Interactions between Ediacaran animals and microbial mats: Insights from Lamonte trevallis, a new trace fossil from the Dengying Formation of South China

    Interactions between Ediacaran animals and microbial mats: Insights from Lamonte trevallis, a new trace fossil from the Dengying Formation of South China Mike Meyer, Shuhai Xiao, Benjamin C. Gill, James D. Schiffbauer, Zhe Chen, Chuanming Zhou, Xunlai Yuan PII: DOI: Reference:

S0031-0182(13)00563-4 doi: 10.1016/j.palaeo.2013.12.026 PALAEO 6706

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

10 July 2013 13 December 2013 16 December 2013

Please cite this article as: Meyer, Mike, Xiao, Shuhai, Gill, Benjamin C., Schiffbauer, James D., Chen, Zhe, Zhou, Chuanming, Yuan, Xunlai, Interactions between Ediacaran animals and microbial mats: Insights from Lamonte trevallis, a new trace fossil from the Dengying Formation of South China, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.12.026

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ACCEPTED MANUSCRIPT Interactions between Ediacaran animals and microbial mats: insights from Lamonte trevallis,

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a new trace fossil from the Dengying Formation of South China

Mike Meyer a,d,*, Shuhai Xiao a, Benjamin C. Gill a, James D. Schiffbauer b, Zhe Chen c,

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Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA

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a

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Chuanming Zhou c, Xunlai Yuan c

24061, USA;

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Department of Geosciences, University of Missouri, Columbia, MO, 65211, USA;

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b

State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and

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Paleontology, Chinese Academy of Sciences, Nanjing 210008, China; Present Address: Geosciences and Natural Resources Department, Western Carolina University,

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Cullowhee, NC, 28723, USA;

*Corresponding author. Tel: 828-227-2950. Email address: [email protected] (M. Meyer), [email protected] (S. Xiao), [email protected] (B. C. Gill), [email protected] (J. D. Schiffbauer), [email protected] (Z. Chen), [email protected] (C. Zhou), and [email protected] (X. Yuan).

ABSTRACT A new ichnogenus and ichnospecies, Lamonte trevallis, is formally described from the Shibantan Member limestone of the upper Ediacaran Dengying Formation, Yangtze Gorges area, South China. It is characterized by horizontal tunnels connected with short vertical burrows and surface trails. The horizontal burrows are elliptical or bilobed in transverse cross-section,

ACCEPTED MANUSCRIPT preserved in full relief, and filled with carbonate intraclasts, micrites, as well as calcite and silica cements. They occur exclusively in silty, crinkled, and microlaminated layers that are interpreted

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as amalgamated cyanobacterial microbial mats; no burrows have been found in intraclastic layers

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adjacent to the microlaminated layers. The vertical traces are filled with the same material as the burrows, but they typically project through the crinkled microlaminae and are exposed on the

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bedding surface. The surface tracks are always preserved in negative epirelief or positive

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hyporelief and consist of two parallel series of either sharp scratch marks or small knobs. The burrow infill has δ18Ocarb and δ13Ccarb values distinct from, but intermediate between,

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microlaminated and intraclastic layers, consistent with petrographic observation that burrow infill consists of a mixture of early carbonate cements, intraclasts, and micrites. Bedding plane

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bioturbation intensity (20–40%)—measured as percentage of bedding plane area covered by L. trevallis traces—is comparable to similar measurements in pre-trilobite Cambrian carbonates.

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The exclusive occurrence of L. trevallis within microbial mats may have both taphonomic and ecological significance. These mats may have provided firm substrates and localized geochemical conditions that contributed to the structural integrity of the burrows, and they may have also facilitated early diagenetic cementation of burrow infill, thus facilitating burrow preservation. The close association of these burrows with microbial mats implies that the trace producers actively mined cyanobacterial mats to exploit oxygen or nutrient resources. The trace makers of L. trevallis were better able to utilize the resources around them than many other Ediacaran trace makers and provide an ichnological record of a flourishing benthic ecology in late Ediacaran oceans at the dawn of the agronomic revolution.

ACCEPTED MANUSCRIPT Key words: Taphonomy, Lamonte trevallis, trace fossil, Ediacaran, Dengying Formation, South

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China

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1. Introduction

The establishment of burrowing behaviors is a key event in the evolution of bilaterian

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animals, significantly impacting paleoecological and geobiological dynamics at the Ediacaran-

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Cambrian transition. Ediacaran burrows and trails are traditionally known for their small size, simple morphology, shallow penetration depth, and relative scarcity when compared to their

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Phanerozoic counterparts (Droser et al., 1999; Droser et al., 2006; Jensen et al., 2005; Jensen et al., 2006; Seilacher, 2007). There are several factors that might have contributed to the limited

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complexity, diversity, and abundance of Ediacaran burrows. First, dissolved oxygen levels were relatively low and uneven in Ediacaran oceans (Fike et al., 2006; Kennedy et al., 2005; McFadden

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et al., 2008; Shen et al., 2008), which would have limited habitable ranges of burrowing animals. Second, the ubiquitous microbial mats of the time (Bottjer et al., 2000; Hagadorn and Bottjer, 1999; Seilacher, 1999) would likely have maintained a sharp redox gradient across the sediment-water interface such that deeper sediments were poorly oxygenated or sulfidic, and thus generally hostile to burrowers (Fedonkin et al., 2007a; Gehling, 1999; Gerdes, 2007). Third, predation pressures were relatively low, thereby allowing animals to forage along the surface of microbial mats or sediments (Dzik, 2005; Fedonkin et al., 2007b; Fedonkin and Waggoner, 1997; Ivanstov and Malakhovskaya, 2002). Finally, it is also possible that Ediacaran animals were not abundant or were physically incapable of making large and complex burrows due to the lack of a strong musculature system (Miller, 2007; Seilacher, 2007). Despite their limited complexity and diversity, Ediacaran

ACCEPTED MANUSCRIPT trace fossils do offer the only paleontological window onto the mounting importance of bioturbation at a key transition in the history of life.

