AN EARLY CAMBRIAN SHALLOW-MARINE ICHNOFAUNA FROM THEPUNCOVISCANA FORMATION OF NORTHWEST ARGENTINA: THE

INTERPLAY BETWEEN SOPHISTICATED FEEDING BEHAVIORS,MATGROUNDS AND SEA-LEVEL CHANGES

LUIS A. BUATOIS AND MARIA GABRIELA MANGANODepartment of Geological Sciences, University of Saskatchewan,

114 Science Place, Saskatoon, SK S7N 5E2, Canada, ,luis.buatois@usask.ca., ,gabriela.mangano@usask.ca.

ABSTRACT—An early Cambrian ichnofauna consisting of Helminthoidichnites tenuis, Helminthopsis tenuis, Multinaisp., Oldhamia alata, and Pilichnus cf. dichotomus is documented from shallow-marine deposits ranging from theupper offshore to the offshore transition in the Puncoviscana Formation of northwest Argentina. Although theichnogenus Oldhamia is more common in Cambrian deep-marine environments, this occurrence provides furtherevidence that it is also present in shallow-marine environments. The burrow network Multina (senior synonym ofOlenichnus) is preserved at the base of tempestites, representing the activity of post-storm colonizers. A drowningsurface separating offshore-transition deposits below from upper-offshore deposits above contains widespreadevidence of trace fossils in direct association with matgrounds. The undermat miners Oldhamia alata and Pilichnuscf. P. dichotomus occur on this surface, revealing exploitation of organic matter in the biomat. Low sediment rateduring drowning and paucity of bioturbation by sediment bulldozers may have promoted the establishment ofthe matground. In comparison with the simpler animal-matground interactions characteristic of the Ediacaran,the combination of Cambrian evolutionary innovations and the presence of microbial mats promoted moresophisticated interactions. Complex feeding trace fossils revealing that systematic undermat mining, as displayed byOldamia alata and Pilichnus cf. dichotomus, is a product of the Cambrian explosion.

INTRODUCTION

TRACE FOSSILS provide significant information to under-stand the timing and nature of the Cambrian explosion.

In addition to critically assessing ichnodiversity levels duringthe Cambrian, recent work has focused on understandingfeeding strategies recorded by trace fossils, particularly exploringthe association between biogenic structures and microbial mats(e.g., Seilacher, 1999; Buatois and Mangano, 2003a).

The Ediacaran–lower Cambrian Puncoviscana Formation ofnorthwestern Argentina hosts one of the most diverseichnofaunas for that critical interval (see review by Buatoisand Mangano, 2004). Traditionally considered as recordingsedimentation in deep-marine environments (Omarini andBaldis, 1984; Jezek, 1990; Acenolaza et al., 1999), this viewhas changed in recent years with the increasing realization thatsome Puncoviscana deposits accumulated in shallow-marineareas (Buatois and Mangano, 2003b, 2004). In this paper wedocument a new trace-fossil locality, referred to as El Mollar, inthe Quebrada del Toro, Salta Province, northwest Argentina(Fig. 1.1–1.3). Shallow-marine deposits at this locality contain avery well-preserved ichnofauna associated with structuresindicative of microbial mats, mostly preserved on a drowningsurface (i.e., flooding surface). The succession exposed issomewhat atypical of the Puncoviscana Formation because itis comparatively less deformed, allowing measuring of acontinuous section. In addition to the trace fossils on thedrowning surface, burrow networks are preserved on the baseof tempestites. The ichnotaxonomic status of burrow networksis still controversial, and therefore a discussion on networkichnotaxa is presented.

The aims of this paper are to: 1) document the trace-fossilcontent of this new locality; 2) review the ichnotaxonomicstatus of Cambrian networks and related ichnotaxa (e.g.,Multina Orłowski, 1968 and Olenichnus Fedonkin, 1985); 3)discuss the role of sea-level changes in the formation of

an ichnofossil-bearing biomat surface; and 4) evaluate theinteractions between sophisticated feeding behaviors andmicrobial mats.

