https://doi.org/10.1130/G46474.1 Manuscript received 7 May 2019 Revised manuscript received 11 August 2019 Manuscript accepted 17 August 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 23 September 2019 Surfing in and on microbial mats: Oxygen-related behavior of a terminal Ediacaran bilaterian animal Shuhai Xiao1*, Zhe Chen2, Chuanming Zhou2 and Xunlai Yuan2* 1 Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24060, USA 2 State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China ABSTRACT and they are spaced with a gap in between Geochemical evidence suggests that terminal Ediacaran (ca. 551–539 Ma) oceans experi- (gap length 10.9–102.2 mm; Table DR1). Gaps enced expansive anoxia and dynamic redox conditions, which are expected to have impacted are featureless and flush with the bedding sur- animal distribution and behaviors. However, fossil evidence for oxygen-related behaviors of face. Segments are 13.0–84.0 mm in length terminal Ediacaran animals is poorly documented. Here, we report a terminal Ediacaran trace and 3.4–8.9 mm in maximum width, straight fossil that records redox-regulated behaviors. This trace fossil, Yichnus levis new ichnogenus (#1–2 in Fig. 2A) or slightly curved (arrow- and new ichnospecies, consists of short and uniserially aligned segments of horizontal burrows head in Fig. 2A), generally smooth (Figs. 2A that are closely associated with microbial mats. Thin-section analysis shows that the trace- and 3A), and taper on both ends. Although making animal moved repeatedly in and out of microbial mats, with mat-burrowing intervals segments can be curved, the ends of neigh- interspersed by epibenthic intermissions. This animal is hypothesized to have been a bilaterian boring segments in a chain match perfectly in exploring an oxygen oasis in microbial mats. Such intermittent burrowing behavior reflects orientation (#1–6 in Fig. 2B). challenging and dynamic redox conditions in both the water column and microbial mats, The segments are preserved as full reliefs highlighting the close relationship between terminal Ediacaran animals and redox dynamics. and can be split with the overlying or underly- ing beds, with the corresponding counterpart INTRODUCTION GSA Data Repository1 for the systematic preserving a negative mold (Figs. 2A, 2B, and Emerging geochemical data indicate an epi- paleontology; ZooBank urn:lsid:zoobank. 2E–2H). Observations made in longitudinal and sode of expansive oceanic anoxia in the terminal org:pub:991964F6-DB19–493A-A91D- transverse thin sections cut perpendicular to the Ediacaran Period (Evans et al., 2018; Tostevin BFB72973B6B1). Y. levis is preserved in bedding plane suggest (1) they are preserved in et al., 2018; Wei et al., 2018; Zhang et al., 2018), limestone of the terminal Ediacaran Shiban- close association with organic- and clay-rich cal- with spatially and temporally dynamic redox tan Member of the Dengying Formation at careous microlaminites interpreted as microbial conditions in shallow oceans (Wood et al., 2015). Wuhe in the Yangtze Gorges area of South mats (labeled “m” in Figs. 2E–2G and 3C–3F); It has been shown that such dynamic redox con- China (Fig. 1; Fig. DR1 in the Data Reposi- (2) they are filled with micritic and intraclastic ditions influenced the distribution of animals tory). Specimens were collected at two hori- sediments, but with a greater amount of cement (Tostevin et al., 2016). However, it has not been zons, ∼20 m and ∼70 m above the base of the than in the peloidal and intraclastic sediment in thoroughly investigated how terminal Ediacaran Shibantan Member. Y. levis is the only trace the matrix (Figs. 3C–3F); (3) they are circular to redox dynamics impacted animal behaviors. Ter- fossil thus far found at the lower horizon, but oblate in transverse cross section (Figs. 2H, 3C, minal Ediacaran ichnofossils are ideal to address it co-occurs with other as-yet-undescribed ich- and 3D), indicating that the burrows experienced this question because, as a record of animal be- notaxa at the upper horizon. Sedimentary fea- some degrees of postdepositional compaction; haviors (Buatois et al., 2016, 2017; Gehling and tures indicate that the Shibantan Member was and (4) there is no discernible lining or back- Droser, 2018), they are often exceptionally pre- deposited on a carbonate shelf, between fair- filled structures. served due to low levels of bioturbation (Droser weather and storm wave base (Meyer et al., To determine whether segments are con- et al., 2002). Here, we describe a new trace fossil 2014). Radiometric dates and biostratigraphic nected with each other through unexposed in- that bears on the behavior of terminal Ediacaran data constrain the Shibantan Member to be trastratal burrows, one of the specimens was bilaterian animals in response to redox dynamics. 