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One of the most important innovations in the early evolution of animal bioturbation was

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the exploration and penetration of the ubiquitous microbial mats that existed during this time (Droser et al., 2006; Gehling, 1999; Gehling et al., 2005; Hagadorn and Bottjer, 1999). These

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microbial mats were a major component of the Proterozoic ecosystem (Eriksson et al., 2007;

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Hofmann, 2000; Seilacher, 1999) and were much more expansive than those found today (Eriksson et al., 2010; Gingras et al., 2011; Seckbach and Oren, 2010). The cohesive nature of

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microbial mats reduces gas transport into and out of sediments and, in combination with vertical stratification of different microbial communities, creates a steep chemical gradient across the

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mat-water interface and in the uppermost centimeters of sediment (Bosak et al., 2012; Moreira et al., 2004). The resulting anoxic sediments would likely have hindered or deterred vertical

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exploration by animals (Seilacher, 1999), although horizontal surface trails and shallow burrows were common in the Ediacaran fossil record (Droser et al., 2005; Jensen et al., 2005; Jensen et al., 2006). However, bioturbation did become more pervasive and, with deeper sediment penetration and therefore greater sediment ventilation, had a greater geobiological impact on Earth systems (Canfield and Farquhar, 2009). Bilaterian animals, in the course of interacting with and exploiting the microbial world (Bottjer et al., 2000; Droser et al., 2002; Hagadorn and Bottjer, 1999; Seilacher, 1999), also left increasingly complex burrow systems (Jensen and Runnegar, 2005; Jensen et al., 2000; Seilacher et al., 2005), indicating a more dynamic Ediacaran ecosystem that can be examined by studying the trace fossil record. In the last decade, the upper Ediacaran Dengying Formation (~551–541 Ma) in the Yangtze Gorges area of South China has become an important stratigraphic unit for the study of

ACCEPTED MANUSCRIPT Ediacaran trace fossils. Numerous trace fossils, including simple taxa such as Helminthoidichnites, Palaeophycus, Planolites, and Torrowangea (Ding et al., 1992; Jensen et

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al., 2006; Weber et al., 2007; Zhao et al., 1988), as well as more complex forms that represent

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under-mat feeding, epibenthic locomotion, and temporary dwelling structures of bilaterian animals (Chen et al., 2013), have been reported from the Dengying Formation. In particular, the

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bituminous limestone of the Shibantan Member in the middle Dengying Formation holds great

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potential to illuminate ecological interactions between late Ediacaran animals and microbial mats, because of the abundance and exceptional preservation of both trace fossils and

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microlaminae which have been interpreted as the remnants of microbial mats. Here, we formally describe a new trace fossil, Lamonte trevallis, which can potentially broaden our understanding

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of trace maker behavior during this time. Building upon our preliminary report of similar trace fossils from the Shibantan Member (Chen et al., 2013), this study synthesizes petrographic and

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geochemical data in order to better understand the preservation and paleoecology of the Shibantan burrows.

2. Geological setting

The geological and stratigraphic background of the Dengying Formation in the Yangtze Gorges area was detailed in Chen et al. (2013). To briefly summarize, the upper Ediacaran Dengying Formation overlies the lower–middle Ediacaran Doushantuo Formation and underlies the Yanjiahe Formation which contains the Ediacaran-Cambrian boundary (Dong et al., 2009; Jiang et al., 2012). It was deposited on a shallow marine carbonate platform (Jiang et al., 2011; Zhou and Xiao, 2007), and its age is constrained between 551 Ma and 541 Ma based on available radiometric dates and stratigraphic correlations (Condon et al., 2005; Jiang et al., 2009; Zhu et

ACCEPTED MANUSCRIPT al., 2009). The Dengying Formation is divided into three units, in ascending age order: the Hamajing, Shibantan, and Baimatuo members (Fig. 1). The Hamajing Member consists of

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peritidal dolostone, with widespread tepee structures (Fig. 2A) and karstification features such as

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dissolution vugs ranging from a few millimeters to a few meters in size (Fig. 2B) (Zhou and Xiao, 2007). The Shibantan Member is composed of dark gray, thin-bedded, bituminous

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limestone interpreted to have been deposited in a subtidal environment (Fig. 2C) (Zhou and

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Xiao, 2007). The Baimatuo Member is composed of light gray massive peritidal dolostone, characterized by abundant dissolution vugs and breccia (Fig. 2G) (Chen et al., 2013; Zhou and

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Xiao, 2007). Although the Shibantan Member is mostly characterized by thin bedded parallel laminae, cross stratification and rip-up clasts are present (Figs. 2E–F), indicating deposition

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above storm wave-base. Crinkled microlaminae consisting of organic-rich, calcareous clays and silts are very common, and are commonly characterized with wrinkled bed surfaces. Crinkled

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microlaminae are often intercalated with thin layers of intraclastic, pelloidal, and oolitic packstone and grainstone (Fig. 2D). These crinkled microlaminae are interpreted as microbial mats (see section 4 below) that trapped silts, clays, and micrites, and were subsequently cemented by diagenetic calcite (Chen et al., 2013). No mud cracks have been observed in the Shibantan Member.

3. Methods and materials The Shibantan Member trace fossils were collected from an active quarry near Wuhe in the Yangtze Gorges area (30º 46’47.74” N, 111º02’28.89” E). Abundant trace fossils were observed on outcrops where stratigraphic orientation (up direction) was unambiguously identified. Numerous loose slabs and blocks containing trace fossils were collected (Figs. 3–4) from active pits where

ACCEPTED MANUSCRIPT thin-bedded limestone of the Shibantan Member is quarried for construction and pavement material. A few slabs were also collected from abandoned roofs, but they can be confidently ascribed to the

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Shibantan Member because of their distinctive lithology (bituminous limestone) and the abundance

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of vendotaenid fossils characteristic of the Shibantan Member. The stratigraphic orientation of the loose slabs can be inferred using sedimentary structures such as low-angle cross stratification and

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truncation of microlaminae. Selected specimens were thin-sectioned perpendicular to bedding

with inferred microbial mats (Figs. 5–6).