STRATIGRAPHIC AND DEPOSITIONAL SETTING

The Puncoviscana Formation is the metasedimentarybasement of northwest Argentina (Turner, 1960; Acenolazaand Toselli, 1981; Ramos, 2008). It consists of a thick, foldedsuccession of wackes and mudstone, with subordinatepresence of conglomerate, limestone and volcanic rocksaffected by very low grade regional metamorphism, rangingfrom slates to schists (Do Campo and Nieto, 2003; Do Campoand Guevara, 2005). As noted in many studies (e.g., Mon andHongn, 1988, 1991; Hongn, 1996; Moya, 1998; Becchio et al.,1999; Mangano and Buatois, 2004), rocks of different type,degrees of metamorphism and tectonic deformation havebeen included under the name ‘‘Puncoviscana Formation,’’suggesting the possibility of further subdivision. Unfortunate-ly, the intense deformation of the Puncoviscana Formationcomplicates establishing a sound stratigraphic subdivision andfor proposing detailed correlations. Although the Puncov-iscana Formation was considered originally as Precambrian(Turner, 1960, 1972), the discovery of the trace fossil Oldhamiaprovided uncontroversial evidence that this unit includesCambrian strata also (Mirre and Acenolaza, 1972; Acenolazaand Durand, 1973). Recent geochronologic studies suggestedthat sedimentation in the Puncoviscana basin comprised theterminal Proterozoic and the early Cambrian (Ramos, 2000,2008; Hongn et al., 2010). TIMS and SHRIMP U-Pb zircongeochronology data indicate that deposition may have startedduring the latest Ediacaran, but took place mainly during theFortunian, coeval with 540–535 Ma calc-alkaline arc volca-nism, with probable slightly younger deposits towards thesouthwest of the basin (Escayola et al., 2011). This age isconsistent with recent ichnologic analysis, which suggested an

Journal of Paleontology, 86(1), 2012, p. 7–18

Copyright ’ 2012, The Paleontological Society

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earliest Cambrian age for the trace fossil-bearing strata(Buatois and Mangano, 2004; Mangano and Buatois, 2004).In contrast to the proposal of Acenolaza (2003), there is noevidence of an Ediacaran age for any of the trace-fossillocalities (Buatois and Mangano, 2005).

The Puncoviscana Basin is represented by a north-southtrending outcrop belt of more than 800 km length and 150 kmwide, extending from southern Bolivia to the surroundings ofthe city of Tucuman, northwestern Argentina (Ramos, 2008)(Fig. 1.1). Historically, the Puncoviscana Formation has beeninterpreted as entirely deposited in deep-marine submarinefans (Omarini and Baldis, 1984; Jezek, 1990; Acenolaza et al.,1999). However, a more recent re-evaluation of its ichnologiccontent and sedimentary facies indicated that shallow-marinedeposits are represented also (Buatois and Mangano, 2002,2003b, 2004; van Staden and Zimmermann, 2003; Lopez deAzarevich et al., 2010). A more complex paleoenvironmentalframework consisting of deep-marine turbiditic deposits alonga western belt and shallow-marine environments affected bywave action along an eastern belt has been proposed (Buatoisand Mangano, 2002, 2003a, 2003b, 2004). In particular,Buatois and Mangano (2004) described lower-offshore tomiddle/lower-shoreface facies, forming coarsening-upwardparasequences and displaying evidence of oscillatory flows.Identical shallow-marine facies have been recently identifiedby Lopez de Azarevich et al. (2010). In addition, van Stadenand Zimmermann (2003) recognized abundant glauconitelayers interbedded with conglomerate in the Rio Corralitooutcrops, suggesting shallow-marine deposition.

SEDIMENTARY FACIES

A bed-by-bed 21 m thick section (Fig. 2) was measuredon one of the flanks of a fold near the El Mollar locality alongthe Old National Road 51 in the Quebrada del Toro, SaltaProvince, northwest Argentina (Fig. 1.3). Two sedimentaryfacies have been recognized in this section.

Facies A consists of mudstone and parallel-laminatedsiltstone interbedded with discrete layers of erosionally based,laterally extensive to rarely lenticular, thin, very fine-grainedsandstone with micro-hummocky cross-stratification (Fig. 3.1)and symmetrical to near-symmetrical ripples (Fig. 3.2). Hum-mocky cross-stratification occurs locally. Mudstone andsiltstone intervals are 5–60 cm thick, and sandstone beds are3–20 cm thick. Sandstone to mudstone ratios range from 1:3 to1:5. Sandstone bases typically contain small tool marks, flutemarks and load casts. Sandstone beds may show patchilydistributed wrinkle marks. Multina isp. occurs at the base ofthin sandstone layers, while Helminthoidichnites tenuis andHelminthopsis tenuis are relatively common at the top.

The symmetrical to near-symmetrical ripples are interpretedas wave ripples and combined-flow ripples, respectively. Thepresence of these structures together with micro-hummockyand hummocky cross-stratification suggests that the sandstonelayers are distal tempestites (Dott and Bourgeois, 1982;Myrow, 1992; Cheel and Leckie, 1993). The interbeddedsiltstone and mudstone intervals mostly record sediment fall-out. Facies A is interpreted to record alternating backgroundsuspension fall-out and distal storm deposition above the

FIGURE 1—Location and geological maps of the study area: 1, general map showing distribution of outcrops of the Puncoviscana Formation andoverall extension of the Puncoviscana Basin (after Ramos, 2008); 2, general location map of Salta Province, Salta city, and the El Mollar; 3, geologicmap of the Quebrada del Toro showing location of the El Mollar (modified from Durand and Acenolaza, 1990).

