia ichnospecies. Oldhamia alata somewhat resembles some of the

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Fig. 14. Variations of Oldhamia antiqua. Antarctic specimens collected by Buggisch. Maine specimens housed at the Geological and Paleontological Institute of the University of Tubingen (Germany).

asymmetrical morphotypes of O. curvata. However, in O. alata probes stay in closer contact with previous ones and the wings are far more complex and commonly alternate. Lobation within wings records an additional fabricational constrain that is typical for O. alata. Oldhamia antiqua Kinahan, 1858 (Figs. 12B, 14, 15) *1858 Oldhamia antiqua Kinahan, p. 69 1895 Oldhamia (Murchisonites) occidens Walcott, pp. 314–315, Fig. 1. 1904 Oldhamia (Murchisonites) occidens Dale (1904), p. 13, Fig. 1 1929 Oldhamia occidens Ruedemann, pp. 47–55, Figs. 28–29 1942 Oldhamia occidens Ruedemann, p. 7, Figs. 1.5, 2.1, 2.2 and 3.4 1942 Oldhamia smithi Ruedemann, p. 10, Fig. 3.5 1962 Oldhamia smithi Neuman, Fig. 2

1990 Oldhamia kernnesraniensis El Hassani and Willefert, pp. 234–235, Figs. 1.1–1.4 and 1.7–1.8. 1990 Oldhamia flabellata El Hassani and Willefert, pp. 234, Figs. 1.5–1.6 1994 Oldhamia cf. antiqua Buggisch et al., pp. 22, Fig. 20b Unit: Puncoviscana Formation (Argentina) and Grand Pitch Formation (Maine). Localities: San Antonio de los Cobres and Quebrada de San Rafael (Cordo´n de Cobres) (Salta Province, Argentina), and Grand Pitch Falls (Maine, United States). Emended Diagnosis: Oldhamia in which diverging curved to straight probings display a palm leaf arrangement or resemble rays of fireworks. Successive probings are separated by narrow strips of unexplored sediment. Remarks: Oldhamia antiqua is the type ichnospecies of Oldhamia. In the Puncoviscana Formation it was documented for the first time by Acen˜olaza and Durand (1982) and has been reanalyzed by Buatois and Ma´n-

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Fig. 15. Large slab containing several specimens of Oldhamia antiqua. Several of these specimens are illustrated in Fig. 14. Grand Pitch Formation, Maine (USA). Cast of Tubingen specimen housed at Yale Peabody Museum (United States), YPM 204587.

from a short stem and curve slightly back at the sides like the leaves of a palm tree or the rays of fireworks. The zigzag arrangement of fans and the axial connecting stem, observed in the type material, are not diagnostic elements of O. antiqua. In fact, they are relatively uncommon in other described occurrences of this ichnospecies. Oldhamia antiqua reflects a search behavior found in many kinds of feeding traces (e.g., Chondrites, Arthrophycus, Phycodes) that approach the nutritious layer from above or below at an oblique angle, but in Oldhamia probings are all in a single bedding plane. If the makers of Oldhamia were undermat miners, we may assume that the stem part opened to the surface and the probings spread below the active biomats, where older mat zones were degraded enough to be readily digestible (Seilacher, 1999). In the construction of each leaf-like structure, probings presumably started from the axis with new ones added on the sides. Some specimens display serial arrangements of such fans (Fig. 7); the same individual could make many such patterns in succession. Other specimens show two stemless fans pointing in opposite directions, indicating a vertical shaft in the center.

gano (2003b). In addition to its occurrence in Argentina and in its type area in Ireland (Forbes, 1849; Kinahan, 1858, 1859; Murchison, 1859; Sollas, 1900; Dhonau and Holland, 1974; Crimes and Crossley, 1968; Crimes, 1976; Holland, 2001), it has been recorded from Lower Cambrian to, very rarely, lower Middle Cambrian strata in Poland (Kowalski, 1987), Belgium (Malaise, 1883; Verniers et al., 2001), Canada (Lindholm and Casey, 1990; Hofmann et al., 1994), United States (Walcott, 1895; Ruedemann, 1929, 1942; Neuman, 1962), Morocco (El Hassani and Willefert, 1990), and Antarctica (Buggisch et al., 1994). With the exception of the Moroccan and Polish occurrences, it is present in folded and otherwise nonfossiliferous deep-marine successions. Oldhamia (Murchisonites) occidens Walcott is a junior synonym of O. antiqua (Lindholm and Casey, 1990). Oldhamia kernnesraniensis El Hassani and Willefert and Oldhamia smithi Ruedemann are also regarded as junior synonyms of O. antiqua. Oldhamia antiqua consists of straight to curved probings in a fan-like arrangement. Tunnels radiate

