Early Cambrian microfossils from the cherts in carbonates of the Kuruktag area, the Tarim block

The basal Cambrian is characteristic for the appearance of small shelly fossils and acanthomorphic acritarchs with relatively small vesicles. In the Tarim Block (NW China) and South China, this microfossil assemblage is dominated by small acanthomorphs and was referred to as the ‘Micrhystridium-Paracymatiosphaera-Megathrix’ (MPM) association, based on microfossils from the lower Cambrian Yurtus and Xishanblaq formations of the Tarim Block. This study provides the new early Cambrian microfossil assemblage preserved in cherts of carbonates in Tikebuladahuang section of the Kruktag area, Tarim Block. These early Cambrian microfossil assemblages come from upper part of the lower Cambrian Xishanblaq Formation in Tikebuladahuang section correlated with the Cambrian Stage 2, and the microfossil association dominated by Heliosphaeridium dissimilare, sponge spicules, Leiosphaeridia spp., and Siphonophycus sp. also occurs. The organic-walled microfossil assemblage found in this study can be compared with the microfossils in the early Cambrian Chert of the Yangtze plate. The Asteridium–Heliosphaeridium–Comasphaeridium (AHC) association in South China and the Tarim block may mix two fossil associations of different ages and fossil compositions.


Introduction
The early Cambrian explosion is one of the most significant evolutionary events in the life history on the earth. Diverse groups originated and/or radiated during this critical time interval (Morris 1993;Shu et al. 1999). One of these groups, micro-phytoplankton, mainly preserved in acritarch forms, changed markedly through this event (Yin 1995;Moczydlowska-Vidal et al. 2008). The basal Cambrian acritarch assemblage is distinguished from the Neoproterozoic assemblage by the world-wide appearance of small acanthomorphic types (with vesicle sizes typically < 20 μm) (Moczydlowska 1991;Yao et al. 2005).
One of these types, Micrhystridium Deflandre 1937, was claimed to be distributed world-wide across several early Cambrian blocks. Due to multiple revisions and several systematic problems, the early Cambrian Micrhystridium was subsequently split into two taxa: Asteridium Moczydłowska 1991 and Heliosphaeridium Moczydłowska 1991. The division was based on the nature of processes: Asteridium has solid processes, while Heliosphaeridium has hollow processes connected to the vesicle interior (Moczydlowska 1991). An organic-walled fossil zone, Asteridium tornatum-Comasphaeridium velvetum was identified from basal Cambrian siliciclastic sequences on the Lublin Slope in southeastern Poland. Later studies then confirmed this zonation pattern in Baltica and Avalonia (Jachowicz-Zdanowska 2013; Palacios et al 2011Palacios et al , 2018. The Yangtze and Tarim blocks are the other two major localities of Heliosphaeridium, which is coeval with the Asteridium tornatum-Comasphaeridium velvetum zone. This assemblage was referred to as Micrhystridium-Paracymatiosphaera-Megathrix (MPM), based on acritarchs and tubular microfossils from the widely distributed basal Cambrian 1 3 32 Page 2 of 10 black chert unit in South China and Tarim. Based on studies of basal Cambrian microfossils from the Tarim Block, Yao et al. (2005) reported the presence of Asteridium tornatum (Volkova, 1968) Moczydłowska, 1991 from the Xishanblaq Formation. The authors also transferred the previous identified Micrhystridium species to Heliosphaeridium, and Paracymatiosphaera Wang 1985to Comasphaeridium Staplin et al. 1965. On the basis of this revision, the authors then proposed a new fossil assemblage: Asteridium-Heliosphaeridium-Comasphaeridium (AHC). This assemblage, has since been confirmed through later studies (Ahn and Zhu 2017;Chang et al. 2020;Dong et al. 2009). There is a superficial indication that this may share some morphologically similar types with the Asteridium tornatum-Comasphaeridium velvetum assemblage of Baltica and Avalonia.
This study seeks to confirm the validity of this assemblage, by investigating chert-permineralized microfossils from the lower-middle Cambrian strata of the Tikebuladahuang section (Kruktag area, Tarim Basin). The study also contributes to improved knowledge of early Cambrian evolution of primary producers and related biostratigraphic correlations. This is important considering that, compared to the coeval metazoan, understanding of diversity and the evolutionary patterns of micro-phytoplankton is still limited, especially for materials from Tarim and South China, for which most relevant literature is only available in Chinese Wang et al. 1982;Wang and Luo 1984;Xie et al. 2015;Xing 1982;Yin 1990Yin , 1987Yin , 1995Yin et al. 1992). This study will also contribute to a better understanding of the paleogeography of the Tarim and Yangtze blocks.