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storm wave base, but well below the fair-weather wave base inan upper-offshore environment. This is the most abundantfacies in the El Mollar section, forming up to 10 m thickintervals.

Facies B consists of regularly interbedded, erosionally based,laterally extensive very fine-grained sandstone, parallel-laminatedsiltstone, and mudstone. Sandstone beds display hummockycross-stratification, micro-hummocky cross-stratification, andsymmetrical to near-symmetrical ripples. Mudstone andsiltstone intervals are 1–13 cm thick, and sandstone beds are5–17 cm thick. Sandstone to mudstone ratios are typically 1:1.Sandstone bases may contain flute marks. Multina isp. locallyoccurs at the base of thin sandstone layers. Abundant, thoughpatchily distributed, wrinkle marks and elephant-skin texturesoccur on the planar tops of sandstone and siltstone beds.Oldhamia alata, Pilichnus cf. P. dichotomus, Helminthoidichnitestenuis, and Helminthopsis tenuis occur on one of these surfaces,marking the top of facies B package.

In thin section across this surface, microsequences of fining-upward siltstone capped by microbial mat are observed (Fig. 4.1).Locally, the mat is deformed (Fig. 4.1). Under magnification, thetypical texture of endobenthic matgrounds is present, consisting offilaments intertwined in angles ranging from 0–45u, is present(Fig. 4.2) (see Noffke, 2010). Framboidal pyrite is common.Floating quartz grains are enveloped within the biomat (Fig. 4.3).Burrows are observed deforming the matground above and below;burrow fill is rich in pyrite (Fig. 4.4).

As in the case of facies A, the symmetrical to near-symmetrical ripples represent wave ripples and combined-flowripples, respectively. The presence of these structures, as wellas of hummocky and micro-hummocky cross-stratification,indicates storm deposition, while the interbedded siltstone andmudstone mostly record sediment fall-out (Cheel and Leckie,1993; Dumas and Arnott, 2006). The increased sandstone-mudstone ratios indicates a slightly more proximal positionthan facies A. Facies B is interpreted to record alternating

FIGURE 2—Stratigraphic section of the Puncoviscana Formation at El Mollar locality, showing the development of two parasequences separated by adrowning surface (DS).

BUATOIS AND MANGANO—EARLY CAMBRIAN SHALLOW-MARINE TRACE FOSSILS 9

background suspension fall-out and distal storm depositionright below the fair-weather wave base in an offshore-transition environment. This facies forms a single discretetwo-meter thick interval.

The facies described are stacked forming two parasequencesseparated by a drowning surface. This low-energy surface clearlyindicates a rapid upward deepening without any associatederosion. The lower parasequence consists of a thick upper-offshore interval capped by a thin offshore-transition interval. Theupper parasequence consists entirely of upper-offshore deposits.

SYSTEMATIC PALEONTOLOGY

Ichnotaxa are listed alphabetically. Specimens are housed atthe Paleontological Museum Egidio Feruglio, Trelew, Argen-tina (MPEF-IC).

Ichnogenus HELMINTHOIDICHNITES Fitch, 1850HELMINTHOIDICHNITES TENUIS Fitch, 1850

Figure 5

Specimens.—Approximately 30 specimens studied in thefield and seven slabs with twelve specimens collected (MPEF-IC 424, 426, 427, 428, 430, 432, 433).

Description.—Simple, unbranched, straight to gently curvedtrails, oriented parallel to the bedding plane. Fill is identical to

the host rock. Overlapping among different individuals iscommon. Diameter is 0.8–2.1 mm and may slightly vary alongthe course of individual trails. Maximum observed length is17.0 mm. Preserved in full relief on sandstone tops.

Discussion.—Helminthoidichnites tenuis is one of the mostcommon components of Ediacaran and lower Cambrianichnofaunas (e.g., Hofmann et al., 1994; MacNaughton et al.,2000; Jensen et al., 2006). This ichnotaxon is interpreted as agrazing trace (pascichnion) most likely produced by vermiformanimals (Buatois et al., 1998).

Ichnogenus HELMINTHOPSIS Heer, 1877HELMINTHOPSIS TENUIS Ksiaz_kiewicz 1968

Specimens.—Approximately ten specimens studied in thefield.

Description.—Simple, unbranched, meandering trails, ori-ented parallel to the bedding plane. Fill is identical to the hostrock. Diameter is 0.7–1.1 mm. Maximum observed length is14.0 mm. Preserved in full relief on sandstone tops.

