2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;
3. Chengdu Center of China Geological Survey, Chengdu 610081, China
The end-Permian mass extinction (EPME) is considered the largest catastrophic event of the Phanerozoic, causing a loss of ~95% of marine species and 81% of terrestrial vertebrate species (Stanley, 2016; Erwin, 2006; Erwin et al., 2002). There has been a consensus that this biotic crisis is closely associated with the devastated palaeoenvironmental conditions triggered by the mass eruption of Siberian Basalt during the Latest Permian (Shen et al., 2011). Due to the long persistence of such stressful environmental conditions in the marine realm, the biotic recovery in the aftermath of the EPME lingered for some 5 Ma (Zhang F F et al., 2018; Tian et al., 2014; Song et al., 2012). It was not until the Early Middle Triassic that fully recovered shallow-marine ecosystems were re-established (Chen and Benton, 2012), which is typically represented by the Luoping Biota of the Middle Triassic Guanling Formation of south-western China (Chen and Benton, 2012; Hu et al., 2011). Prolific vertebrate and invertebrate fossils from this biota revealed a highly developed shallow marine ecosystem after the EPME (Liu et al., 2014; Benton et al., 2013; Chen and Benton, 2012; Hu et al., 2011). In addition to the abundantly preserved body fossils, trace fossils were also widely distributed in the fossil-bearing units with high diversity and abundance (Luo et al., 2017a, b; Zhang et al., 2014; Benton et al., 2013). A comparison of trace fossil assemblages from the Luoping Biota and those from the Early Triassic suggests that trace makers might well have recovered immediately after the EPME (Luo et al., 2017b). Among the traces produced by decapod crustaceans (e.g. Rhizocorallium, Spongeliomorpha, and Thalassinoides) in Luo-ping, new trace fossil materials found recently revealed the occurrence of another branching trace with highly regular sinuosity, justifying a reasonable assignment to the crustacean burrow Sinusichnus sinuosus, which has not been discovered from South China until recently (Luo et al., 2017b). The aim of this paper is to describe the morphology of this new trace fossil in detail and discuss its palaeoecology. A summary of all reported occurrence of Sinusichnus has also been generated, with emphasis on the spatial and temporal distribution of this highly regularly branching trace.1 GEOLOGICAL SETTING AND STRATIGRAPHY
During the early Middle Triassic, the Luoping area was located on the southwestern part of the Yangtze Block and was separated from the Nanpanjiang Basin by a shoal complex (Enos et al., 2006; Lehrmann et al., 2005; Feng et al., 1997; Fig. 1a). Several spatially and temporally separated intraplatform basins or depressions (e.g., the Panxian, Luoping, Xingyi, and Guanling) have been discovered from the Late Anisian, Late Ladinian and Carnian intervals, respectively, yielding exceptionally preserved reptile fossils (Benton et al., 2013; Hu et al., 2011). The preservation of those fossil Lagerstätten was aided by particular palaeoenvironmental conditions in these basins, such as restricted circulation, density stratification of the water column, and dysoxic to anoxic bottom waters during the burial of the faunas (Benton et al., 2013). In Luoping, abundant marine reptile faunas and invertebrate fossils (including trace fossils) were preserved in dark-colored, micritic limestones representing the upper part of the Member Ⅱ of the Guanling Formation (Hu et al., 2011). The Guanling Formation is subdivided into two members. Member Ⅰ is composed of siliciclastic sediments, representing deposition in subtidal to intertidal environments (Hu et al., 1996). Member Ⅱ consists of micritic limestones, bioclastic limestones, oncoidal limestones and dolomites in the lower and middle parts, and black muddy limestones, cherty limestones, and grey dolomite in the upper part. The Guanling Formation in the Luoping area, overall, records a deepening upward sequence (Zhang et al., 2008).
Three excavated sections, namely the Dawazi (or Daaozi), Shangshikan, and Xiangdongpo revealed the well preservation of the Luoping Biota, which have been studied extensively in terms of the stratigraphy, sedimentology, palaeontology and taphonomy (e.g., Luo et al., 2017a, b, 2013; Feldmann et al., 2015, 2012; Liu et al., 2014; Schweitzer et al., 2014; Zhang et al., 2014; Benton et al., 2013; Huang et al., 2013; Wen et al., 2012a, b; Chen and Benton, 2012; Hu et al., 2011; Zhang et al., 2009, 2008; Fig. 1b). A thickly bedded limestone unit among these three sections aided the correlation among the three sections. The trace fossil studied herein was discovered from the Xiangdongpo Section. Three stratigraphic units were defined in this section, which represent deposition in shallow subtidal to offshore environment (Luo et al., 2017b; Fig. 2).
