Chen Et Al, 2018 Late Ediacaran Trackways Produced by Bilaterian An
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SCIENCE ADVANCES | RESEARCH ARTICLE PALEONTOLOGY Copyright © 2018 The Authors, some Late Ediacaran trackways produced by bilaterian rights reserved; exclusive licensee animals with paired appendages American Association for the Advancement 1,2 1,3 2,4 1,2,3 5 of Science. No claim to Zhe Chen *, Xiang Chen , Chuanming Zhou , Xunlai Yuan , Shuhai Xiao * original U.S. Government Works. Distributed Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors. under a Creative Thus far, however, this fossil record has been limited to simple surface trails and relatively shallow burrows. We Commons Attribution report possible trackways, preserved in association with burrows, from the terminal Ediacaran Shibantan Member NonCommercial (ca. 551 to ca. 541 million years ago) in the Yangtze Gorges area of South China. These trace fossils represent the License 4.0 (CC BY-NC). earliest known trackways. They consist of two rows of imprints arranged in poorly organized series or repeated groups. These trackways may have been produced by bilaterian animals with paired appendages, although the phylum-level phylogenetic affinity of the trace makers remains unknown. It is possible that the trackways and associated burrows were produced by the same trace maker, indicating a complex behavior involving both walking and burrowing. Together, these trackways and burrows mark the arrival of a new era characterized by an increasing geobiological footprint of bilaterian animals. Downloaded from INTRODUCTION panarthropods) and represent an important category of ichnological Molecular clocks indicate that the kingdom Metazoa diverged in the architecture, are hitherto unknown in the Ediacaran Period [(9), Tonian Period [~1000 to 720 million years (Ma) ago], major superphyla but see Chen et al.(13)]. This pattern is in sharp contrast to the early of animals in the Cryogenian Period (~720 to 635 Ma ago), and most Cambrian ichnological record, where trackways are common and some bilaterian animal phyla in the Ediacaran Period (635 to 541 Ma ago) (1). seem to record locomotion activities of early arthropods (10, 16, 17). However, although molecular fossils of sponges have been reported Here, we report new trace fossils from the terminal Ediacaran http://advances.sciencemag.org/ from the Cryogenian Period (2), unambiguous bilaterian animals do Shibantan Member of the upper Dengying Formation (ca. 551 to not occur in the fossil record until the late Ediacaran Period (3). ca. 541 Ma ago) in the Yangtze Gorges area of South China (Fig. 1; Traditionally, many Ediacaran body fossils have been interpreted as see the Supplementary Materials for a description of the geological representatives of living animal phyla (4). In this traditional view, ar- and stratigraphic setting). These trace fossils include burrows and pos- thropods and annelids, both of which are expected to have paired ap- sible trackways that are preserved in close proximity and are apparently pendages, are featured prominently in the Ediacara biota. For example, connected. They were probably made by millimeter-sized animals with Dickinsonia and Spriggina have been interpreted as polychaetes, and bilateral appendages and can provide important insights into early Parvancorina has been interpreted as an arthropod (4). However, di- bilaterian evolution and behaviors. agnostic anatomical features supporting such interpretations—that is, parapodia that are inferred to be present in the last common ancestor on June 6, 2018 of annelids (5) and jointed appendages in the last common ancestor of RESULTS arthropods (6)—are either absent or not preserved in these fossils. The trace fossils described in this paper are preserved in dark gray, Thus, these traditional interpretations have been questioned (7), and thin-bedded, bituminous limestone of the ca. 551-Ma to ca. 541-Ma the current view is that few Ediacaran animal fossils are phylogeneti- Shibantan Member deposited in a subtidal environment between cally resolved at the phylum level (8). Thus far, no Ediacaran body fos- fair-weather and storm wave bases (15, 18). Like other trace fossils from sils have been convincingly shown to bear paired appendages. the same locality and horizon, they exclusively occur in beds character- The trace fossil record provides another archive that can be ex- ized by crinkled microlaminae, which are interpreted as microbial plored to gain insights into the early evolution of animals and ap- mats, suggesting intimate ecological interactions between the trace pendages (9). In general, most Ediacaran trace fossils are simple, makers and the mats (13, 15, 19). These