Discussion.—Helminthopsis tenuis is particularly abundantin Ediacaran–Early Cambrian ichnofaunas (e.g., Buatois andMangano, 2003a; Jensen et al., 2006). It is interpreted as agrazing trace (pascichnion) probably produced by vermiformanimals (Ksiaz_kiewicz, 1977; Buatois et al., 1998).

FIGURE 3—Upper-offshore tempestites in the Mollar section showing evidence of oscillatory flows: 1, sharp-based sandstone bed withmicrohummocky cross-stratification; 2, sharp-based sandstone bed with symmetrical to near-symmetrical ripples. Lens cap is 5.5 cm wide.

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Ichnogenus MULTINA Orłowski, 1968MULTINA isp.Figure 6.1–6.3

Specimens.—Approximately 50 specimens studied in thefield and one slab with one specimen collected (MPEF-IC419).

Description.—Irregular overlapping networks having straight,meandering to winding strings. Network size is 28.1–190.4 mmand string diameter is 2.0–4.0 mm. Preserved in full relief onsandstone bases.

Discussion.—The ichnogenus Multina was introduced byOrłowski (1968) based on specimens from upper Cambrianshallow-marine deposits of the Holy Cross Mountains inPoland. These specimens, included in the type ichnospeciesM. magna, were subsequently redescribed by Orłowski andZylinska (1996). This ichnotaxon consists of approximately1 mm wide curved epichnial grooves that display commonoverlap and bifurcate forming 2–5 cm wide irregular polygons.Orłowski and Zylinska (1996) noted indistinct transversefurrows in some of the specimens, suggestive of backfill andperistaltic movements. Another occurrence of M. magna wasdocumented from Lower Ordovician turbidites of northwestArgentina (Buatois et al., 2009). A second ichnospecies, M.minima, was introduced by Uchman (2001) based on specimensfrom Eocene deep-marine deposits of Spain. Multina minima is

characterized by overlapping strings, containing abundantswellings and forming highly irregular and very small (lessthan 5 mm wide) networks. It shares with M. magna thepresence of common overlap. The type specimen of M. minimaand a second occurrence in Lower Cretaceous relatively deep-water deposits of Bulgaria (Uchman and Tchoumatchenco,2003) are preserved as positive hyporeliefs. However, recentlydescribed specimens from the Eocene of Spain are preserved asfull reliefs on the base of turbidites (Rodrıguez-Tovar et al.,2010). Specimens from the Puncoviscana Formation havestrings that are morphologically highly variable, being locallymore meandering than those of M. magna, which are typicallycurved. In contrast, networks are more irregular than in M.magna. Also, Multina from the Puncoviscana Formation lacksthe abundant swellings that typify M. minima, and isremarkably larger. Partially preserved networks consisting ofisolated strings may be confused with grazing traces, such asHelminthopsis or Helminthorhaphe Seilacher, 1977 (fig. 5-3).

As noted by Orłowski and Zylinska (1996), Multina displaymorphologic similarities with some graphoglyptid ichnotaxa,most notably Paleodictyon Meneghini, 1850, Protopaleodic-tyon Ksiaz_kiewicz, 1958, and Megagrapton Ksiaz_kiewicz,1968. However, although the morphology of the networksmay be superficially similar, these three ichnotaxa are opengalleries that are passively filled (i.e., they are agrichnialstructures of farmers and trappers rather than fodinichnial

FIGURE 4—Microbial-mat textures in thin-section across the drowning surface: 1, microsequences of fining-upward siltstone capped by microbial mat;note mat deformation (arrows) and enrichment in organic matter; scale bar51 mm; 2, texture of endobenthic matground, consisting of filamentsintertwined in angles ranging from 0–45u; scale bar51 mm; 3, floating quartz grains (arrows) enveloped within the mat; scale bar50.5 mm; 4, burrow incross section, displaying spreite and visibly deforming the matground above and below; note abundant pyrite within the burrow; scale bar51 mm.

BUATOIS AND MANGANO—EARLY CAMBRIAN SHALLOW-MARINE TRACE FOSSILS 11

structures of deposit feeders). According to Uchman (1998),many lower Paleozoic occurrences of Megagrapton may in factrepresent Multina, noting that these specimens displaycommon overcrossing of the strings, which is not common,if present at all, in graphoglyptids. The same may be true ofmany lower Paleozoic recordings of Protopaleodictyon.