Biostratigraphical study suggests that the Luoping Biota is of Anisian Age. This is supported by both conodont zonation and bivalve assemblages. The Nicoraellakockeli conodont Zone has been found from the fossil bearing unit at the Dawazi, Shangshikan, and Xiangdongpo sections (Zhang et al., 2014; Bai et al., 2011a, b; Hu et al., 2011; Huang et al., 2009). This conodont zone includes elements such as Nicoraellagermanicus, Nicoraellakockeli and Cratognathodus sp., indicative of a Pelsonian Age of the Middle Anisian (Huang et al., 2009; Zhang et al., 2009). Meanwhile, the underlying Member Ⅰ of the Guanling Formation yields bivalve assemblages of Anisian Age in South China (Zhang et al., 2008). Further, several clay beds discovered from Member Ⅰ of the Guanling Formation were regarded as marker beds of the base of the Anisian in southwestern China (Zhang et al., 2009; Enos et al., 2006).2 DESCRIPTION OF TRACE FOSSIL MATERIALS 2.1 Morphology
Sinusichnus occurs at two horizons of the Xiangdongpo Section (Fig. 2). The first horizon shows scarce presence of Sinusichnus in association with large sized Thalassinoides (Fig. 2). This layer exhibits a nodular appearance on the field exposure. Sinusichnus burrows at this horizon are characterized by branched burrow systems, with burrow fillings having a darker color than the host rock. Burrow fillings of Thalassinoides are silicified, but fragments of shells and gastropods can be discerned. At the second horizon, Sinusichnus are present in the basal part of bioturbated wackestones with silicified nodules (Fig. 2). The well-preserved specimens at this horizon facilitate a detailed examination of trace fossil morphology. Specimens studied were preserved as positive or negative hyporeliefs and can be found at areas spanning several square decimeters. Horizontal burrows are unlined and show regular sinuous tunnels, while less regular to straight tunnels are also observed in the same branching system (Figs. 3a-3d). Burrow width is uniform in a single branching system. Y- or T-shaped branching pattern dominates (Figs. 3a-3b). On some occasions, two closely emplaced triple junctions form an H-like configuration (Fig. 4b). Branching points with four branches are also observed occasionally (Fig. 4a). The burrowing system penetrates shallowly into the sediments at very shallow depths (no more than 1.5 cm). Burrow surfaces are smooth to knobby. In the vertical profile, retrusive spreiten are not observed. Associated large Rhizocorallium are characterized by their long, tongue-shaped burrows, with occasional primary successive branching. These features, combined the dumbbell-like cross-section view justify them as R. commune (Knaust, 2013; Rodríguez-Tovar and Pérez-Valera, 2008). U-shaped R. commune is more abundant than Sinusichnus on the bedding plane. The marginal tubes of Rhizocorallium range from 18 to 25 mm in diameter. Sinusichnus burrows were locally cross-cut by Rhizocorallium or vice versa. There is no distinct color difference between the burrow filling of Sinusichnus, Rhizocorallium, and their host rocks.