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surfaces in order to petrographically characterize the occurrence of fossils and their relationship

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Polished thin sections and slabs were examined using scanning electron microscopy (Figs. 7–8), conducted on an FEI Quanta 600 field emission environmental scanning electron microscope

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(ESEM) in high vacuum mode using secondary electron (SE) or backscattered electron (BSE) detectors. Energy dispersive X-ray spectroscopic (EDS) point spectra and elemental maps were

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generated using an integrated Bruker AXS QUANTAX 400 with a high-speed silicon drift detector. Identical operating conditions were maintained for all EDS analyses: 20 keV accelerating voltage, 5.0 spot size (a unitless measure of beam current and probe diameter), 11.5–12 mm working distance, and X-ray signal count-rates of 25–35 kcps. All elemental maps were acquired for 600 seconds live-time, and individual point spectra were collected for 100 seconds live-time. Elemental peaks from point spectra were identified and quantified (with zaf and Au-Pd coating corrections applied) using Bruker Esprit 1.9.2 software. Initial X-ray point spectra were collected from the burrow infill, intraclastic layers, and clayey/silty microlaminated layers (Table 1, Supplemental Table 1, Supplemental Fig. 1). Elemental maps were subsequently generated (Figs. 7–8) for all elements found at >1% (normalized weight percentage; nwp) in point analysis. Composite elemental maps (Figs. 7C and 8C–D) consist of a montage of individual maps that may vary in

ACCEPTED MANUSCRIPT relative scales; thus brightness values do not necessarily correspond to the same quantitative abundance across individual maps.

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Freshly cut surfaces that intersect burrow infill, intraclastic layers, and microlaminated

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layers were microdrilled for carbon and oxygen isotope analysis (Fig. 9), using a 1 mm drill bit on a bench press drill. Veins and vugs were avoided when powders were microdrilled. The

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carbonate powders were then allowed to react with 100% phosphoric acid at 70 °C in a

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MultiFlow-Geo headspace sampler device connected to an Isoprime 100 continuous flow isotope ratio mass spectrometer. Carbon and oxygen isotope compositions are reported in standard delta

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notation as per mil (‰) deviations from Vienna Pee Dee Belemnite (VPDB). Reproducibility from repeated analysis of international standards IAEA CO-1, IAEA CO-9, and NBS 18 was

2).

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better than 0.10‰ for δ13Ccarb and 0.22‰ for δ18Ocarb (Supplemental Table 2, Supplemental Fig.

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To measure bedding plane bioturbation intensity (or horizontal trace fossil density), burrows on slabs were photographed and then traced digitally using Adobe Illustrator CS5 ®. The digital traces (Fig. 3) were then imported to ImageJ (Schneider et al., 2012) to measure the area of bedding surface occupied by burrows and to calculate the percentage of bioturbated bedding surface (i.e., bedding plane bioturbation intensity). This method is essentially similar to that of Marenco and Bottjer (2011), however this study discarded the use of a grid. Vertical bioturbation intensity was measured in a similar way, but on surfaces cut perpendicular to bedding surfaces.

4. Microbial mats in the Shibantan Member Microbial mats are a ubiquitous feature in the Precambrian (Bottjer et al., 2000; Droser et al., 2002; Hagadorn and Bottjer, 1999; Seilacher, 1999) and have been recognized in the Shibantan

ACCEPTED MANUSCRIPT Member limestone (Chen et al., 2013; Weber et al., 2007). The trace fossils described in this paper come from a facies in the Shibantan Member that is characterized by parallel-bedded intraclastic

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limestone intercalated with clayey/silty, crinkled microlaminated layers. The interpretation of these

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crinkled microlaminated layers as remnants of amalgamated microbial mats has been discussed previously by Chen et al. (2013), and similar structures have been described from other Ediacaran

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successions (Bouougri and Porada, 2011). Originally referred to as “growth bedding’ by Pettijohn

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and Potter (1964), “biolaminated deposits” by Gerdes and Krumbein (1987), or more recently “biolaminates” by Noffke et al. (1996), these microbial structures are suggested to have formed

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during episodic establishment and growth of microbial mats. The process that creates biolaminates includes periods of non-deposition in which a microbial mat is established on a sediment surface,

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followed by periods of sedimentation in which the mat is covered by a thin layer of sediment, through which mat-constructing organisms can move upwards to establish a new microbial mat and

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repeat the cycle (Gerdes et al., 2000; Gerdes et al., 1991). Microbial mats, particularly cyanobacterial mats, contain a copious amount of extracellular polysaccharide substance and can trap fine-grained sediments such as clays and silts. Also, because of the high contents of water and organic carbon in microbial mats, it is expected that fossilized microbial mats would contain organic laminae that are sometimes rich in pyrite and can be locally deformed during compaction to generate crinkled lamination. The opaque material in the microlaminated layers of the Shibantan Member, as seen in petrographic thin sections (Figs. 2D, 5A), is likely organic carbon and/or pyrite. In particular, the presence of pyrite and its weathering products (iron oxides) is consistent with the field observation that freshly excavated bedding surfaces of Shibantan limestone are commonly rusty in color due to the abundance of iron oxides. The pyrite was likely derived from the degradation of organic matter in the microbial mats (Kilburn

ACCEPTED MANUSCRIPT and Wacey, 2011; Laflamme et al., 2011; Schieber, 2007). The interpretation of the crinkled microlaminae in the Shibantan Member as amalgamated microbial mats is supported by the

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wrinkled bedding surfaces (hence crinkled microlaminae), the preferential concentration of clays

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and silts, and the preferential presence of organic carbon and pyrite. Given the thickness of these crinkled and microlaminated layers (see Section 6 below for reconstruction of pre-compaction

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thickness), evidence for their cohesiveness, and the depositional environment of the Shibantan

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Member (above storm wave base), it is likely that they represent cyanobacterial mats. The preferential occurrence of clays and silts in the crinkled microlaminae of the Shibantan

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Member is intriguing. Although we believe that these clays and silts were probably detrital in origin and they were trapped in microbial mats, we cannot rule the possibility that some of these clays

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may have been authigenic in origin. Authigenic aluminosilicate minerals have been suggested to play an important role in the preservation of soft-bodied organisms (Anderson et al., 2011;

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Butterfield, 1990; Cai et al., 2012; Laflamme et al., 2011; Meyer et al., 2012; Orr et al., 1998) and it has been demonstrated experimentally that microbial mediation can facilitate the formation of authigenic clays in microbial mats or biofilms (Darroch et al., 2012). Given the abundance of clays in the crinkled microlaminae of the Shibantan Member, however, it is likely that most of them were detrital in origin. The intervening intraclastic layers have little to no aluminosilicates and show sharp boundaries with the microlaminae (Figs. 5–6), probably representing episodes of rapid sedimentation that did not allow for the establishment of microbial mats (Bouougri and Porada, 2011; Schieber, 2004). Such cycles have been observed in Proterozoic sediments and modern subtidal biolaminates and have been hypothesized to be the result of seasonal sediment fluxes (Bouougri and Porada, 2011; Eriksson et al., 2005a; Eriksson et al., 2005b; Gerdes et al., 2000; Murphy and Sumner, 2008; Porado and Bouougri, 2007).