The ichnogenus Olenichnus, with O. irregularis as its typeichnospecies, was proposed by Fedonkin (1985) based onearliest Cambrian specimens from shallow-marine deposits.Olenichnus comprises networks consisting of sinuous stringsforming irregular networks preserved as negative hyporeliefs.The type specimen and additional material from the lowerCambrian of the Mickwitzia Sandstone in Sweden, have beenrevised by Jensen (1997), who noted the presence of knobsindicative of vertical components. Olenichnus has beenregarded as a junior synonym of Multina by Uchman andAlvaro (2000). There are slight differences in the morphologyof the networks that warrant keeping Olenichnus irregularisas a valid ichnospecies of Multina, M. irregularis. Thisichnospecies is characterized by irregular networks of veryvariable size that do not form polygons as in M. magna. Theabsence of swellings distinguishes M. irregularis from M.minima.

Uchman (1998) noted similarities between Multina andPseudopaleodictyon Pfeiffer (1968), which was proposed basedon Palaeophycus hartungi Geinitz (1867), becoming Pseudo-paleodictyon hartungi. Based on the original description andillustration, the true nature of Pseudopaleodictyon is hard toevaluate. Although the exact date of publication of Pseudo-paleodictyon is unclear (varying between 1966, 1968 and 1969,depending on source), the use of Pseudopaleodictyon is notrecommended, and Multina is preferred instead.

Two other network ichnogenera have been described fromcontinental deposits. These are Vagorichnus described byBuatois et al. (1995) from Jurassic lacustrine turbidites of

China and Labyrintichnus described by Uchman and Alvaro(2000) from Miocene marginal-lacustrine deposits of Spain.The former was first regarded as a junior synonym of Multinaby Uchman and Alvaro (2000) and Schlirf et al. (2001), but

FIGURE 5—Helminthoidichnites tenuis preserved as full relief at the topof a sandstone tempestite; field photo; scale bar52 cm.

FIGURE 6—Multina isp. preserved as full relief at the base of sandstonetempestites: 1, general view of networks consisting of meandering andwinding strings, field photo, note branching (arrow); 2, close-up ofnetworks, field photo; 3, individual string resembling a meanderinggrazing trail rather than a burrow network, MPEF-IC 419, scale bar51 cmlong. Lens cap is 5.5 cm wide.

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subsequently retained as a valid ichnotaxon by Uchman et al.(2007). As in Multina, Vagorichnus is a feeding structureinvolving active fill. However, in contrast to Multina, Vagor-ichnus is characterized by a wider morphologic variability, acomplex branching pattern (including also secondary successivebranching), and abundant aligned knobs that indicate verticalmeandering movements (Buatois et al., 1995). Labyrintichnus ismorphologically very similar to Multina but it is passively filled(Uchman and Alvaro). Both Vagorichnus and Labyrintichnusare regarded here as valid ichnotaxa.

Ichnogenus OLDHAMIA Forbes, 1849OLDHAMIA ALATA Seilacher, Buatois and Mangano, 2005

Figure 7.1–7.6

Specimens.—Thirteen slabs with forty five specimenscollected (MPEF-IC 422 to 430 and 432 to 435).

Description.—Horizontal complex burrow systems consist-ing of closely spaced tunnels (i.e., probings) that form up toeight wing-like units resembling small Lophoctenium struc-tures. Individual tunnels are 0.1–1.6 mm wide. Wing-like unitsare 4.4–20.1 mm wide. As noted by Seilacher et al. (2005),probings proceed to the convex side commonly starting atthe center and protruding outward, but a centripetal patternhas been detected in some lobes. Although opposite, 180usymmetrical wings, 29.5–32.5 mm wide, locally occur (Fig. 7.3),more complex asymmetrical patterns defined by successivewings alternating along an imaginary axis are more common.These cumulative structures are 17.3–35.0 mm. Successivewings are adjacent to each other, and do not overlap (Fig. 7.4).Abrupt changes in the length of individual tunnels reveallobation within wings, having two to four, 3.5–8.5 mm widelobes in each wing.

Discussion.—This ichnospecies was originally referred to as‘‘a new ichnospecies of Oldhamia’’ by Buatois and Mangano(2004), and subsequently formally proposed by Seilacher et al.(2005), who described and interpreted this ichnotaxon indetail. Poorly preserved and isolated tunnels may be confusedwith grazing trails (e.g., Helminthoidichnites). In contrast toother ichnospecies of Oldhamia, probings in O. alata not onlybend back on previous ones, but follow them so closelyresembling the spreite of a tiny Lophoctenium Richter, 1850(Seilacher et al., 2005). Oldhamia alata is reminiscent of theasymmetrical morphotypes of O. curvata (Fig. 7.5) but probesin O. alata stay in closer contact with previous ones and thewing-like units, in cases displaying lobation, are far morecomplex and commonly alternate (Seilacher et al., 2005).Oldhamia is a feeding trace (fodinichnion) produced byvermiform animals (Seilacher, 1999). Oldhamia is essentiallya Cambrian ichnogenus, being particularly abundant duringthe early to middle Cambrian (Lindholm and Casey, 1990;Herbosch and Verniers, 2011). Although it is more common indeep-marine environments (e.g., Hofmann et al., 1994), it alsooccurs in shallow-marine deposits (e.g., Goldring and Jensen,1996) (see Paleoenvironmental significance of Oldhamia).