Several mathematical approaches have been applied to the study of geometry and regularity of Sinusichnus (e.g., Belaústegui et al., 2014; de Gibert et al., 1999). These approaches include the measurement of wavelength (λ) and amplitude (A) of the sinuous tunnels within Sinusichnus burrow system. The diameter (d) is also measured (see Fig. 5a for details). Measurements of the former two were plotted in a bivariate diagram to investigate their correlations (Fig. 5b). Data of 18 measurements show a significant correlation between λ and A (R2= 0.724 5), which has been observed in other reported Sinusichnus (e.g., Belaústegui et al., 2014; de Gibert et al., 1999). The only difference is the value of slope. De Gibert et al. (1999) reported S. sinuosus materials from the Lower Pliocene strata in France and Spain, with a slope value at 5.19. Belaústegui et al. (2014) measured Sinusichnus specimens from six localities in Spain and revealed three slope values at around 1.1, 2.5 and 4.05. Diameters of Sinusichnus remain identical in each particular burrow system but varied slightly between different specimens. A total measurement of 102 specimens reveals a burrow width ranging from 4 to 16 mm, with an average value at 8.8 mm (Fig. 5c).3 DISCUSSIONS 3.1 Taxonomic Discussion
The newly discovered trace fossil materials are extremely similar to the ichnogenus Sinusichnus established by de Gibert (1996). This is revealed by its regular sinuous and branching morphology of horizontal tunnels. Except the type ichnospecies S. sinuosus established by de Gibert (1996), Kappel (2003) proposed another ichnospecies S. priesti based on trace fossil materials from the Upper Cretaceous strata in Germany. The only feature distinguishing S. sinuosus from S. priesti is the presence of bioglyphs on the latter. No scratching marks or bioglyphs have been observed in the Luoping specimens, thus allowing a reasonable assignment to S. sinuosus. It is noted, however, that the specimens studied here are less regular in some part of the burrow system when compared with Sinusichnus from the Pliocene and Miocene strata (e.g., Belaústegui et al., 2014; Buatois et al., 2009). While the positive correlation between wavelength (λ) and amplitude (A) of the Luoping specimens has also been found in typical S. sinuosus (de Gibert et al., 1999), further supporting the studied trace fossils are S. sinuosus.
Several trace fossils are comparable to Sinusichnus in morphology and this has been discussed in detail in previous studies (e.g., Belaústegui et al., 2014; Buatois et al., 2009; de Gibert et al., 1999). These traces include members of the Thalassinoides group (e.g., Thalassinoides, Ophiomorpha; Seilacher, 2007) and the graphoglyptid trace Protopaleodictyon. In addition, the similarity between Sinusichnus and the horizontal meandering burrows such as Cochlichnus and Cosmorhaphe has also been mentioned (Belaústegui et al., 2014). The newly discovered specimens from the Middle Triassic Guanling Formation reveal several characteristics that distinguish it from all the above-mentioned traces. Firstly, it shows the typical Y-shaped branching, which ruled out its possible assignment to the sinuous Cochlichnus or Cosmorhaphe. The latter two sinusoidal burrows are both smooth, unbranched grazing traces although the latter occasionally branched (Bordy et al., 2011; de Gibert et al., 1999; McCann and Pickerill, 1988). The regular sinuous morphology in the new specimen (Figs. 3a–3d, 4a–4b) was never present in Thalassinoides. Additionally, the absence of pelleted lining in branching tunnels in our specimens distinguishes it from Ophiomorpha.
It has also been mentioned that Sinusichnus resembles certain graphoglyptid traces, in particular, Protopaleodictyon and Megagrapton (Belaústegui et al., 2014; de Gibert et al., 1999; de Gibert, 1996). However, the found specimen did not displayshort, blind apical elements or appendages at each undulation typical of Protopaleodictyon incompositum (Uchman, 1998). For the latter, the branching burrows of Megagrapton are instead slightly curved or straight and the branching angles are nearly 90 (Häntzschel, 1975), which morphology is very different from the materials reported herein.3.2 Palaeoecology of Sinusichnus from Luoping, Southwestern China
It has been noted that Sinusichnus presents morphological similarities with other branching burrows now clustered within the Ophiomorphid traces (e.g. Thalassinoides, Ophiomorpha, Spongeliomorpha, Pholeus and Gyrolithes, sensu, Seilacher, 2007). These Ophiomorphid traces are currently interpreted as dwelling or feeding traces (Ekdale, 1992), which suggests a similar ethological mode for Sinusichnus trace makers. On the other hand, de Gibert et al. (1999) pointed out that the highly regular sinuosity of branches within Sinusichnus burrow systems is very characteristic of agrichnial structures, indicating possible farming or trapping strategies. Further, de Gibert et al. (1999) discussed four proposed ethological modes of Sinusichnus and favored an ethological change during the ontogeny of the producer from juvenile to adult. For instance, in the Lower Pliocene BaixEbre Basin in Spain, the small and large Sinusichnus burrowing systems were preserved together (de Gibert et al., 1999; de Gibert, 1996).