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5. Systematic paleoichnology

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The Shibantan Member is the most fossiliferous member of the Dengying Formation

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(Weber et al., 2007; Zhao et al., 1988), with not only trace fossils, but also macroscopic Ediacaralike fossils such as Paracharnia (Sun, 1986) and Yangtziramulus (Shen et al., 2009; Xiao et al.,

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2005). Additionally, the correlative Gaojiashan Formation in southern Shaanxi Province, contains

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tubular fossils such as Cloudina, Conotubus, Gaojiashania, Shaanxilithes, and Sinotubulites (Cai et al., 2010; 2011, 2012; Chen et al., 2008; Hua et al., 2005; 2003; Meyer et al., 2012). Of these,

(Chen and Wang, 1977; Chen, 1999).

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Sinotubulites has also been found in the base of the Baimatuo Member in the Yangtze Gorges area

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Until recently, only simple trace fossils have been reported from the Shibantan Member. The most common forms are Palaeophycus/Planolites-type traces (Weber et al., 2007). These traces

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are horizontal, generally 5–8 mm in width, and sometimes extremely flat. These are thought to be shallow feeding burrows of early metazoans (Weber et al., 2007). Additionally, Torrowangea and Helminthoidichnites have also been found in the Shibantan Member. Torrowangea is gently curved traces with a width of ~1 mm and irregularly spaced transverse constrictions (Weber et al., 2007). Helminthoidichnites-type trace fossils have a larger size range, 2–7 mm in width and reaching several decimeters in length. They are straight to sinusoidal traces, lack any ornamentation, and can be preserved as negative epi- or hyporeliefs. Helminthoidichnites in the Neoproterozoic is thought to be broadly comparable to the ichnogenera Helminthopsis and Gordia, in that they all represent foraging behavior or undermat mining activities (Jensen, 2003). Recently an assemblage of trace fossils has been discovered in the Shibantan Member, in preferred association with lithified microbial substrates (Chen et al., 2013). This assemblage

ACCEPTED MANUSCRIPT includes three types of trace fossils: surface trackways, vertical traces, and horizontal tunnels. It is inferred that the same bilaterian animal was responsible for creating these three types of traces

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representing locomotion, resting, and undermat mining behaviors, respectively. Below we provide a

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systematic description of these trace fossils, incorporating the three different types of traces into a single paleoichnological species based on their shared trace maker in order to better understand the

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ecological behavior of the trace maker. These trace fossils and associated thin sections are

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reposited in the Virginia Polytechnic Institute Geoscience Museum (VPIGM).

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Ichnogenus Lamonte igen. nov.

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Diagnosis. As for type species.

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Type and only ichnospecies. Lamonte trevallis igen. and isp. nov.

Etymology. Ichnogenus name derived from Italian la monte (mountain) with reference to the

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mountainous region near the fossil locality.

Lamonte trevallis igen. and isp. nov. Figures 3–8

2013 Unnamed bilaterian animal trace fossils; Chen et al., figs. 2, 3, 5, 6, 8.

Diagnosis. A network of millimeter-sized surface tracks and trails (parallel series of sharp scratch marks or shallow pits), horizontal lined burrows (composed of straight to gently curved elements filled with intraclasts and cements), and vertical traces found in association with crinkled and microlaminated layers.

ACCEPTED MANUSCRIPT Etymology. Ichnospecies epithet derived from Latin tres (three) and vallis (valley or gorge) with reference to the Three Gorges near the fossil locality and the tripartite nature of the trace fossil.

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Material. Five slabs (VPIGM-4642–4672) and numerous fragmentary specimens from the middle part of the Shibantan Member of the Dengying Formation collected in a quarry just west of

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Wuhe, Hubei Provence, South China (30º 46’47.74”N, 111º02’28.89”E).

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Holotype. The arrowed specimen in Fig. 4E (VPIGM-4643) is designated as the holotype.

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Description. The trace fossil consists of three different morphotypes (horizontal burrows, surface trails, and vertical traces) that are sometimes connected with each other (Chen et al., 2013).

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The horizontal burrows are straight to gently curved, 3–8 mm in width, 1.5–4 mm in depth, up to 30 cm long, and elliptical or bilobed in transverse cross-section. An individual burrow maintains a

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constant width along its length, although it may taper at both ends when the burrow moves out of the plane of preservation. Burrows have a smooth or rough surface, and some seem to have paired

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knobs (Chen et al., 2013). Some burrows feature a prominent central longitudinal groove (Chen et al., 2013), though this feature is not present in all horizontal burrows and could have formed during collapse of the tunnel (see fig. 7 in Chen et al. 2013). Burrows commonly overlap each other (Figs. 6C, 8) and rarely cross-cut each other (Chen et al., 2013). They may pass through or terminate at nodes or vertical traces (fig. 4A of Chen et al., 2013). The vertical traces are cylindrical and about 5 mm in depth, and 3–9 mm in diameter (Chen et al., 2013). Vertical traces project through the crinkled laminae and are exposed on the bedding surface (fig. 6A, B, D of Chen et al., 2013). The vertical traces often truncate or distort microlaminae (fig. 6F of Chen et al., 2013). Nodes have flat tops and bottoms (Figs. 6D–E) and are similar to the vertical traces in size (up to 6 mm wide and 6 mm deep), but they may not project above the crinkled laminae layers. Occasionally, multiple burrows appear to radiate from a node

ACCEPTED MANUSCRIPT (Fig. 4A, 4C, 4E), but this is an artifact due to the compaction of overlapping burrows; thin sections cut perpendicular to bedding plane show that the burrows are instead stacked upon each other,

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creating a feature that is superficially similar to a node (Figs. 6C, 8).