Ichnogenus PILICHNUS Uchman, 1999PILICHNUS cf. P. DICHOTOMUS Uchman, 1999

Figure 8.1, 8.2

Specimens.—One slab with three specimens collected (MPEF-IC 426).

Description.—Tunnel systems consisting of horizontal,straight to locally winding strings commonly displayingdichotomous branches. T-shaped branchings are very rare.Tunnel fill is the same than the host rock. One of thespecimens locally shows sinusoidal strings (Fig. 8.2). Strings

are 0.1–0.2 mm wide. Preserved in full relief on sandstonetops.

Discussion.—Pilichnus was proposed based on specimensfrom Cretaceous turbidites in Germany (Uchman, 1999). Thestratigraphic and paleoenvironmental range of Pilichnus havebeen recently expanded to include Cambrian (e.g., Zhanget al., 2007; Mangano, 2011) and Ordovician shallow-marinedeposits (e.g., Mikulas, 2003), and Carboniferous turbidites(e.g., Mikulas et al., 2004). It is most likely a feeding trace(fodinichnion). Structures similar to Pilichnus are produced inmodern shallow-marine environments by the polychaetesHeteromastus filiformis and Capitella cf. aciculata (Hertwecket al., 2007). The main difference between the specimensstudied and P. dichotomus is the absence of a pyritizedfill, which is characteristic of the latter ichnospecies (seeUchman, 1999) and the local presence of sinusoidal strings inthe Puncoviscana material. Apparently branching sinusoidalspecimens illustrated by Seilacher et al. (2005, their fig. 8B) areeither overlapping Cochlichnus Hitchcock, 1858 or fragmen-tary preserved Pilichnus cf. P. dichotomus.

DISCUSSION

Paleoenvironmental significance of Oldhamia.—The ichno-genus Oldhamia is a common component of Cambrian deep-marine environments worldwide (see Herbosch and Verniers,2011 and references therein). As a result, it has been assumed insome studies that Oldhamia is an indicator of deep-marineenvironments (e.g., Tacker et al., 2010). However, thisichnogenus has also been recorded in shallow-marine facies(Kowalski, 1987; Goldring and Jensen, 1996; Buatois andMangano, 2004). While Oldhamia is invariably the dominantichnotaxon in deep-marine deposits where it occurs in relativelylow-diversity assemblages, in shallow-marine settings it istypically an accessory element in more diverse assemblages(Goldring and Jensen, 1996; Buatois and Mangano, 2003b,2004; Seilacher et al., 2005). At ichnospecific level, Oldhamiaalata and O. geniculata are only known from shallow-marinedeposits whereas O. flabellata and O. curvata have only beenrecorded in deep-marine environments; O. antiqua and O.radiata have been recorded in both shallow- and deep-marinesettings (Seilacher et al., 2005).

Microbial mats and sea-level changes.—In recent years therehas been increased interest on the interplay between sequencestratigraphy and matground development. A number ofstudies documented that in Precambrian rocks, microbialmat textures are preferentially preserved in the lower portionof highstand systems tracts (Banerjee and Jeevankumar, 2005;Sarkar et al., 2005; Catuneanu, 2007). It has been argued thatPrecambrian sequences are characterized by very thin trans-gressive systems tracts and well-developed highstand systemstracts; even in some cases, transgressive systems tracts areabsent and highstand systems tracts are stacked (Sarkar et al.,2005). Within this framework, the absence of transgressivepackages is related to the presence of gently dipping shelves,which facilitated rapid transgressions in combination witha generally low sediment supply. Sarkar et al. (2005) alsoproposed that binding of clastic particles resulting from theprolific growth of microbial mats reduced the effects of waveand current reworking, allowing sediment aggradation in spiteof the low sediment supply.