microbial mats comprised a horizontal, unbranched trails and burrows, representing simple be- few millimeters of sediment and may have also helped trace fossil pres- haviors of cnidarian-grade and simple worm-like animals that lived ervation (20, 21). close to the sediment surface (10, 11). Only a few Ediacaran trace fos- The material consists of co-occurring trackways (labeled TW1 and sils record moderately complex behaviors of trace-making animals TW2 in Fig. 2) and burrows (labeled UB1 to UB3 in Fig. 2) that were (12–15). Trackways, which are a series of impressions left by locomotion excavated in situ with known stratigraphic orientation and with both appendages (for example, parapodia of annelids and appendages of parts and counterparts. Thus, there is no ambiguity with regard to the identification of epirelief versus hyporelief preservation. 1State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Trackways Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. 2Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, The trackways are straight or slightly curved, each consisting of two Nanjing 210008, China. 3University of Chinese Academy of Sciences, Beijing 100049, rows of shallow depressions on the top bedding surface (that is, nega- China. 4CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing tive epireliefs; Fig. 2A) and two rows of projections on the correspond- Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing ing bottom surface (that is, positive hyporeliefs; Fig. 2B). Other than 210008, China. 5Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA. these depressions and projections, there is no discernible topographic *Corresponding author. Email: [email protected] (S.X.); [email protected] (Z.C.) relief or evidence for sediment disturbance between and beyond the Chen et al., Sci. Adv. 2018; 4 :eaao6691 6 June 2018 1of8 SCIENCE ADVANCES | RESEARCH ARTICLE Shuijingtuo Fm Cambrian Yanjiahe Fm Cambrian SSFs Tarim (~541 Ma) North China Mbr Baimatuo Sinotubulites Yangtze Cathaysia Ar-Pt Vendotaenia sp. Ediacaran Trackways Huangling Dengying Fm Ediacara fossils & anticline Shibantan Mbr trace fossils HMJ 551.1 ± 1.0 Ma (U-Pb) Miaohe biota Ar-Pt Acanthomorphic acritarchs 632.5 ± 1.0 Ma (U-Pb) Zigui Doushantuo Fm 635.3 ± 1.1 Ma (U-Pb) Wuhe 50 m 10 km Yangtze Yichang Cryogenian Nantuo Fm Downloaded from River 0 m Ar-Pt Chert and Dolostone Diamictite Cryogenian- Huangling Archean- Fault phosphorite Ediacaran granite Paleoproterozoic ABBlack shale Limestone Fig. 1. Geological map and stratigraphic column. (A) Generalized geological map of the Yangtze Gorges area, showing the distribution of Ediacaran strata and the http://advances.sciencemag.org/ fossil location at Wuhe (triangle). Inset map shows the location of the Yangtze Platform in South China. (B) Stratigraphic column of the Ediacaran Doushantuo and Dengying formations, showing stratigraphic occurrences of fossils, including the trackways (triangle) reported in this paper. Mbr, Member; Fm, Formation; SSF, small shelly fossil; HMJ, Hamajing Formation. Zircon U-Pb ages from the work of Schmitz (40). Figure produced by S.X. using Adobe Illustrator. two rows of depressions. Furthermore, TW2 crosses over but does not facing series that are at an acute angle (~15°) with respect to the midline. disrupt UB1 and UB2 (Fig. 2, A and C). Thus, TW1 and TW2 cannot Each series is 17 to 18 mm in length and consists of four or more tracks, be trails, partially preserved or partially excavated burrows, or collapsed although the exact number of tracks is difficult to determine. The op- burrows (fig. S2), because trails or burrows would disrupt the sediment posingly facing series are asymmetrical across the midline and are between the two lateral walls (fig. S2, A and C), they would cut into slightly staggered (or overlap) along the length. The apparent “stride” UB1 and UB2 when crossing over the latter (fig. S2, E and F), and (distance between two successive series along length) is ~13.7 mm. Dis- on June 6, 2018 the lateral walls of collapsed burrows would be preserved as positive continuous impressions seem to be present along the midline and epireliefs rather than negative epireliefs (fig. S2C). For the same rea- oblique scratches on one side of the trackway (Fig. 3, C and D; green sons, the depressions in TW2 cannot be outlets emerging from a hor- color in Fig. 3E), but it is uncertain whether these are distinct features. izontal burrow, for example, epichnial version of a treptichnid (10), as TW2 has a preserved trackway length of ~91 mm, an external width hasbeenproposedfortheCambriantracefossilBicavichnites that was of~16mm,andaninternalwidthof~4mm(Fig.4,AtoC).Tracksize otherwise interpreted as a trackway