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The horizontal burrows are preserved as full relief tunnels, with the interior filled by micrite (Figs. 4D, 5A, 6B, F, 8) or intraclasts (Figs. 5E, H, 6D, H), and sometimes lined with the remnants

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of a thin organic layer (Fig. 5G, I). Regardless, the infill is cemented by diagenetic calcite and

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silica. Typically, silica cement is present in the center of the burrow, suggesting that it represents a later stage cementation than calcite cement. Some burrows have bitumen within their centers

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(arrows in Figs. 5A, G, 6B). Such bitumen probably derived from oil migration that filled the residual voids following calcite and silica cementation. The bitumen is optically visible (Fig. 5A),

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and it can be observed in the Ca map (Fig. 7) although not easily observed in the C map because the abundance of calcite in the burrow infill.

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Burrows occur exclusively in clayey and silty, crinkled, microlaminated layers that are interpreted as amalgamated microbial mats (Chen et al., 2013). In all cases, burrows either truncate or split microlaminae (Figs. 5F, G, I, 6A, G), indicating that sediments were excavated and disrupted within amalgamated microbial mats (see below). Microlaminae above and below the burrows warp around the tunnels, indicating that the burrow casts are more resistant to compaction than microlaminated and intraclastic layers. Thus, the burrows did not collapse immediately after the passage of the trace maker, which suggests a certain degree of structural integrity to allow diagenetic cementation of the open tunnels and passive sediment infilling. The surface tracks/trails are 4–6 mm wide and always preserved in negative epirelief or positive hyporelief. Trails consist of two parallel grooves, whereas tracks consist of two parallel series of either sharp scratch marks (fig. 5A–D of Chen et al., 2013) or small knobs in hyporelief

ACCEPTED MANUSCRIPT (fig. 5F of Chen et al., 2013). The sediments between the parallel series surface marks are leveled with the bedding surface (fig. 5A–B of Chen et al., 2013). The two series of knobs and scratch

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marks in a surface track are separated by 3–6 mm, and they are not arranged symmetrically (fig.

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5A–D of Chen et al., 2013). The surface tracks are sometimes connected with horizontal tunnels (figs. 3A, 5A of Chen et al., 2013).

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Discussion. Lamonte trevallis’ morphologies include similar characteristics observed in

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some other Ediacaran and Cambrian traces. Comparison of L. trevallis to Archaeonassa, Bilinichnus, Diplichnites, and Psammichnites has been discussed previously by Chen et al. (2013).

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To briefly summarize, the bilobed tunnels of Lamonte trevallis are similar to Archaeonassa and Psammichnites. However, Archaeonassa is preserved in epirelief (Jensen, 2003; Yochelson and

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Fedonkin, 1997), unlike the full relief preservation found in L. trevallis. Psammichnites is similar to L. trevallis in its full relief preservation and its bilobed transverse profile, but L. trevallis lacks the

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finer transverse ridges characteristic of Psammichnites (Seilacher and Hagadorn, 2010). L. trevallis burrows sometimes have a medial ridge/groove, which can be seen in Psammichnites where such a medial groove is interpreted as the moving trace of an infaunal trace maker’s siphon or proboscis extending above the sediment surface (Seilacher and Hagadorn, 2010), but it is uncertain whether in L. trevallis this medial groove represents a physical collapse of the tunnel roof (Chen et al., 2013) or the presence of a siphon. The surface tracks and trails are somewhat similar to Diplichnites, which consists of two series of closely spaced, elongate, ellipsoidal to circular imprints (Buatois et al., 1998; Pollard et al., 2008; Seilacher, 2007). However, the knobs and scratch marks of L. trevallis are less distinctive and less regularly arranged than those of Diplichnites, and there is no evidence relating any of the observed L. trevallis features to arthropod appendages.

ACCEPTED MANUSCRIPT Broadly, the unornamented horizontal burrows appear similar to Planolites, Palaeophycus, and Helminthoidichnites (Pemberton and Frey, 1982; Weber et al., 2007). Lamonte trevallis is

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similar to Planolites in that their burrows are filled with material (e.g., calcite and silica cements)

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different from the surrounding host rock, but the presence of intraclasts (derived from surrounding sediments) in the burrow infill is similar to Palaeophycus (Pemberton and Frey, 1982).

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Helminthoidichnites, like L. trevallis, can also be preserved in full relief and is commonly

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associated with microbial fabrics, both in the Shibantan limestone and elsewhere in the late Ediacaran and early Cambrian (Buatois and Managano, 2004; Buatois and Mángano, 2012; Weber

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et al., 2007). Unlike Helminthoidichnites, Planolites, or Palaeophycus, however, L. trevallis burrows are connected to vertical traces and surface tracks consisting of two series of knobs and

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scratch marks (fig. 5 of Chen et al., 2013).

Branching treptichnids from uppermost Ediacaran strata in Namibia are somewhat similar to

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L. trevallis in that they both have a (near) circular cross section, well defined knobby structures, and vertical elements (Jensen et al., 2000). However L. trevallis burrows and tracks are not made of a single row of elongate cylindrical or spindle-shaped elements as in treptichnids. Some Ediacaran and early Cambrian trace fossils described as Olenichnus have a vertical shaft connected to a burrowing system (Buatois and Mángano, 2012; Fedonkin, 1990; Jensen, 1997; Smith et al., 2003), but the burrow network of Olenichnus is more complex than the Shibantan tunnels (Chen et al., 2013). Though not found near the Ediacaran-Cambrian boundary, Thalassinoides shares some characteristics with L. trevallis, such as vertical shafts connected to an infaunal burrow network, ‘turn arounds’ which are larger, sub-surface cavities for dwelling within the horizontal burrow network (possibly similar in structure to L. trevallis nodes), and tunnel linings (seen in the related Granularia) (Myrow, 1995; Seilacher, 2007). These similarities are superficial, as Thalassinoides is

ACCEPTED MANUSCRIPT predominately associated with decapod arthropods, its burrow networks can be massive in scale (>1 m scale) and commonly have Y- and T-shaped junctions, and Thalassinoides in Cenozoic deposits

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is typically found in intertidal rather than subtidal environments (Myrow, 1995).

6. Analytical results

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The bedding plane bioturbation intensities on the five analyzed specimens are 20–40%

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(average 30%). In vertical cross-section, the percentage of bioturbated area is ~5%; the lower percentage relative to bedding surface analysis is due to the fact that the burrows are exclusively

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restricted to the microlaminated layers and thus many layers are undisturbed. The degree of bioturbation, as measured on the bedding surfaces, is comparable to pre-trilobite Cambrian

al., 2008; Skinner, 2005).