The studied S. sinuosus specimens herein might suggest certain deposit-feeding behavior of the trace makers. This is supported by the evidence that the sinuous branching burrow systems are distributed in very shallow tiers. Further, it was found that S. sinuosus from the Guanling Formation in Luoping are in close association with R. commune. The cross-cutting relationships of these two ichnotaxa show that they most likely belong to the same tier. Thus, they might be produced by organisms sharing similar feeding habits and colonize at the same time. The deposit feeding behavior of trace maker producing R. commune has been recognized by several researchers (e.g., Knaust, 2013). The trace makers producing S. sinuosus and R. commune probably lived at shallow depths to feed on bacteria within sediments. The search for bacteria as food resources of S. sinuosus trace makers is further supported not only by the net-work shaped specimens, but the abundant preservation of reticulated ridge structures in strata (Luo et al., 2013). The latter represent the former presence of microbial mat, implying a prolific distribution of microbes in sediments serving as food resources for the deposit-feeding organisms producing S. sinuosus and other deposit feeders.
Several animals have been proposed as the potential trace maker of Sinusichnus, which include decapod crustaceans and isopods (Knaust, 2016; Belaústegui et al., 2014; Buatois et al., 2009; de Gibert, 1996). S. sinuosus from Luoping might also be produced by decapod crustaceans. In fact, the body fossil record suggested that decapod crustaceans were abundantly preserved in the Luoping Biota, which includes a newly established family and several new species (e.g., Feldmann et al., 2017, 2015, 2012; Huang et al., 2013).3.3 Temporal and Spatial Distribution of Sinusichnus
Since de Gibert (1996) established the ichnospecies Sinusichnus sinuosus based on Lower–Middle Pliocene specimens from Spain and France, more and more similar structures assigned to Sinusichnus have been reported from different localities and stratigraphical ranges (Belaústegui et al., 2014; Buatois et al., 2009; Kappel, 2003). Table 1 summarizes discovered trace fossil Sinusichnus from all over the world. These data also include some unpublished data. The oldest known occurrence of reported ichnotaxa could be traced back to the Middle Triassic (Knaust et al., 2016). The study on the ichnology of the Middle Triassic Udelfangen Formation established a new ichnospecies Sinisichnus seilacheri, which was probably produced by isopod crustaceans (Knaust et al., 2016; Table 1). Other reported occurrence includes S. sinuosus from the Upper Cretaceous Hidden Lake Formation, and several more examples from Miocene and Pliocene (Belaústegui et al., 2014; Buatois et al., 2009; de Gibert et al., 1999; de Gibert, 1996; Table 1). Our reported trace fossil materials from the Middle Triassic strata at Luoping in South China represent the first report from South China, and further supports the long stratigraphic range of Sinusichnus sinuosus starting from the Anisian. In addition, recent studies suggest that Sinusichnus might have an even longer stratigraphic range dating back to the Late Devonian (Table 1, unpublished data). These occurrences include one occurrence from the Late Devonian Wutong Formation in Wuhan, China, and another one from the Lower Triassic Jialingjiang Formation in Sichuan, China. Whether these older examples are produced by decapod crustaceous or isopod crustaceans remains to be answered.
New Middle Triassic trace fossils were found from the Guanling Formation in Luoping, Yunnan Province. The trace fossil materials were associated with the Luoping Biota assigned to the Anisian in age. Various morphological features suggest that these traces can be tentatively assigned to the decapod trace Sinusichnus sinuosus. Thus, these new trace fossil materials represent the first reported Sinusichnus from South China, strengthening its early appearance since Anisian. However, the global record of Sinusichnus might suggest that these burrows could have an older history back to the Late Devonian.ACKNOWLEDGMENT
This study was partly supported by the ARCDP discovery grant to G. R. Shi (No. DP150100690). This research has also been supported by a CAS Pioneer Hundred Talents Program (to ML). This research is also supported by a grant from the State Key Laboratory of Biogeology and Environmental Biology (No. GBL21410 to ML), China University of Geosciences (Wuhan), and a grant from the National Natural Science Foundation of China (No. 41502013). This research has also been supported by for China Geological Survey projects (Nos. DD20160020, 1212011140051, 12120114030601, and 1212010610211). This paper is a contribution to the IGCP 630 "Permian–Triassic climatic and environmental extremes and biotic response". The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0794-4.
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