Geological settings
The Tarim Block is suggested to be located adjacent to East Gondwana in low latitudes during the Early Cambrian (Huang et al. 2000). The Tarim Block is composed of a Neoarchean to Neoproterozoic basement and middle Neoproterozoic to Cenozoic marine-continental cover series (Xu et al. 2013). The Kruktag area is located the northeastern margin of Tarim Basin. The Cambrian successions are well developed and exposed in the Kruktag area and consists of four formations-in ascending order, these are the Xishanblaq, Xidashan, Moheershan, and Tursaktag formations. These upward shallowing successions are dominated by black shales and cherts in the Xishanblaq Formation, while carbonates in the Moheershan, and Tursaktag formations. The gradual transition occurs in the Xidashan Formation. The lower Cambrian Xishanblaq Formation unconformably or disconformably overlies the Neoproterozoic glaciogenic Hankalchough Formation. The cherts from the upper Xishanblaq Formation bear numerous Heliosphaeridium microfossils, while the cherts from the lower part are unfossiliferous (Yao et al. 2005). The lower Cambrian Xidashan Formation conformably overlies the Xishanblaq Formation. The Xidashan Formation bears the macro-tubular organic fossils Sabellidites cambriensis Yanichevsky 1926 (Yang et al. 2005) and trilobites, with three zones in ascending order, Metaredlichioides-Chengkouia, Tianshanocephalus, and Arthricocephalus-Changaspis. The middle Cambrian Moheershan Formation conformably overlies the lower Cambrian Xidashan Formation and conformably underlies the Late Cambrian Tursaktag Formation. The Moheershan Formation bears trilobites, including Lejopyge and Ptychagnostus (Yang et al. 2005).
Recent precise zircon U-Pb dating constrained one tuff layer below the bottom of the chert unit of the Xishanblaq Formation at 520.3 ± 2.9 Ma (Yang et al. 2021). This result suggests the chert unit of the Xishanblq Formation correlates to the Cambrian Stage 3.

Sampling
The investigated materials were collected from the Tikebuladahuang section at the northern margin of Lopnur (= Yuli) County, Xinjiang Province, NW China (Fig. 1). The study section is located approximately 100 km southeast of Lake Besteng, with GPS coordinates of 41°17′39.32"N, 87°45′27.29"E. Sampling started with the lowest exposed horizon of the Xishanbulaq Formation and ended at the top of the Moheershan Formation (Fig. 2). A total of 32 samples were collected, including 29 black cherts from the Xishanblaq Formation, one black chert from the Xidashan Formation, and two cherts from the Moheershan Formation. One petrographic section was made for each sample.

Microscopic work
All specimens described in this paper are examined using an Axio Imager 72 microscope, and reposited at China University of Mining and Technology. All fossil specimens presented in this paper were photographed using one × 100 oil immersion lens. The codes and England Finder references are given for each specimen, with slide labels oriented to the right. Fossil are measured from scaled thin section photos, using CorelDRAW X5 software. The diagrams of morphologic parameters were created using Origin 2017 software, and other figures were produced or adjusted in CorelDRAW X5 software.

Terminology
To avoid ambiguity, the acritarch terms used in this study are specified herein. 'Vesicle' is used to refer to the major and central body encompassed by an organic wall, 'vesicle cavity' refers to the interior space of the vesicle, 'process' refers to the protrusion attached to or extended from the vesicle wall (Fig. 3).

Results and discussion
The tikebuladahuang microfossil assemblage The composition of the Tikebuladahuang microfossil association is summarized in Fig