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sediments (Droser and Bottjer, 1988; Droser et al., 1999; Marenco and Bottjer, 2011; Marenco et

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EDS spectral data of burrow fills, microlaminated layers, and intraclastic layers are presented in Supplemental Table 1 and Supplemental Fig. 1. As expected of limestone sediments, Ca, O, and C are the major elements. Minor constituents, <1% in total nwp, include Al, K, Fe, Mg, P, and S. Microlaminated layers exhibit higher levels of Si, Al, Fe, and K than intraclastic layers, consistent with the presence of silts and clays in the microlaminated layers. The burrow infill shows higher levels of Si than both the intraclastic and microlaminated layers, likely due to the presence of silica cements in the burrows. The elemental distribution patterns were also confirmed by elemental maps (Figs. 7–8). Stable isotope data from burrow fills, microlaminated layers, and intraclastic layers are presented in Fig. 9 and Supplemental Table 2. Overall, intraclastic layers have the highest δ13Ccarb and δ18Ocarb values (average at 3.9‰ and –3.8‰, respectively), microlaminated layers have the

ACCEPTED MANUSCRIPT lowest values (average at 2.0‰ and –6.0‰, respectively), and burrow infill falls in between. The combined data array shows a positive correlation between δ13Ccarb and δ18Ocarb (R2 = 0.8),

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suggesting a two-component mixing between a diagenetic cement end-member (with lower δ13Ccarb

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and δ18Ocarb values) and intraclastic/micritic end-member (with higher values recording primary

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marine signatures).

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7. Taphonomy

The exclusive occurrence of Lamonte trevallis traces with microlaminated layers suggests a

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close association between the trace makers and microbial mats, and has important taphonomic and paleoecological implications. The microbial mats probably facilitated the preservation of the

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burrows in several ways. It has been argued that the prevalence of microbial mats and the limited sediment mixing by shallow burrowers resulted in firm substrates and sharp geochemical gradients

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across the sediment-water interface, permitting exceptional preservation of shallow tiers of burrows in the Ediacaran and early Cambrian periods (Droser et al., 2002; Schieber, 2007; Seilacher, 1999; Seilacher et al., 2005). This model is certainly applicable to the Shibantan traces. The firm substrate of the microbial mats, as well as possible linings (Figs. 5I, 7C), would have aided the burrows structural integrity even after they were buried. Subsequently, the burrows were partially filled by intraclastic (and micritic) sediments that were introduced from the overlying intraclastic layers through the vertical traces. This initial infilling made the burrows relatively more resistant to compaction compared to the surrounding microlaminated layers. Finally, after the burrow infill and surrounding microlaminated layers were differentially compacted, diagenetic calcite cementation occurred in both burrow infill and microlaminated layers, and residual void space in the burrows was filled by later diagenetic silica cementation.

ACCEPTED MANUSCRIPT The sequence of taphonomic events described above is supported by carbon and oxygen isotope data (Fig. 9). We propose that the positive δ13C and mildly negative δ18O values of the

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intraclastic layers represent normal marine signatures; similar values are typical of late Ediacaran marine carbonates elsewhere in the world (Halverson et al., 2010). Carbonate in the microlaminated

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layers has significantly lower δ13C and δ18O, at least in part due to the relative abundance of calcite

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cements (under the influence of organic degradation and meteoric fluids) compared with marine

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micrites and intraclasts. The intermediate δ13C and δ18O values of the burrow infill fall along a mixing line in the δ13C–δ18O cross-plot between the intraclastic and microlaminated layers,

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suggesting both intraclasts and diagenetic cements contributed to the burrow infill, an inference in perfect agreement with petrographic observations and the taphonomic scenario proposed above.

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The taphonomic model proposed above also allows us to estimate the original (precompaction) thickness of the microlaminated layers. Assuming the burrows were originally

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cylindrical, their elliptical profile in transverse cross-section suggests that they were compacted, although the degree of compaction is less than that of the surrounding microlaminae that warp around the burrows. The original diameter of the burrows can be estimated by taking the geometric mean of the major and minor axes of the elliptical transverse cross section, which then allows an estimate of the compaction ratio of the burrow to be calculated (~1:1.5). A comparison of the restored diameter of burrows with the thickness of surrounding microlaminae extending beyond the burrows allows an estimate of the compaction ratio of the microlaminated layers (~1:9.5). Assuming that the microlaminated layers (mostly ~0.5–1.5 mm thick) represent amalgamated microbial mats, this compaction ratio indicates that the amalgamated microbial mats were ~2–14 mm in original thickness. This is within the thickness range of both modern subtidal cyanobacterial mats (Gingras et al., 2011; Jahnert and Collins, 2011, 2012; Murphy and Sumner, 2008) and

ACCEPTED MANUSCRIPT Neoproterozoic examples in subtidal marine settings similar to the Shibantan Member (Bouougri

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and Porada, 2011; Préat et al., 2011; Schieber, 1986).

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8. Paleoecology

It may prove to be a challenging task to determine the paleoecological motives underlying

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the association of L. trevallis traces with microbial mats. However, several possibilities are worth

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consideration. Traditionally, microbial mats have been regarded as an important nutrient resource for foragers in the Ediacaran Period (Seilacher, 1999). Evidence of undermat mining can be found

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in modern environments (Tarhan et al., 2013) and their traces are common in the late Ediacaran Period, recording the activities of foraging animals to feed on concentrated organic carbon within

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and underneath microbial mats (Gehling, 1999; Seilacher, 2007). If nutrients were the only ecological drive, however, there would be no need for the feeding animals to tunnel within the mats.

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They could have instead grazed on the surface of microbial mats, as many modern and Phanerozoic animal grazers do, particularly when predation pressure and competition for nutrients in Ediacaran benthic communities were minimal. However, the surface tracks and trails of L. trevallis appear to be locomotive, lacking evidence indicative of a surficial foraging behavior (Seilacher, 2007; Seilacher et al., 2005).