Oldhamia curvata Lindholm and Casey, 1990 (Fig. 16) 1965 Oldhamia sp. Churkin and Brabb, Fig. 4 1972 Oldhamia sp. Mirre´ and Acen˜olaza (1972), pp. 75–76, Fig. ac 1973 Oldhamia radiata Acen˜olaza and Durand, Figs. 1B, E 1978 Oldhamia radiata Acen˜olaza (1978), p. 28, Fig. 13 1981 Oldhamia radiata Acen˜olaza and Toselli (1981), p. 51, Figs. 2, 3 1982 Oldhamia antiqua Acen˜olaza and Durand, p. 709, Fig. 8 1984 Oldhamia antiqua Acen˜olaza and Durand (1984), Figs. 1D, 2D 1986 Oldhamia Acen˜olaza and Durand (1986), Fig. 3J 1987 Oldhamia antiqua Acen˜olaza and Durand (1987), pl. 1, Fig. F 1989 Oldhamia antiqua Lindholm and Casey (1989), p. 6, Fig. 3C, D

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Fig. 16. Variations of Oldhamia curvata. Based on specimens illustrated by Lindholm and Casey (1990), Sweet and Narbonne (1993), and Hofmann et al. (1994).

*1990 Oldhamia curvata Lindholm and Casey, pp. 1276–1278, Fig. 8D–G 1990 Oldhamia antiqua Durand and Acen˜olaza, p. 94, Figs. 3.43.5 1993 Oldhamia smithi Sweet and Narbonne, p. 71, Fig. 3a 1993 Oldhamia antigua (lapsus calami) Durand (1993) , pl. 1, Fig. H 1996 Oldhamia antiqua Durand (1996), pl. 1, Fig. D 1999a Oldhamia antiqua Acen˜olaza et al., Fig. 9C 1999b Oldhamia radiata Acen¨olaza et al. (1999b), pl. 2, Fig. 6 Unit: Suncho Formation. Localities: Sierra de la Ovejerı´a (Catamarca Province, Argentina) (see Acen˜olaza and Durand, 1982, for locality details). Emended Diagnosis: Oldhamia with thin, uniformly curved, unbranched probings, commonly forming almost symmetrical fan structures and leaving a gap in the center. Probings commonly regularly spaced,

maintaining the curvature along their trajectory. Asymmetric forms rare. Remarks: This ichnospecies has been defined by Lindholm and Casey (1990) based on specimens from the Early Cambrian of Newfoundland. Subsequently, Sweet and Narbonne (1993) and Hofmann et al. (1994) documented the same ichnospecies from Que´bec and Arctic Canada, respectively. In addition, Hofmann et al. (1994) noted that some of the specimens referred to as O. radiata and O. antiqua from the Puncoviscana Formation in previous studies (Acen˜olaza and Durand, 1973, 1982) should be included in O. curvata (see also Buatois and Ma´ngano, 2003a) and that Oldhamia isp. described from Alaska by Churkin and Brabb (1965) should also be placed in O. curvata. Oldhamia curvata is characterized by its thin and uniformly curved rays, commonly displaying regular spacing (Lindholm and Casey, 1990). The central gap and the curvature suggest that the probings were made in a centripetal succession.

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Fig. 17. Variations of Olhamia flabellata. Puncoviscana Formation, northwest Argentina. Based mostly on specimens housed at the Miguel Lillo Institute (San Miguel de Tucuma´n, Argentina).