On the other hand, Noffke (2010) suggested that microbiallyinduced sedimentary structures are particularly widespreadduring transgressive phases. She based her view on actualisticgrounds, namely the widespread development of coastal areasduring the Holocene transgression, leading to extensive

BUATOIS AND MANGANO—EARLY CAMBRIAN SHALLOW-MARINE TRACE FOSSILS 13

FIGURE 7—Oldhamia alata preserved as full relief at the top of a sandstone tempestite: 1, general view showing overall high density of specimenscovering the bedding plane; corrugations on the left represent evidence of microbial mats; 2, close-up of specimens displaying typical branching pattern;note associated corrugations on the bedding surface; MPEF-IC 423; 3, detailed view of specimens showing symmetrical arrangement of wing-like lobes;upper specimen is the holotype; MPEF-IC 424; 4, close-up of specimens showing the non-overlapping nature of wing-like lobes; isolated tunnels, such asthose on the right, may be confused with grazing trails; MPEF-IC 425; 5, detailed view of specimens that are somewhat reminiscent of O. curvata; non-overlapping nature of the lobes, however, indicates affiliation with O. alata; MPEF-IC 430; 6, close-up view of specimens in close association withmicrobial mat textures. MPEF-IC 431. Scale bars51 cm.

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microbial mat ecosystems flourishing in tidal-flat, lagoon andshelf settings.

Faults affecting the succession preclude placing the ElMollar section within a broad sequence-stratigraphic frame-work. Despite this, the association of an ichnofossil-bearingmatground and the drowning surface is clear. In El Mollarsection, the best-preserved trace-fossil suite is in directconnection with wrinkle marks and elephant-skin texturesand it occurs at the siltstone mantling the planar top of asandstone tempestite. This layer is capped by a low-energydrowning surface, and the top of the tempestite thereforerepresents an omission surface. Buatois and Mangano (2004)noted that low sediment rate during drowning and paucity ofbioturbation by sediment bulldozers may have promoted theestablishment of the matground. Interestingly, the ichnofaunaof this bed resembles that of the deep-marine deposits,particularly with respect to the dominance of Oldhamia.

Tiering structure.—The tiering structure of the 5 cm thickoffshore-transition bed right below the drowning surface isvery simple (Buatois and Mangano, 2004). Three ichnoguildshave been recognized, from shallow- to deep-tier, Helminthop-sis, Oldhamia and Multina. The Helminthopsis ichnoguildconsists of Helminthopsis tenuis and Helminthoidichnitestenuis. This ichnoguild represents the activity of transitory,surface to near-surface, mat-grazer structures produced by

vagile vermiform animals. The Helminthopsis ichnoguild isubiquitous in both Ediacaran and early Cambrian shallow-and deep-marine deposits, revealing one of the simplestfeeding strategies associated with biomats (Buatois andMangano, 2003a).

The Oldhamia ichnoguild consists of Oldhamia alataand Pilichnus cf. P. dichotomus. This ichnoguild includessemi-permanent, very shallow-tier, undermat-miner structuresproduced by stationary vermiform organisms. The Oldhamiaichnoguild is particularly common in lower to middleCambrian deep-marine deposits but it has been recorded alsoin shallow-marine deposits, albeit rarely (see Paleoenviron-mental significance of Oldhamia). The presence of Oldhamia inEdiacaran rocks has been questioned (Tacker et al., 2010) andaccordingly this ichnoguild is most likely a product of theCambrian explosion.

The Multina ichnoguild consists of semi-permanent, shallow-to middle-tier, deposit feeder structures produced by vagilevermiform organisms. It is represented by Multina isp., which ispreserved at the base of the tempestite. Burrow networksinvariably cross-cut tool marks, indicating that Multina is apost-event burrow emplaced after storm deposition. Theproducers burrowed into the tempestite and moved along thesand-mud interface. This ichnoguild serves as a proxy toevaluate burrowing depth in offshore settings by the earlyCambrian, pointing to incipient colonization of the infaunalecospace. The tracemakers moved along the lithologic interface,therefore producing negligible bioturbation and little or nodisturbance. As in the case of the Oldhamia ichnoguild, theMultina ichnoguild seems to have been a product of theCambrian explosion with no equivalents in the Ediacaran.Possible networks compared with Olenichnus have beenrecorded in the Huns Member of the Urusis Formation inNamibia by Jensen and Runnegar (2005). However, these arevery shallow-tier structures. The Multina ichnoguild occurs inTremadocian deep-marine deposits of the Puna area innorthwest Argentina, suggesting a possible onshore-offshorepattern for this ichnotaxon (Buatois et al., 2009).

The ichnofauna of the offshore-transition beds unrelatedto the drowning surface (i.e., those offshore-transition layersbelow the uppermost bed) is very simple, and consists only ofMultina isp. preserved at the base of sandstone tempestites,ranging from 5 to 8 cm thick. The tiering structure of theupper-offshore deposits is very similar to that of the offshore-transition bed below the drowning surface. However, theichnoguild represented by undermat miners (Oldhamia alataand Pilichnus cf. P. dichotomus) is not present, and only twoichnoguilds have been recorded: the Helminthopsis andMultina ichnoguild. This provides further evidence that theconditions associated with the drowning event were instru-mental in promoting development and preservation of mat-grounds and associated trace fossils.