551–538 Ma (see the Data Repository). cut across the gap between two segments. No Yichnus levis consists of disconnected fu- intrastratal structure was found in either the FOSSIL DESCRIPTION siform or spindle-shaped segments that are part or the counterpart slab (Figs. 2C and 2D). The fossil is described as Yichnus levis new either isolated (arrowhead in Fig. 2A) or This was also obvious upon close inspection of ichnogenus and new ichnospecies (see the aligned to form a curved uniserial chain (#1–6 the terminal ends of the segment: On both the in Figs. 2A and 2B). As many as six fusiform part and counterpart slabs, the segment appears *E-mails: [email protected]; [email protected]. segments are preserved in a chain (Fig. 2B), to direct away from the sediment (Figs. 2A and 1GSA Data Repository item 2019367, stratigraphic setting, methods, and systematic paleontology, is available online at http://www.geosociety.org/ datarepository/2019/, or on request from [email protected]. CITATION: Xiao, S., Chen, Z., Zhou, C., and Yuan, X., 2019, Surfing in and on microbial mats: Oxygen-related behavior of a terminal Ediacaran bilaterian animal: Geology, v. 47, p. 1054–1058, https://doi.org/10.1130/G46474.1 1054 www.gsapubs.org | Volume 47 | Number 11 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1054/4852010/1054.pdf by guest on 28 September 2021 111°E Shuijingtuo Fm surface, with segments exposed in troughs but N 526.4±5.4 Ma Cambrian SSFs buried beneath crests (e.g., Jensen, 1997, his Cam. Yanjiahe Fm Ediacaran-Cambrian Tarim boundary (~539 Ma) North figure 46A). Similarly, apparently disjointed China burrow segments could result from a continu- Mbr ous intrastratal burrow undulating in and out of Baimatuo Sinotubulites Yangtze Cathaysia the plane of preservation; this model has been proposed to explain the apparently disjointed 31°10'N 111°15'E 31°10'N Vendotaenia sp. Ar-Pt burrows of Treptichnus (Archer and Maples, Y. levis 1984; Buatois and Mángano, 1993; Jensen Dengying Fm Ediacara-type fossils et al., 2000). When these continuous burrows are Huangling & trace fossils Shibantan Mbr split, they may appear disjointed (Figs. DR2A– anticline Y. levis DR2B). However, combined observation of the Ediacaran 31°N 31°N HMJ part and counterpart should still reveal a contin- 551.1±0.7 Ma Miaohe biota uous burrow, whereas our thin-section observa- Ar-Pt tion revealed that the burrow segments of Y. levis are genuinely disjointed. Thus, the stratinomic Acanthomorphic acritarchs models that could make continuous burrows ap- 632.5±0.5 Ma pear discontinuous can be conclusively ruled out 30°50'N Zigui 30°50'N Doushantuo Fm 635.2±0.6 Ma for Y. levis. A third possibility is that the burrow Wuhe . 50 m 10 km Yangtze Yichang Nantuo Fm segments of Y. levis were made discontinuous River Cry by erosional processes (Figs. DR2C–DR2D). 0 m However, erosion would likely partially or en- Ar-Pt tirely remove microbial mats surrounding the chert and dolostone diamictite Cryogenian- Huangling Archean- Fault phosphorite burrow, whereas Y. levis is surrounded by in- Ediacaran Granite Paleoproterozoic ABblack shale limestone tact microbial mats above and below. Also, if a burrow were erosionally exposed before it was Figure 1. Geological map and stratigraphic column. (A) Generalized geological map of filled, it would be cast from above and preserved Yangtze Gorges area, China, showing distribution of Ediacaran strata and fossil location at Wuhe (star). Inset map shows major tectonic units (Yangtze, Cathaysia, North China, and as semireliefs rather than full reliefs. Finally, Tarim cratons) and Yangtze Gorges area (rectangle). (B) Stratigraphic column of Ediacaran to completely remove a segment of horizontal Doushantuo and Dengying Formations, showing stratigraphic range of fossils. Stars mark burrow, several millimeters of strata would have stratigraphic horizons of Yichnus levis. Mbr—Member; Fm—Formation; HMJ—Hamajing been differentially eroded, considering the size Member; SSF—small shelly fossil; Cry.—Cryogenian; Cam.—Cambrian. Zircon U-Pb ages in Shuijingtuo Formation are from Okada et al. (2014), and those in Doushantuo Formation of Y. levis. However, there is no evidence for are from Condon et al. (2005). erosion of this magnitude; the bedding surface is flat and smooth, largely defined by microbial laminae undisrupted by erosion. Thus, the most 2B). In other words, the segment terminates uniserial chains, and when multiple chains are likely scenario is that the burrow segments were at both ends, rather than going off-plane into found in the same bedding surface, the chains originally disjointed when they were made (Fig. either the part or counterpart slab; the termi- do not have consistent orientation (Fig. 3A), DR2E). Such short burrow segments with an nation is also clearly seen in longitudinal thin inconsistent with cylindrical body fossils that opening to the water column can also be more sections (Figs.