Reported 'Asteridium tornatum' in South China and tarim blocks
Specimens assigned to A. tornatum have been reported from the upper Xishanblaq Formation of the Mochia-Khutuk section (Yao et al. 2005, Fig. 1.1-1.4). Typical A. tornatum extracted from siliciclastic rocks is characteristic for its solid and thorn-like processes. However, the description and illustrations in the publication clearly showed that the processes are in the shape of short straight or curved cones. Specimens with spherical vesicles covered by numerous evenly distributed short conical processes also appeared in the Upper Xishanblaq Formation examined in this study ( Fig. 4i-l). These specimens are similar to A. tornatum from Mochia-Khutuk in terms of vesicle and process morphology and measurement data (Fig. 5). The processes of the specimens in this study are hollow and connected to the vesicle interior (Fig. 4l). Through a change of focus level, it was shown that the hollow and relatively small processes are sometimes opaque and exhibit a pseudomorph of solid   Fig. 5a), H. dissimilare was artificially subdivided into these two different subgroups: processes > 2.7 μm (Fig. 6a) and processes < 2.7 μm (Fig. 6b) to conduce a comparison study. Morphological measurement data (Fig. 5a) showed that these two groups largely overlap and are continuous in size distribution and process width distribution. It is therefore not necessary to divide H. dissimilare into two morphological taxa. Process length and width gradually increase as vesicle size increases (Fig. 5b). This suggests that, specimens with shorter, narrower processes and smaller vesicles may represent early development stages of those with longer, wider processes and larger vesicles. Ahn and Zhu (2017) reported A. tornatum from the Terreneuvian Yanjiahe Formation near Jijiapo Village, Yangtze Platform. However, these specimens are poorly preserved and do not show intact processes. All processes are relatively wide and truncated, and according to statistical data on process length, some are even > 3.5 μm. A. tornatum possesses thorn-like numerous processes, while H. ampliatum has much less densely distributed long spine-like processes. Based on illustrations on the same publication, the distribution pattern and number of processes are completely indistinguishable from H. ampliatum. These specimens therefore represent H. ampliatum with broken processes rather than A. tornatum. Xie et al. (2015) and Yin et al. (2016) reported A. tornatum from basal Cambrian black chert from Majiang County, Guizhou Province, South China. An interior vesicle exists inside the outer vesicle to which the processes are attached. This feature has never been reported from any uncontroversial A. tornatum specimens. Wang and Luo (1984) identified an algal genus from basal Cambrian black chert in Abazhai, Guizhou Province, South China, the species Asterococcoides inconspicus. A. tornatum from Majiang County meets the diagnostic features of this species and should hence be considered a synonym of Asterococcoides inconspicus.
To date, no uncontroversial A. tornatum has been reported from the basal Cambrian in South China and Tarim. This does not exclude the possibility to future A. tornatum reports; however, based on currently published data, it does not appear likely that this is a common species in these areas.

Assemblage correlation and comparison
In Tarim (NW China) and South China, the widely distributed basal Cambrian MPM microfossil assemblage (Fig. 7) is of Terreneuvian age as confirmed by carbon isotopic data and/or small shelly fossils (Ahn and Zhu 2017;Yao et al. 2005).  (Moczydlowska 1991), also suggests this species has a higher stratigraphic occurrence than the MPM assemblage (Fig. 7). The AHC assemblage thus may mix two microfossil associations of different age and completely different compositions: the older MPM association and the younger H. dissimilare association. Given this, together with the age and A. tornatum identification problems discussed above, the AHC assemblage cannot be accepted as a valid fossil zone/assemblage.

Implication for paleogeography
According to the similarity of trilobite assemblages, the Yangtze plate (South China) was very close to the Tarim plate in paleogeography during the early Cambrian period. The early Cambrian trilobite assemblage of platform facies from Tarim basin include, such as Bathynotus, Chittidilla, Jingyangia, Kepingaspis, Kunmingaspis, Meitanella, Paokannia, Redlichia, Tsuyidiscus, and Ushbaspis (see Zhang 1981;Lin et al. 1990). The assemblages are also found in the contemporaneous trilobite fauna of the Yangtze platform. A cluster analysis using trilobite taxa list, suggested that the two blocks may belong to the same biogeographic unit during the early Cambrian (Zhou et al. 2008). The organicwalled microfossil assemblage found in this study also supports the close biogeographic proximity between them, and  Microfossils from the Xishanblaq Formation, Tikebula section, for each illustrated specimen, thin section number (TK-, referring to the Tikebuladahuang section, the first number following TK-referring to the sampling order number, the second number referring to the photograph number, the third number referring to the specimen number), and coordinates are given. a-c Heliosphaeridium dissimilare, same specimen at different focal levels to show the nature of processes; the black arrow in a points to a process smoothly curves down-leftwards; the black arrows in b show a process rapidly tapers toward the distal part, resulting in a conical base and a long needlelike end, TK-32-08-01, L-33-2; d Heliosphaeridium dissimilare, TK-32-03-01, U-32-4; e-g Heliosphaeridium dissimilare, same specimen at different focal levels to show the nature of processes, TK-32-06-01, L-33-4; h, Heliosphaeridium irregulare n. sp., TK-32-01-01, S-32-2; i-j Heliosphaeridium dissimilare, same specimen at different focal levels to show the nature of processes; the black arrow points to an apparently hollow process, TK-32-09-01, L-33-2; k-l Heliosphaeridium dissimilare, same specimen at different focal levels to show the nature of processes; the white arrow points to a hollow process which connects to the vesicle interior, TK-32-11-01, G-27-4; m, Leiosphaeridia sp., Heliosphaeridium dissimilare also appears at the upper and left right corners, TK-32-06-02, L-33-4; n Leiosphaeridia sp., Heliosphaeridium dissimilare also appears to the lower left and right, TK-32-07-01, L-33-2; o Siphonophycus sp. ponited by black arrows, TK-31-03-01, O-20-1; p sponge spicules, TK-18-01-01, A-34-1; q sponge spicules, TK-22-03-01, Y-14-3; R, sponge spicules, TK-22-05-01, R-17-1. Scale bar in k represents 10 μm in a-h, 5 μm in i-l, 10 μm in m-o, 47.89 μm in p, 45.19 μm in q, 116.83 μm in r can be compared with the microfossils in the early Cambrian Chert of the Yangtze plate (Fig. 8).

