Gilbert et al.—Himalayan Cambrian microfossils Taken at face value, the known ranges of Chancelloria,
Archiasterella, and Igorella maidipingensis might imply an early Cambrian depositional age for the Parahio Formation. However, given the well-constrained trilobite and brachiopod biostratigraphy, we conclude that these microfossil taxa simply ranged higher than previously recorded. Presently, small shelly fossils are poorly described from middle Cambrian strata worldwide, whereas macrofossils such as trilobites and brachiopods have been studied intensely throughout the Cambrian. Furthermore, other species within Igorella, Chancelloria, and Archiasterella range into the middle Cambrian. In addition, the long local range of the most common small shelly fossil, A. dhiraji n. sp., within the Parahio Formation (Fig. 2) itself suggests a stratigraphic range that is markedly longer than that of associated trilobites. Accordingly, the stratigraphic occurrence of the material described in this study can be reconciled and integrated with the trilobite-based biozonation given reasonable upward range extensions of some species of the small shelly fauna. This results provides a demonstration of why it was unwarranted to assign other Himalayan rocks to the Sinosachites flabelliformis-Tannuolina zhangwentangi Assemblage zone based on chancelloriids alone (see Hughes et al, 2005; contra Kumar et al., 1987). Long temporal ranges for small shelly taxa are complemented
by wide geographical distributions for these taxa, with occurrences of Archiasterella dhiraji n. sp. occurring in Laurentia and inboard Gondwana, and a widespread inboard peri-Gondwanan, South Chinese, and Siberian distribution for Igorella cf. maidipingensis. Cupitheca and Chancelloria likewise have global distributions.
Integration with trilobite biostratigraphy.—The biostratigraphic zonation for the Cambrian of the Parahio Formation proposed by Peng et al. (2009) was based on the local occurrence of well-characterized trilobite taxa, integratedwith original work at the same sites, namely the Parahio Valley section in the Spiti region (Hayden, 1904; Reed, 1910) and the Zanskar Valley in the Ladakh region (Jell and Hughes, 1997). Peng and colleagues (2009) recognized six trilobite zones, three levels, and six unzoned intervals for the Cambrian System of the Parahio and Zanskar valleys (Figs. 2–4) spanning an interval from the upper informal global Stage 4 through the lower half of the Guzhangian Stage of the Cambrian System. The Peng and colleagues (2009) study also provided a link with well-established successions in China and Australia, enabling a quite precise global correlation. The occurrence of microfossils is discussed below within the context of the trilobite zonation.
Haydenaspis parvatya level.—This level contains both chancelloriids, Archiasterella dhiraji n. sp. and Chancelloria sp. and is the oldest body fossil fauna yet collected in situ from Hayden’s section, occurring at 78.07m above the base of the Parahio Valley section (Figs. 2, 4). Trilobite data (Peng et al., 2009) suggests that it is equivalent to the base of the Maochuangian of North China, or within the top part of the Duyunian Stage of South China. Globally, this level lies within the upper part of the informal Stage 4 of the Cambrian System, and thus to the uppermost part of the second Series of the Cambrian System.
15
Kaotaia prachina Zone.—Mircofossils from this zone include both chancelloriids, Archiasterella dhiraji n. sp. and Chancelloria sp., as well as an indeterminate hyolith, and were collected at 439.44m above the base of the Parahio Valley section as collection PO15 (Figs. 2, 4). Its stratigraphic position, above the inferred level of Oryctocephalus indicus and below that of Paramecephalus defossus, suggests that it lies within the lower part of the informal global Stage 5 (Peng et al., 2009).
Paramecephalus defossus Zone.—This zone has the most diverse microfossil fauna recorded in this study, containing all taxa described herein: hexactinellid sponge spicules, Archiasterella dhiraji n. sp. and Chancelloria sp., an indeterminate hyolith, Cupitheca sp. Igorella cf. maidipingensis, a single early meraspid ptychopariid, and indeterminate spines. The collections PO21 (765.14m), PO24 (775.41m), and PO25 (776m) represent this zone. This is the middle part of the Parahio Formation in Hayden’s section, and is specificallyinferredtobeHayden’s (1904) level 6 (Fig. 4). Peng and colleagues (2009) correlated this zone with the middle of the Taijiangian Stage of South China. The PI13 site is the oldest body fossil bearing collection in the Zanskar Valley. Brachiopod biostratigraphy (Popov et al., 2015) locates PI13 stratigraphically within the P. defossus Zone (Figs. 3, 4). Its microfossil fauna contains only Archiasterella dhiraji n. sp., which occurs in both the oldest and youngest body fossil collections in the Parahio Valley section (Fig. 2) and has a long range globally. Although the age of the PI13 collection is poorly constrained biostratigraphically by small shelly fossils, a biostatigraphically diagnostic brachiopod species allows precise correlation between this level and a horizon approximately 500m below the top of the Parahio Formation in the Parahio Valley (Popov et al., 2015).
Oryctocephalus salteri Zone.—This zone contains hexactinellid sponge spicules, Archiasterella dhiraji n. sp., Chancelloria sp., and a indeterminate hyolith collected at PO31 (836.36 m) and PV880 (880.93 m) (Figs. 2, 4). Peng and colleagues (2009) correlated this zone with the middle Taijiangian Stage of South China, and with the late Early or early Late Templetonian stage of Australia.
Above Iranoleesia butes level/below Sudananomacarina sinindica Zone.—The Parahio Valley PO9 collection (1242.4m) yielded chancelloriid spicules only. It was made a little over an hundred meters below the top of the Parahio Formation and is the inferred site of Bhatt and Kumar’s (1980) collection (Fig. 2).
Resolution of the conodont conundrum
Two of the putative conodont specimens figured as Oneotodus by Bhatt and Kumar (1980, pl. 1 figs. 1, 3) resemble individual rays of Archiasterella dhiraji n. sp. The figured specimen reported as GSI19604 is similar to an isolated recurved abapical ray, or possibly an ascending horizontal ray, and that reported as GSI19606 is also similar to the linear adapical ray. Oddly, however, the specimens presently reposited in the Geological Survey of India with these specimen numbers are evidently not those originally figured, nor are they certainly fossils.
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