Features described as ‘double crossbars’ or ‘blades’ by Tang et al. are identical to the lanceolate folds that form in flattened leiospheres (Fig. 1; see also Moczydłowska, Reference Moczydłowska2010, fig. 1; Riedman and Porter, Reference Riedman and Porter2016, fig. 11.12; Loron and Moczydłowska, Reference Loron and Moczydłowska2018, plate 2.8, 2.9; Slater et al., Reference Slater, Harvey and Butterfield2018, fig. 9N–Q; Slater and Willman, Reference Slater and Willman2019, fig. 5M, O, U, V, X, Z). The linear structures termed ‘crossbars’ by Tang et al. (Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019) are better interpreted as pyritized folds in the carbonaceous vesicle wall. The folds that form in the walls of collapsed Leiosphaeridia vesicles (and other acritarchs) can often be outlined with pyrite, where dense clusters of pyrite euhedra line the creases and margins of these folds (Fig. 1.8, 1.10, 1.15; see also Slater and Willman, Reference Slater and Willman2019, fig. 5AC). Similarly, the ‘irregular organic traces’ described by Tang et al. (Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019) can instead be viewed as sinuous wrinkles in the vesicle wall (Fig. 1; see also Slater and Willman, Reference Slater and Willman2019, fig. 5E, 5G, 5AB). The proposed hollow nature of the ‘crossbars’ is, in our view, more likely the result of a fold or crease in the vesicle wall having been incompletely filled with a growth of diagenetic barite and pyrite crystals. Prominent lanceolate folds are possessed by leiospheres such as Leiosphaeridia crassa (Naumova, Reference Naumova1949) and L. jacutica (Timofeev, Reference Timofeev1966), whereas forms such as L. minutissima (Naumova, Reference Naumova1949) and L. tenuissima (Eisenack, Reference Eisenack1958) display a higher proportion of thin, sinuous folds (see Javaux and Knoll, Reference Javaux and Knoll2017). The ‘crossbars’ or ‘outgrowths’ that Tang et al. (Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019) interpret to protrude beyond the ‘margin of the fossil’ can simply be viewed as pyritized folds (Fig. 1.9, 1.10), tears in the vesicle wall, or, in some cases, excystment structures (compare for example the features described as outgrowths in fig. 3.6 of Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019 with split examples of Appendisphaera in Sergeev et al., Reference Sergeev, Knoll and Vorob'Eva2011, fig. 5.1–5.3). Further, the specimens compared to bivalved arthropods (Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 4) with ‘V-shaped clefts’ or ‘offset margins’ are identical to leiospheres exhibiting medial release splitting (Fig. 1.16–1.23; see also specimens of Leiosphaeridia wimanii [Brotzen, Reference Brotzen1941] in Butterfield et al., Reference Butterfield, Knoll and Swett1994, fig. 13E, 13F and specimen of Valeria lophostriata [Jankauskas, Reference Jankauskas1979] with medial split in Loron et al., Reference Loron, Rainbird, Turner, Greenman and Javaux2019, fig. 4E). The ovoid or fusiform specimens (e.g., Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 4.7) are indistinguishable from fusiform leiosphere acritarchs (Fig. 1.15; see also Strother et al., Reference Strother, Battison, Brasier and Wellman2011, fig. 2B; Tang et al., Reference Tang, Pang, Yuan, Wan and Xiao2015, fig. 4.G2; Loron and Moczydłowska, Reference Loron and Moczydłowska2018, pl. 3).

Figure 1.

(1–23) Acid-extracted Cambrian Leiosphaeridia displaying the range of features seen in ‘Cambrowania ovata.’ Arrows in (2) show displaced margin (compare Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 4.3, 4.4); (5, 6) schematic outlines of specimens (1) and (2), respectively (compare to Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 2.2); arrows in (9) and (12) point to apex of folds comparable to structures termed ‘outgrowths’ in Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 3.6. Compare fusiform specimen (15) with Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 2.7; note the extensively pyritized folds in (8, 10, 15); specimens (16–23) display medial splits (arrow in 16), interpreted as release structures in leiospheres and comparable to the ‘clefts’ seen in ‘Cambrowania ovata’ (Tang et al., Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019, fig. 4.1, 4.2). (1–4, 7–14, 16) from early Cambrian (Fortunian–Stage 2) Lontova Formation, Estonia; (15) from Cambrian Stage 5 Borgholm Formation, Närke, Sweden; (17–23) from early Cambrian Stage 4 File Haidar Formation, Sweden. Scale bar = 100 μm for (1, 2, 5, 6, 9–12, 15, 17–23), 200 μm for (3, 4, 7, 8, 13, 14, 16). Refer to cited sources for repository information.

Tang et al. (Reference Tang, Hu, Xie, Yuan, Wan, Zhou, Dong, Cao, Lieberman, Leys and Xiao2019) briefly raise the similarity of Cambrowania ovata to the carbonaceous spheroid Chuaria circularis Walcott Reference Walcott1899 (Tang et al., Reference Tang, Pang, Yuan and Xiao2017), but dismiss any deeper comparison on the grounds that no Chuaria are known with an interlaced truss of crossbars. Chuaria does exhibit occasional linear folds (see Sharma and Shukla, Reference Sharma and Shukla2016, fig. 5) and can even display splitting reminiscent of the ‘V-shaped clefts’ in the Hetang specimens (e.g., Sharma and Shukla, Reference Sharma and Shukla2016, fig. 5F, 5G), however, Chuaria possesses a relatively thick vesicle wall and compacts to form concentric folds around the periphery of the vesicle (Butterfield et al., Reference Butterfield, Knoll and Swett1994; Samuelsson and Butterfield, Reference Samuelsson and Butterfield2001). Spheroidal acritarchs visible at the bedding-plane scale tend to have thick walls (e.g., Chuaria, Cerebrosphaera), whereas Leiosphaeridia, although known from bedding-plane compressions (e.g., Butterfield et al., Reference Butterfield, Knoll and Swett1994, fig. 13F), have substantially thinner walls and therefore do not usually leave a prominent impression within the sediments. The Hetang specimens are therefore interesting for their taphonomic expression; we argue that the Hetang fossils likely derive from thin-walled leiosphere vesicles, which are unusually prominent at bedding-plane scale owing to their outlining in pyrite and barite.

The Hetang leiospheres are larger than most acid-extractable forms because larger specimens tend to fragment during acid extraction, but even specimens isolated by palynological processing can frequently reach ~1 mm in length (e.g., Slater et al., Reference Slater, Harvey and Butterfield2018, fig. 9; see also the large Leiosphaeridia wimanii in Butterfield et al., Reference Butterfield, Knoll and Swett1994, fig. 13D, 13E). In any case, the size range of leiospheres is extremely variable owing to their undoubtedly polyphyletic nature. The Hetang specimens are smaller than many bedding-plane examples of Chuaria, and for that matter other compression fossils of vesicular organic spheroids such as Beltanelliformis (e.g., Jensen et al., Reference Jensen, Palacios and Mus2007, fig. 3). Interestingly, the large size of the Hetang specimens is suggestive that some Leiosphaeridia were filled with cell-aggregates in life. As with other leiospheres, the construction of a simple organic-walled vesicle is a widely convergent character (Javaux and Knoll, Reference Javaux and Knoll2017), and therefore little can be determined about the precise phylogenetic affinity of the Hetang specimens at present, although a metazoan affinity can be ruled out.