Oldhamia flabellata Acen˜olaza and Durand, 1973 (Figs. 12C, 17) ?1859 Oldhamia antiqua Kinahan, partim, Fig. 4, 9. *1973 Oldhamia flabellata Acen˜olaza and Durand, p. 49, pl. 1, Figs. C, D, F 1981 Oldhamia radiata Hofmann and Cecile, Fig. 40.3F 1981 Oldhamia? sp. Hofmann and Cecile, p. 281, Fig. 40.3C Unit: Puncoviscana Formation. Localities: San Antonio de los Cobres (Salta Province, Argentina). Emended Diagnosis: Oldhamia with irregular, sinuous, occasionally discontinuous probings closely packed to form a leaf-line structure. Probings were made in a centrifugal succession to both sides. Remarks: Oldhamia flabellata was originally described from northwest Argentina by Acen˜olaza and Durand (1973) and is also known from Canada (Lindholm and Casey, 1990; Hofmann et al., 1994). Specimens from Morocco included in O. flabellata (El

Hassani and Willefert, 1990) more likely belong to O. antiqua. Oldhamia flabellata comprises irregular, rectilinear, irregular and somewhat discontinuous tunnels. Commonly, the point of origin of the tunnel system is not apparent. While being one-sided like O. antiqua, this ichnospecies differs by the more irregular course of its probings and their tendency to hug previous ones. By adding new probings on either side of the midline, the patterns become leaf-shaped and cover efficiently a given surface, being optimal in feeding strategy. Serial and opposite arrangements may also occur. Oldhamia geniculata isp. n. (Figs. 12D, 18) 1997 Oldhamia recurvata Seilacher, p. 29 (nomen nullum) 1998 Oldhamia Almond, cover photo and Figs. unnumbered Holotype: MPEF-IC 431 (Paleontological Museum Egidio Feruglio, Argentina), Los Chorrillos (Queb-

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Fig. 18. Variations of Oldhamia geniculata. Puncoviscana Formation, northwest Argentina. Based on the holotype (Fig. 18b, MPEF-IC 431) housed at the Paleontological Museum Egidio Feruglio (Trelew, Argentina) and field specimens.

rada del Toro) (Salta Province, Argentina), Punoviscana Formation. Units: Puncoviscana Formation (Argentina) and Besonderheid Formation (South Africa). Localities: Los Chorrillos (Quebrada del Toro) (Salta Province, Argentina) and Western and Northern Cape Provinces (South Africa) (see Almond, 1998, for South African locality details). Diagnosis: Relatively large ichnospecies of Oldhamia, in which probings radiate in a centrifugal sequence and turn the tips back 1808 on the free side to fill the sectors in between, forming a hook-like structure. Instead of a fixed point of central branching, there is a curved spreite-like structure in the center. Description: Relatively large systems oriented parallel to the bedding plane, in which branches radiate in a centrifugal sequence and turn the tips back 1808 on the free side. A curved spreite-like structure in the center occurs rather than a fixed point of branching. Remarks: This new ichnospecies was informally introduced in a drawing as Oldhamia recurvata (Seilacher, 1997) and is formally defined herein as

Oldhamia geniculata in order to avoid confusion with O. curvata. Oldhamia geniculata, so far only known from South Africa (Almond, 1998) and northwest Argentina, met the requirement of filling radial sectors without branching: a certain distance from the main shaft, each probe turns by 1808 on the free side and stops at about 1 / 2 to 1 / 3 of the way back. Strangely, the hook is always made in foresight on the advancing side of the probe series, rather than backwards, where guidance would have been provided by the previous probe. The direction of the swing can be derived from the kinks in loops 5 and 6 of Fig. 18; they show that there was a near-collision with the hook of the previous probe. Another striking feature, namely the segmentation of individual branches, is a tectonic artifact. It occurs only in probes running at approximately right angles to a faint schistosity. The segmentation is therefore a product of tectonic stress that deformed the rock together with the trace fossils and broke up the tube fillings at regular intervals. Their higher competence may have been due to an original mucus content or

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Fig. 19. Oldhamia recta. Holotype. YPM.204453 (Yale Peabody Museum). Floyd Church Member, McManus Formation (Albermarle Group), Saint Martin quarry, Carolina Slate Belt (North Carolina, United States).

early diagenetic cementation. In any case it proves that the tunnels were actively backfilled and therefore behaved unlike the matrix. Recognition of the stress direction also allows to retrodeform the feeding patterns to their original shapes (Fig. 18). Oldhamia radiata Kinahan, 1858 Unit: Puncoviscana and Suncho Formations. Localities: San Antonio de los Cobres and Cuesta Mun˜ano (Salta Province, Argentina) and Sierra de la Ovejerı´a (Catamarca Province, Argentina). Emended Diagnosis: Oldhamia with probings that branch and radiate to all sides from a central point in a Chondrites-like fashion, but except from the central area they all remain in a single bedding plane.