The onset of sophisticated feeding strategies and thepersistence of matgrounds.—Interactions between organismsand matgrounds were widespread during Ediacaran times, butremain relatively simple due to the absence of organismscapable of developing complex feeding structures (Buatois andMangano, in press). These were dominated by mat grazerstructures of the Helminthopsis ichnoguild and mat scratcherstructures of the Radulichnus ichnoguild, the latter in directassociation with the mollusk-like Kimberella (Fedonkin, 2003;Gehling et al., 2005; Seilacher et al., 2005; Fedonkin et al.,2007; Seilacher and Hagadorn, 2010). Interactions are alsoevidenced by the presence of serially repeated resting tracefossils of Dickinsonia and the related genus Yorgia preserved

FIGURE 8—Pilichnus cf. P. dichotomus preserved as full relief at the topof a sandstone tempestite; corrugations represent evidence of microbialmats; MPEF-IC 426: 1, general view showing overall branching pattern;scale bar51 cm; 2, various branching systems; note local development ofsinusoidal morphology of elements in the system on the lower right; scalebar51 cm.

BUATOIS AND MANGANO—EARLY CAMBRIAN SHALLOW-MARINE TRACE FOSSILS 15

on top of microbial mats (Ivantsov and Malakhovskaya, 2002;Fedonkin, 2003; Gehling et al., 2005; Sperling and Vinther,2010).

The combination of persistent matgrounds, albeit morepatchily distributed, and the evolutionary innovations of theCambrian explosion allowed for more complex interactions todevelop (Buatois and Mangano, in press). The sophisticatedbehavioral programs exhibited by Oldhamia alata and, to alesser extent, Pilichnus cf. P. dichotomus in the PuncoviscanaFormation clearly illustrate an increase in the complexityof organism-matground interactions with respect to thosedisplayed by their Ediacaran counterparts. The closely guidedprobings in O. alata, leaving no unexplored sediment betweensuccessive elements within a lobe, evince an improved feedingprogram even if compared to other Oldhamia ichnospeciesmore common in deep-sea deposits (Seilacher et al., 2005).Pilichnus cf. P. dichotomus, with its systematic dichotomousbranching pattern, reflects efficient exploration of the organicmatter preserved below the microbial mat. In fact, it may beargued that systematic undermat mining is an evolutionaryinnovation which resulted from the Cambrian explosion. Thebest candidates for Ediacaran undermat miners are thestructures illustrated in the Vingerbreek Member of theNudaus Formation of Namibia by Bouougri and Porada(2007), which await further description.

CONCLUSIONS

An early Cambrian shallow-marine (upper offshore tooffshore transition) ichnofauna is documented from thePuncoviscana Formation of northwest Argentina. The ichno-fauna consists of Helminthoidichnites tenuis, Helminthopsistenuis, Multina isp., Oldhamia alata and Pilichnus cf. P.dichotomus. This occurrence provides further evidence thatthe ichnogenus Oldhamia, although much more commonin Cambrian deep-marine environments, is also present inshallow-marine environments.

A drowning surface separating offshore-transition belowfrom upper-offshore deposits above contains widespreadevidence of trace fossils in direct association with matgrounds.The undermat miners Oldhamia alata and Pilichnus cf. P.dichotomus are particularly well preserved on this surface. Lowsediment rate during drowning and paucity of bioturbation bysediment bulldozers may have promoted the establishment ofthe matground.

While the trace-fossil record of animal-matground interac-tions during the Ediacaran is relatively simple, the combina-tion of Cambrian evolutionary innovations and the presenceof microbial mats were conducive to more complex interac-tions. Systematic undermat mining, as displayed by Oldamiaalata and Pilichnus cf. P. dichotomus, is a product of theCambrian explosion.

ACKNOWLEDGMENTS

Financial support for this study was provided by theAntorchas Foundation, and Natural Sciences and EngineeringResearch Council (NSERC) Discovery Grants 311727-05/08and 311726-05/08 awarded to Mangano and Buatois, respec-tively. I. Sabino assisted during the field work. N. Carmonaprovided detailed comments on the microbial mat structures.M. Bertling, D. Seilacher, A. Uchman and A. Zylinskaprovided feedback on various ichnologic aspects. B. Nova-kovski did the thin-sections. D. Dawson helped us to track theelusive publication date of the Pseudopaleodictyon paper.Reviewers S. Jensen and D. McIlroy, and Editor B. Prattprovided very useful comments. E. Ruigomez (Museo Egidio

Feruglio) curated the specimens and assisted us withphotography.

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