Conclusions
1. The cherts in carbonates of the Cambrian strata (Xishanblaq, Xidashan and Moheershan formations) of the Tikebuladahuang section (Kuruktag area, Tarim Block) were carefully examined. Only the Upper Xishanblaq Formation yielded microfossils. This assemblage is dominated by a newly identified species, H. dissimilare, sponge spicules, Leiosphaeridia spp., and Siphonophycus sp. also occur. 2. The AHC assemblage mixes two fossil associations of different ages and fossil compositions, and the revision work conducted by Yao et al. (2005) also contains several morphological problems. The AHC association therefore cannot be accepted as a valid fossil association.
Discussion. Similar to H. dissimilare, H. lubomlense (Kirjanov, 1974) Moczydłowska, 1991 is also characterized by the hollow processes widened at the bases, but differs by possessing a regularly spheroidal vesicle outline (Moczydlowska 1991;Sergeev et al. 2020). H. ampliatum (Wang 1985) Yao et al. (2005) is another Micrhystridium complex acritarch, which commonly appears in the chert of pretrilobitic units in Tarim and South China. It also has a smooth, single-layered spherical vesicle, the size distribution of which is similar to H. dissimilare. However, the process length of H. ampliatum is always larger than vesicle diameter. Its processes are robust, either in the shape of relatively long and straight cones or broken (Fig. 5C). Conversely, the processes of H. dissimilare are flexible and much shorter (Fig. 5A, B). H. cf. lublomse has been reported from the chert of the Upper Xishanblaq Formation in the Mochia-Khutuk section, approximately 15 km north of the used section in this study ( Fig. 1; Yao et al. 2005). Its open nomenclature is due to a relatively small vesicle and shorter processes. Re-examination of the description and published illustration shows that this species fits all diagnostic criteria of H. dissimilare, and these should therefore be considered as the same species. Braun and Chen (2003) reported M. ssp. from the Hetang Formation of the Xintangwu Section, Western Zhejiang Province. At least one specimen (Braun and Chen 2003) resembles H. dissimilare, in terms of vesicle size and process nature. However, the preservation, fossil image, and simple fossil description do not allow for further examination of the specimen and synonyms therefore cannot be determined.
Distribution. Cambrian Stage 3 Shabakta Formation of the phosphorite-bearing Maly Karatau Range, South Kazakhstan (Sergeev et al. 2020 6 Morphological measurements of well-preserved Heliosphaeridium dissimilare from the Xishanblaq Formation of the Tikebula section. a Cross-plot and frequency distribution of vesicle diameter and process length of well-preserved Heliosphaeridium dissimilare in this study, the solids and empty circles respectively represent specimens similar to what were assigned to Asteridium tornatum and Heliosphaeridium. cf. lublomse in Yao et al. (2005), the areas circled by dotted lines represent the morphological measurements of Yao et al. (2005); b cross-plot and frequency distribution of vesicle diameter and process width; c histogram of the process length/vesicle diameter ratio; d histogram of the process width/vesicle diameter ratio The lower Cambrian Fucoid beds (Downie 1982), Scotland. The lower Cambrian Tokammane Formation, Spitsbergen (Knoll and Swett 1987). The lower Cambrian Bastion (Downie 1982) and Buen (Wallet et al. 2022) formations, Greenland. The lower Cambrian Gog and Mt Whyte Formation, North America (Downie 1982). The lower Cambrian Ratcliffe Brook Formation, southern New Brunswick, Canada (Palacios et al. 2011). The middle Cambrian Sosnowiec Formation, Poland (Moczydłowska 1998 Subgroup Sphaeromorphitae Downie, Evitt and Sarjeant, 1963Genus Leiosphaeridia Eisenack, 1958, emend. Downie and Sarjeant, 1963 Type species. Leiosphaeridia baltiea Eisenack, 1958, Ordovician, Baltica Leiosphaeridia sp. Figure 4M-N Description. Organic-walled microfossil with a singlelayered, smooth, collapsed vesicle and thick wall. Excystment and processes not observed. Dimensions. Vesicle diameter 21.65-29.11 μm (n = 2), vesicle wall thickness 0.74-0.75 μm (n = 2).
Dimensions. A specimen about 1.09 μm in diameter.