Remarks: In the Puncoviscana Formation, Oldhamia radiata was documented for the first time by Acen˜olaza and Durand (1973) and has been recently reanalyzed by Buatois and Ma´ngano (2003b). In addition to its occurrence in Argentina and in its type area in Ireland (Forbes, 1849; Kinahan, 1858, 1859; Sollas, 1900; Dhonau and Holland, 1974; Crimes and Crossley, 1968; Crimes, 1976; Holland, 2001), Oldhamia radiata is also known from Spain (Crimes et al., 1977), Canada (Lindholm and Casey, 1990; Hofmann et al., 1994), Mongolia (Goldring and Jensen, 1996) and Antarctica (Buggisch et al., 1994). With the exception of the occurrences from Mongolia and Spain, it is invariably present in deepmarine deposits. Oldhamia radiata consists of rectilinear to slightly curved rays forming a radial pattern from a central area of origin. This ichnospecies reaches even coverage by means of making probes radiate to all sides of a vertical shaft and filling the sectors between them by branching. This principle is also used by plant roots and the trace fossil Chondrites. Yet the present ichnospecies should better be affiliated with Oldhamia because it spreads in one plane and is similar in fine morphology, size and preservation. On the other hand, branching required a change of burrowing behavior: probes could not be entirely backfilled with processed or introduced sediment until all branches had been finished. Oldhamia recta isp. n. (Figs. 19, 20) 1989 Syringomorpha sp. Gibson, p. 5, Figs. 4.3–4.4 1992 Oldhamia cf. flabellata Seilacher and Pflu¨ger, Fig. 1 1997 Oldhamia recta Seilacher, p. 29. 1999 Oldhamia simplex Omarini et al., p. 89 (nomen nullum) Holotype: YPM.204453 (Yale Peabody Museum), Saint Martin quarry, Carolina Slate Belt (North Carolina, United States), Floyd Church Member, McManus Formation (Albermarle Group). Unit: Floyd Church Member of the McManus Formation (Albermarle Group). Localities: various localities in the Carolina Slate Belt (North Carolina, United States) (see Gibson, 1989, for locality details).

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Fig. 20. Variations of Oldhamia recta. Floyd Church Member, McManus Formation (Albermarle Group), North Carolina, United States. Based on specimens housed at the Yale Peabody Museum (United States).

Diagnosis: Oldhamia with straight backfilled tunnels parallel to the bedding plane that are arranged in non-intercutting and seemingly unconnected bundles. Description: Bundles of straight, subparallel, unbranched and closely spaced tunnels oriented parallel to the bedding plane. Tunnels are a few centimeters long and up to 1.5 mm in diameter. Preserved as positive as well as negative reliefs on the same bedding plane. Remarks: These tunnel systems were formerly assigned to Syringomorpha by Gibson (1989). However, they were subsequently placed in Oldhamia by Seilacher and Pflu¨ger (1992) based on the fact that the tunnels were confined to the bedding plane instead of being vertical as in Syringomorpha (see also Jensen, 1997). The tunnels must have been actively backfilled, because they are preserved either as positive or as negative hyporeliefs. Therefore these structures made by worm-like undermat miners can be affiliated with the ichnogenus Oldhamia, which evolved much more

sophisticated search behaviors in the lowermost Cambrian (Fig. 7). The pattern shows that the tracemaker had a well developed central nervous system that allowed it to sense the proximity of previous probings. Psammichnites saltensis (Acen˜olaza and Durand, 1973) (Figs. 21, 22A, B, D) *1973 Nereites saltensis Acen˜olaza and Durand, p. 49–50, Fig. 2A 1974 Nereites Seilacher, Fig. 2 (Cambrian example) 1977 Nereites Seilacher, Fig. 4 (Cambrian example) 1978 Nereites saltensis Acen˜olaza (1978), p. 28. Fig. 12 1986 Nereites saltensis Acen˜olaza and Durand (1986), Fig. 3.F 1987 Nereites saltensis Acen˜olaza and Durand (1987), pl. 1, Fig. B 1990 Nereites saltensis Durand and Acen˜olaza, p. 92–92, Fig. 3.1

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Fig. 21. Variations of Psammichnites saltensis. Note size differences. Guachos Formation, northwest Argentina. Based on material studied in the field.