While it is possible that the trace makers of Lamonte trevallis may have exploited organic nutrients within mats, it seems that the utilization of oxygen resources within cyanobacterial mats can better explain most features of L. trevallis traces (Gingras et al., 2011; Seilacher, 1999). Gingras et al. (2011) provided an actualistic study of microbial mat conditions that may have been analogous to those during the late Ediacaran of South China, such as the presence of microbial mats, a low-oxygen water column, and relatively low levels of bioturbation (Canfield and Farquhar,

ACCEPTED MANUSCRIPT 2009; Canfield et al., 2007). Gingras et al. (2011) found that, in anoxic waters, microbial mats create an “oxygen oasis”, or localized enrichment in dissolved oxygen within the mat. However,

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such oases are limited in space and time. They exist only a few millimeters below the surface of the

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mats during the day time. During the night, dissolved oxygen levels in the mat drop below levels in the overlying water column due to the cessation of photosynthesis and consumption of oxygen

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through the oxidation of reduced species (e.g. hydrogen sulfide) produced by heterotrophs in the

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mat (Gingras et al., 2011). These fluctuating conditions present a challenge for animals burrowing into the microbial mat to exploit the oxygen oases. The burrowing animals may need to ventilate or

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even escape from the tunnel during the night when conditions in the microbial mats became potentially lethal due to the buildup of hydrogen sulfide. Therefore, the need to ventilate, vacate, or

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otherwise mitigate sulfidic conditions within the mat may explain the frequent occurrence of vertical elements in L. trevallis traces; these vertical elements may have functioned as a chimney to

unfavorable.

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ventilate the burrow system or as an exit from the burrow system when conditions became

When a microlaminated layer of the Shibantan Member is burrowed, the bedding surface bioturbation intensity (22–41%) is comparable to that of pre-trilobite Cambrian sediments (Desjardins et al., 2010; Dornbos et al., 2004; Marenco and Bottjer, 2008). However, bioturbation intensities measured perpendicular to bedding surfaces are low (~5%), roughly equivalent to ichnofabric index 2 of Droser and Bottjer (1986). This discrepancy is largely due to the shallow penetration of L. trevallis traces and the low frequency of burrowed beds. Future research should investigate whether the shallow penetration depth and the low frequency of burrow beds were controlled by low and fluctuating oxygen levels in Ediacaran oceans.

ACCEPTED MANUSCRIPT 9. Conclusions Previous reports of trace fossil assemblages from the Dengying Formation have concluded

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that they are predominately characterized by simple horizontal traces. In this paper we describe the

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new ichnotaxon Lamonte trevallis, which is a network of three trace fossil elements (a horizontal burrow, a vertical trace, and a surface track/trail) that are likely made by the same bilaterian

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animals. The horizontal burrow is generally elliptical or bilobed in transverse cross-section, which

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sometimes passes through or terminates at nodes or vertical traces. Horizontal burrows are preserved in full relief, infilled by intraclasts, micrites, diagenetic calcite and silica cements. These

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burrows occur exclusively in clayey and silty crinkled microlaminae that are interpreted as the remnants of cyanobacterial mats. Surface tracks/trails consist of two parallel grooves or two parallel

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series of sharp scratches or shallow pits that correspond to small knobs in hyporelief. The close association of Lamonte trevallis with microbial mats has important taphonomic

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and ecological significance. On the basis of petrographic observations and stable isotope data, we propose a series of taphonomic events that contributed to the preservation of L. trevallis, including the maintenance of burrow structural integrity by firm mat-grounds, burrow infill with intraclastic and micritic sediments, sedimentary compaction, and final cementation by diagenetic calcite and silica. We also propose that L. trevallis trace makers may have tunneled into the microbial mats mainly to exploit both oxygen and nutrient resources. Lamonte trevallis trace makers may have developed behaviors more advanced than most other Ediacaran animals, including under-mat tunneling, epibenthic locomotion, and temporary dwelling. Although L. trevallis burrows are relatively small in size, shallow in penetration depth, and infrequent in stratigraphic distribution, the bedding plane bioturbation intensity on burrowed beds is comparable to that of pre-trilobite Cambrian carbonates. When combined with other finds of late Ediacaran traces representing more

ACCEPTED MANUSCRIPT complex behaviors (Gehling et al., 2001; Jensen and Runnegar, 2005; Jensen et al., 2000), L. trevallis provides additional evidence that a major change in the benthic ecosystem was under way

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during the late Ediacaran Period.

Acknowledgements

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This work was supported by National Science Foundation, Chinese Academy of Sciences,

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National Natural Science Foundation of China, and Chinese Ministry of Science and Technology. Constructive comments by Lidya Tarhan, Dave Bottjer, and an anonymous reviewer greatly

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improved this manuscript.

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tectonic units in China and Ediacaran paleogeographic map of the Yangtze Craton showing facies

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distribution. Adopted from Jiang et al. (2011). (B) Stratigraphic column of the Doushantuo and Dengying formations showing stratigraphic occurrences of trace fossils in the Shibantan

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Member of the Dengying Formation. Modified from Chen et al. (2013). Radiometric dates from

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Condon et al. (2005), Jiang et al., (2009), and Zhu et al., (2009). HMJ: Hamajing Member. (C) Google Earth view showing quarry location west of Wuhe. Rectangle denotes fossil site. (D) View

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looking southeast to fossil site. Scale bar in C = 0.25 km.

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Fig. 2. Sedimentary structures of the Dengying Formation. (A) Tepee structures from the Hamajing Member. (B) Dissolution vugs from the Hamajing Member. (C) Closer view of the thin-bedded

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limestone of the Shibantan Member in the quarry where most of the trace fossils were collected. (D) Transmitted light photomicrograph of microlaminated (white bracket) and intraclastic layers (black bracket). (E) Cross stratification in the Shibantan Member. (F) Rip-up clast and convoluted bedding in the Shibantan Member. (G) Breccia and dissolution vugs in the Baimatuo Member. Scale bars in A–C, E–G = 10 cm, and in D = 1 mm.

Fig. 3. Light photographs and corresponding interpretive drawings of trace fossil-bearing slabs, on inferred top bedding surface. Lamonte trevallis traces are marked by solid black lines filled with gray color (apparent discrepancies between line drawings and their respective photographs are due to lighting angle), smaller trace fossils are marked in black lines (e.g., Fig. 3A and 3D). Rectangles

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(D–E) VPIGM-4670. Scale bars = 5 cm.

