1. Composition of the microfossil assemblage
The composition of the Tikebuladahuang microfossil association is summarized in Fig. 2. Only eight chert samples from the Upper Xishanblaq Formation are fossiliferous (TK-18, TK-19, TK-21, TK-22, TK-25, TK-26, TK-31, TK-32). Sponge spicules (Fig. 4 P-R) dominate the middle part of the Upper Member, while H. irregulare n. sp. (Fig. 4 A-L) dominates the middle-upper part of the Upper Member. Leiosphaeridia sp. (Fig. 4 M-N) and Siphonophycus sp. (Fig. 4 O) occasionally appear in the upper part of the upper formation.
2. Morphology of Heliosphaeridiumirregulare n. sp.
Similar to H. irregulare, H. lubomlense (Kirjanov, 1974) Moczydłowska, 1991 is also characterized by the hollow processes widened at the bases.
While organic-walled microfossils are normally extracted using conventional palynological methods (Vidal 1988), small specimens (< 10 μm in diameter) may be lost during sieving through a mesh screen, which is also about 10 μm in diameter. Size measurement of extracted specimens using palynological methods may therefore result in an overestimate of size distribution. To date, uncontroversial H. lubomlense specimens have all been extracted by applying palynological methods, and the total combined size of vesicles and all processes has consistently > 17 μm (Moczydlowska 1991). However, other organic-walled fossils reported from the same publications as H. lubomlense have total sizes much smaller than 10 μm (Moczydlowska 1991). The lack of H. lubomlense specimens < 10 μm therefore cannot be explained by loss during sieving, and its size distribution as described in these publications therefore be considered acurate. Conversely, direct size measurement of spherical acritarchs from a petrographic section may lead to underestimates. In fact, a 10 μm spherical acritarch in a 25 μm thick petrographic section is expected to be underestimated within 0.5 μm (corresponding author, unpublished data). This limited underestimation of the size distribution of H. irregulare can, however, be ignored. H. lubomlense has more or less homomorphic cylindrical processes of approximately equal length in each discrete specimen, widened at the bases (Fig. 5 D). Both cylindrical and conical processes can be present in a single H. irregulare specimen (Fig. 5 A-B). The base width and total length of the processes can also vary in a single specimen. The heteromorphic processes of H. irregulare may result from differential growth but their evolutionary significance is not yet clear. Based on the above, H. irregulare cannot be considered a synonym of H. lubomlense.
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. irregulare. 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. irregulare are flexible and in the shape of much shorter cones (Fig. 5A-B). Thus, it is thus not difficult to distinguish between them.
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 the triangular process bases similar to those of H. lubomlense, but with a relatively small vesicle. Re-examination of the description and published illustration shows that this species fits all diagnostic criteria of H. irregulare, and these should therefore be considered as the same species.
A specimen described as H. dissimilare (Volkova 1969) Moczydłowska, 1991 was reported from the chert of the Cambrian Stage 3 Shabakta Formation in the Maly Karatau Range, South Kazakhstan (Sergeev et al. 2020, Fig. 5.7–5.8). However, H. dissimilare has homomorphic cylindrical processes, while both the optical photomicrograph and confocal laser scanning micrograph of that specimen clearly exhibit both conical, cylindrical/hair-like and thorn-like processes. Despite heteromorphic processes, which are also confirmed in the description (Sergeev et al. 2020), the flexible nature of the processes and vesicle size also fit the diagnostic features of H. irregulare; this specimen should hence be considered as belonging to latter species. Other Heliosphaeridium fossils described in the same publication have a vesicle size similar to that of H. irregulare, but the nature of their processes was not clear enough to enable identification. In the absence of more detailed information, it is difficult to determine whether there are synonyms of these taxa solely, on the basis of available photographs.
Braun and Chen (2003) reported M. ssp. from the Hetang Formation of the Xintangwu Section, Western Zhejiang Province. At least one specimen (Braun & Chen 2003) resembles H. irregulare, 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.
3. Reported ‘Asteridiumtornatum’ 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 clastic rocks is characteristic for its solid and thorn-like processes. However, the description and illustrations in the publication clearly show that the processes are in the shape of short straight or curved cones and are of unequal width within a single specimen. Fortunately, specimens similar to A. tornatum from Mochia-Khutuk also appeared in the Upper Xishanblaq Formation examined in this study (Fig. 4I-L). A careful examination showed that the processes 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 somewhat opaque and exhibit a pseudomorph of solid processes (Fig. 4K). Thus, the specimens of A. tornatum identified from Mochia-Khutuk should be considered H. irregulare with shorter processes.
H. cf. lublomse and A. tornatum from Mochia-Khutuk can be roughly distinguished by process length, with processes < 2.7 μm considered Asteridium tornatum and longer processes considered H. cf. lublomse. In this study, H. irregulare was subdivided into these two different subgroups: processes > 2.7 μm and processes < 2.7 μm. Morphological measurement data (Fig. 6A) 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. irregulare into two morphological taxa. Process length and width gradually increase as vesicle size increases (Fig. 6B). 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.
Anh et al. (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 > 3.5μm. Based on illustrations on the same publication, the distribution pattern and number of processes are completely the same as H. ampliatum. These specimens therefore represent poorly preserved H. ampliatum 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. However, a re-examination of the same published thin sections reveals that processes are hollow and distributed regularly in the shape of a diamond mesh (corresponding author, unpublished data). An interior vesicle exists inside the outer vesicle to which the processes are attached. These features have never been reported from any uncontroversial A. tornatum specimens. Luo identified an algal genus from basal Cambrian black chert in Abazhai, Guizhou Province, South China, the species Asterococcoides inconspicus (Wang & Luo, 1984). 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.
4. 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 & Zhu, 2017; Yao et al., 2005). Common elements of this MPM assemblage are H. ampliatum, P. regularis, and Megathrix longus Yin 1987. The overlying Cambrian stage 3 microfossil assemblage, dominated by H. irregulare, is completely different from the MPM assemblage. The appearance of H. irregulare in the Cambrian Stage 3 Shabakta Formation also suggests this species has a higher stratigraphic occurrence than previously expected (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. irregulare association. Yao et al. (2005) and Dong et al. (2009) denied the existence of the outer envelope of Paracymatiosphaera, and thus revised this genus to Comasphaeridium. However, new materials from South China clearly show that this outer envelope exists, and this revision should hence not be accepted (corresponding author, unpublished data). Given this, together with the age and A. tornatum problems discussed above, the AHC assemblage cannot be accepted as a valid fossil zone/assemblage.
5. Implication for paleogeography
According to the similarity of trilobite assemblages, the Yangtz 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, which proves that the two blocks are very close in paleogeography (Zhou et al. 2008). The organic-walled microfossil assemblage found in this study also support the paleogeography proximity between them, and can be compared with the microfossils in the early Cambrian Chert of the Yangtze plate (Yin, unpublished data, Fig. 8).