1993 Nereites saltenis (lapsus calami) Durand (1993), pl. 1, Fig. F 1994 Helminthorhaphe isp. Hofmann et al., p. 773, Fig. 3A, C 1994 Nereites saltensis Durand (1994), pl. 1, Fig. F 1996 Nereites saltensis Durand (1996), pl. 1, Fig. H 1999a Nereites saltensis Acen˜olaza et al., p. 103, Fig. 2F, I 1999b Nereites saltensis Acen˜olaza et al., pl. 2, Fig. 2 2001 Nereites saltensis Acen˜olaza and Alonso, Fig. 3.2 2004 Guided meandering trace fossil Buatois and Ma´ngano, Fig. 2D Unit: Guachos and Puncoviscana Formation. Localities: Los Guachos quarry, Cachi, Rancagua, Rı´o Capillas, and Campo Quijano, (Salta Province, Argentina). Remarks: Because of its bilobed profile and regular meandering, this Puncoviscana trace fossil has originally been affiliated with Nereites (Acen˜olaza and Durand, 1973; Seilacher, 1974). It was only through the specimen shown in Figs. 21 and 22D that this

assignment could be corrected. This unique slab is interesting in various respects: (1) there is an arthropod trackway with oblique series of appendage imprints (Diplichnites); (2) the impressions are not blurred, so they were not made on a biomat; (3) because the appendage imprints stick out as ridges, we deal with a sole face; and (4) in crossing a large meandering trail, the trackway contours its negative profile, but is neither obliterated, nor does it change its course. This is surprising, because arthropod trackways, even if preserved as undertracks, belong to a shallower tier and were therefore made earlier than the meandering traces. The contradiction resolves if one takes the soft margins of the trace in Fig. 21 into account: this bedding plane was not touched by the meandering Psammichnites animal, but simply lifted up, as the hydrostatic creature wedged itself along at a deeper level. This is why the pre-existing arthropod trackway did not become wiped out. However, the Psammichnites overtrace would have collapsed again, had the tunnel not

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Fig. 22. Grazing traces and arthropod trackways from the Guachos Formation, northwest Argentina. A. Psammichnites saltensis. Enhanced with chalk on Salta sidewalk. B. Psammichnites saltensis. Cast at the Geological and Paleontological Institute of the University of Tubingen. C. Psammichnites cf. gigas. Field photograph. Preserved as hyporelief. D. Psammichnites saltensis deforming Diplichnites isp. Field photograph. E. Tasmanadia cachii. Field photograph.

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Fig. 23. Variations of Tasmania cachii. Guachos Formation, northwest Argentina. Specimens studied in the field.

been backfilled with extra sediment probably introduced from the surface (Seilacher-Drexler and Seilacher, 1999). As the backfill of Nereites consists always of sediment scraped away in front of the head, it is concluded that the present trace should better be affiliated with Psammichnites. Meandering is not uncommon in Psammichnites. Examples are known from Lower Cambrian and Carboniferous shallow-marine facies (Ma´ngano et al., 2002). However, none of them has the kink in each loop, by which the maker of P. saltensis appears to have assured itself that it was close to the previous turn. Because the kink also triggered the next turn, it was performed even if the contact with the previous turn had failed (Seilacher, 1986). This behavioral program remained the same through all growth stages (Fig. 21), but as it avoided overcrossing, it was more efficient than the looping behaviour of the contemporaneous P. gigas (Torell).

Psammichnites cf. gigas (Torell, 1868) (Fig. 22C) 2001 Nereites saltensis Acen˜olaza and Alonso, Fig. 2.3 Unit: Guachos Formation. Localities: Los Guachos quarry (Salta Province, Argentina). Remarks: One of the most impressive Lower Cambrian trace fossils is the lasso trail Psammichnites gigas. It is known from lowermost Cambrian shallow-marine sandstones in Sweden (Torell, 1868, 1870), Canada (Hofmann and Patel, 1989), Australia (Walter et al., 1989), Greenland (Pickerill and Peel, 1990), Spain (Seilacher and Ga´mez-Vintaned, 1995, 1996), Zhu ´ lvaro and Vizcaı¨no, 1999), United (1997), France (A States (Jensen et al., 2002), Iran (Seilacher, unpublished data), and Sardinia (Fro¨hler, 1994). Considering the kind of backfill, the transversal sculpture on the ventral, and a conspicuous sinusoidal line on the