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Fig. 4. Photographs of trace fossils on inferred top bedding surface. (A) Magnified view of box area in Figure 3C showing several burrows overlapping at one spot, VPIGM-4668. (B) Magnified view

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of box area in Figure 3D (rotated counter clock-wise 90 degrees), VPIGM-4670. (C) Slab with

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multiple burrows, VPIGM-4653. (D) Oblique view of tunnels on bedding surface and in crosssection, showing burrows enveloped by light-colored clay layer, VPIGM-4653. (E) Magnified view

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of box area in Figure 3A showing overlapping burrows. Labeled lines denote cross sections featured

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VPIGM-4642. Scale bars = 1 cm.

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in Figs. 5 and 6. Arrow points to holotype of Lamonte trevallis, VPIGM-4643. Slab pictured is

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Fig. 5. Transmitted light photomicrographs of burrows and vertical traces in cross-sections perpendicular to bedding plane, all shown with inferred stratigraphic up direction on top. (A) Bilobed burrow filled with micritic sediments, cements, and bitumen (arrow). Boxes denoting areas magnified in Fig. 5B–D, VPIGM-4644. (B) Closer view of grains in intraclastic layers. (C) Closer view of crinkled microlaminae (white arrow) in microlaminated layers (white bracket). (D) Micritic burrow infill (arrow) with some cements (arrowhead). (E) Burrow filled with intraclasts. Outline of burrow in dashed line. Box denotes area magnified in Fig. 5H, VPIGM-4669. (F) Two overlapping burrows (outlined by dashed lines), one of which is obliquely cut and filled with intraclasts (black arrow), whereas the other is filled mostly with micrites (white arrow). Burrows are separated by a microlaminated layer, VPIGM-4645. (G) Burrow with micrites, cements, and bitumen (arrow). Box denotes area magnified in Fig. 5I, VPIGM-4646. (H) Magnified view of Fig. 5E showing intraclasts

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(arrows). Scale bars in A = 5 mm, B–D = 100 µm, E–G = 5 mm, H–I = 1 mm.

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Fig. 6. Transmitted light photomicrographs of burrows and vertical traces in cross-sections perpendicular to bedding plane, all shown with inferred stratigraphic up direction on top. (A)

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Bilobed burrow filled with micrites and cements. Box denotes area magnified in Fig. 6G, VPIGM-

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4647. (B) Partially exposed burrow filled with micrites, some intraclasts, cements, and bitumen (arrow), VPIGM-4654. (C) Overlapping burrows, with the lower unexposed burrow cut obliquely.

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Arrows point to breaks in the unexposed burrow, probably broken during compaction, VPIGM4648. (D–E) Different views of the same node showing connection with horizontal burrow to the

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left. Dashed lines represent node outlines. Arrows point to microlaminae truncation (black), splitting (white), and covering node (yellow). (D) Node, VPIGM-4655. Box denotes area magnified

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in Fig. 6H. (E) Outer edge of node with an irregular outline (dashed line), VPIGM-4656. (F) Unexposed burrow filled with micrites and cements. Dashed lines represent burrow outline, VPIGM-4671. (G) Magnified view of box in Fig. 6A, showing truncation of microlaminae (arrow). (H) Magnified view of box in Fig. 6D, showing intraclasts, micrites, and cement (arrowhead) in burrow infill. Scale bars in A–E = 5 mm, F–H = 1 mm.

Fig. 7. SEM images and EDS elemental maps of Lamonte trevallis burrow in cross-section (see also Fig. 5A), VPIGM-4644. (A) SE image. (B) BSE image. (C) Composite elemental maps. Thin dashed lines represent boundaries of individual elemental maps. Thick dashed lines in Mg map outline microlaminae and burrow. All scale bars = 1 mm.

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bedding plane, all shown with inferred stratigraphic up direction on top, VPIGM-4657. Slab

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illustrated in Figure 4C. (A) Transmitted light image. (B) BSE image. Solid and dashed lines denote sedimentary layers and burrows, respectively. Boxes denote elemental map areas in Fig. 8C–D. (C)

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Composite elemental maps of labeled boxes in Figure 8B. (D) Elemental map of labeled box in

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Figure 8B. Dashed lines represent boundaries of individual elemental maps. Scale bars in A–B = 1

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cm, in C–D = 1 mm.

Fig. 9. δ13C and δ18O of microlaminated layers, intraclastic layers, and Lamonte trevallis infill

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carbonates.

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Table 1. Average elemental concentrations (in normalized weight percentages) from EDS point analyses of intraclastic layers, burrow infill, and microlaminated layers. EDS measurements of carbon concentrations are not exact due to the low X-ray energy of carbon (Kα = 0.277).

Table 1 Average ± standard error (1σ) Type

Intraclastic layer L. trevallis infill Microlaminated layer All Samples

C

O

Mg

Al

Si

P

S

K

Ca

Fe

1.5 ± 0.1 2.5 ± 0.4 2± 0.4 2.1 ± 0.3

47 ± 0.6 48.1 ± 0.5 47.5 ± 0.8 47.6 ± 0.3

0.7 ±0 0.6 ±0 0.7 ±0 0.6 ±0

0.3 ± 0.1 0.2 ± 0 1± 0.2 0.5 ± 0.2

0.5 ± 0.1 4.9 ± 1.1 2.4 ± 0.6 2.8 ± 1.3

0.3 ± 0.1 0.1 ± 0 0.3 ± 0 0.2 ± 0

0.2 ± 0 0.3 ± 0.1 0.3 ± 0.1 0.3 ± 0

0.2 ± 0 0.2 ± 0 0.7 ± 0.1 0.4 ± 0.2

49.2 ± 0.7 42.9 ± 1.4 44.8 ± 1.2 45.4 ± 1.9

0.1 ± 0 0.1 ± 0.1 0.4 ± 0.1 0.2 ± 0.1

Tota l 100 ±0 100 ±0 100 ±0

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Highlights Lamonte trevallis is described from the Ediacaran Dengying Formation, S. China. L. trevallis occurs exclusively within remnants of microbial mats. Bedding plane bioturbation intensity is comparable to early Cambrian carbonates. Burrowers may have actively exploited oxygen and nutrient resources. L. trevallis provides evidence of a flourishing benthic community in the Ediacaran.

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