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dorsal side, P. gigas has been interpreted as the work of a slug-like animal that bulldozed inside the sediment and collected food from the surface with a pendulating siphon (Seilacher-Drexler and Seilacher, 1999). None of these details can be observed in the Guachos specimen (Fig. 22C). Yet, the lasso-like loops and relatively large size leave little doubt that we deal with a similar behavioral program as performed by the P. gigas organism. Tasmanadia cachii Durand and Acen˜olaza, 1990 (Figs. 22E, 23) Unit: Guachos and Puncoviscana Formation. Localities: Los Guachos quarry and Cachi (Salta Province, Argentina). Remarks: In addition to ordinary arthropod undertracks (Diplichnites), the Guachos facies contains abundant trackways that are difficult to interpret in terms of arthropod locomotion. In trilobite trackways, one can distinguish series of appendage impressions that become more obvious if the animal moved at a slight angle to its body axis. Each of these series corresponds to one wave of activation passing along the body from the rear to the front legs. These metachronal series must overlap, because each wave displaces the body only by a fraction of its length. In Tasmanadia cachii, subsequent series of imprints do not overlap. Instead, they form individualized patterns, whose bracket shapes probably correspond to the general outline of the tracemaker. This means that the animal was not continuously supported; rather it must have moved in jumps, driven by the simultaneous action of all appendages. In agreement with this model, subsequent patterns may be out of line, because the animal was displaced during the jump by a lateral current (Fig. 23). But what kind of an animal produced these tracks? It got about 7 cm long, had an oval outline and 10 pairs of appendages. Not all of these must have left an impression, because smaller legs near the front or rear ends, which cannot readily be distinguished, because there are no pushback heaps, may not have penetrated to the interface, on which the undertrack is preserved. So some kind of arthropods would be a possibility. True legs, however, touch and scratch the substrate with their tips, while each appendage imprint of T. cachii consists of 56 small bifid impressions in a radiating row. Another possibility is a polychaete

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worm, similar to modern Aphrodite, that had setate parapodia on each body segment. Or could there be an arthropod in Burgess-type biota that fits our ichnological profile? cf. Treptichnus pedum (Seilacher, 1955) (Fig. 11C) Unit: Guachos Formation. Localities: Los Guachos quarry (Salta Province, Argentina). Remarks: This structure, consisting of two series of knobs, might at the first sight be mistaken for an arthropod trackway. More likely we deal with outlets emerging from a horizontal tunnel in an alternating fashion. Similar, but smaller, burrow systems are known from shallow-marine Middle Cambrian sandstones of the Grand Canyon (Bicavichnites Lane et al., 2003). Like the latter, the Guachos specimen is here tentatively considered an epichnial expression of Treptichnus pedum. The multiple exits require a special explanation, because in an open tunnel system, active ventilation would have been easier in a U-shaped structure with only two openings to the surface. Most likely, the multiple exits served for passive ventilation and/or for trapping organic particles that drifted along the sediment surface. Alternatively, they could have become actively backfilled upon completion of the next exit. Treptichnus pedum was more three-dimensional than the burrows of typical undermat miners. Nevertheless, it could have exploited the same niche. References Acen˜olaza, F.G., 1978. El Paleozoico inferior de Argentina segu´n sus trazas fo´siles. Ameghiniana 15, 15 – 64. Acen˜olaza, F.G., Alonso, R.N., 2001. Icno-asociaciones de la transicio´n Preca´mbrico–Ca´mbrico en el noroeste de Argentina. Journal of Iberian Geology 27, 11 – 22. Acen˜olaza, F.G., Durand, F.R., 1973. Trazas fo´siles del basamento cristalino del noroeste argentino. Boletı´n de la Asociacio´n Geolo´gica de Co´rdoba 2, 45 – 55. Acen˜olaza, F.G., Durand, F.R., 1982. El icnoge´nero Oldhamia (traza fo´sil) en Argentina. Caracteres morfolo´gicos e importancia estratigra´fica en formaciones del Ca´mbrico inferior de Argentina. 5to Congreso Latinoamericano de Geologı´a (Buenos Aires). Actas 1, 705 – 720. Acen˜olaza, F.G., Durand, F.R., 1984. The trace fossil Oldhamia: its interpretation and occurrence in the Lower Cambrian of Argentina. Neues Jahrbuch fur Geologie und Pala¨ontologie, Monatshefte H 12, 728 – 740.

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