The Cambrian palaeontological record of the Indian subcontinent

Abstract The Indian subcontinent's biota and biostratigraphy is amongst the least well known internationally of any Cambrian succession worldwide. Recent revision of previously described type material and a substantial number of new finds reveal a typical Cambrian skeletonized fauna and an organic-walled biota, as well as various trace fossils. This biota, reviewed here synoptically, currently contains 51 non-agnostoid trilobite genera belonging to 50 species; 15 genera and species of agnostoids; one species of bradoriid arthropod; 18 brachiopod genera containing 20 species; echinoderm thecal and columnal plates; a soft-bodied eldoniid; representatives of three hyolith genera; other small shelly fossils some of which are identified to species level; acritarchs; and a variety of ichnotaxa. Regional biostratigraphic zonations for trilobites, brachiopods, small shelly fossils and organic-walled, and trace fossils are herein combined into an integrated Cambrian biostratigraphical scheme that permits correlation along and across the lithotectonic zones of the Himalayan margin and southward onto cratonic India. These consist of 13 named biostratigraphic units for trilobites, seven for brachiopods, three for small shelly and organic-walled fossils, and one for trace fossils. The basal boundary of the Cambrian (~ 541 Ma) is biostratigraphically localised in the Lesser Himalaya between Ediacaran carbonate-rich beds bearing the organic-walled tubular Shaanxilithes ningqiangensis, and dark, highly stratigraphically condensed, phosphate-rich shale bearing Fortunian (~ 535 Ma) and Cambrian Stage 2 age (~ 524 Ma) small shelly fossil assemblages. The oldest macrofossils presently known are somewhat younger, and represent early Cambrian Stage 4. The Redlichia noetlingi trilobite Zone/Botsfordia granulata brachiopod Zone (~ 512 Ma), is widely represented in siliciclastic rocks across and along the Himalaya, spanning the Tethyan, Lesser, and sub-Himalayan lithotectonic zones, and possibly also extending onto the craton. Stage 5 Cambrian fauna are the most diverse and best biostratigraphically characterised, much of which are relatively well preserved in limestone, but presently known only in the Tethyan Himalaya where species-level correlation between the Zanskar and Parahio valleys has been established. There, rates of sediment accumulation were notably high, with some 2000 m of rock when compacted deposited within approximately 13 Myr. During this interval, the ranges of some trilobite and brachiopod species (and their zones) were apparently less than one million years long. The indicus trilobite zone has recently been localized, shortly below the prachina zone. Furongian (late Cambrian, ~ 493 Ma) fossils are known only in the Bhutanese Himalaya. Except where faulted, the Cambrian succession is capped by an unconformity throughout the Himalaya. In the Salt Range and Lesser Himalaya this unconformity is sub-Permian, but in the Tethyan Himalaya it is sub-Ordovician. The regional Cambrian can now be correlated globally with reasonable precision, and all parts of the Himalayan margin south of the Yarlung-Tsangpo suture have an equatorial Gondwanan biota most similar to North China, and particularly, to South China. Links with Australia are suggested by non-cosmopolitan species but Indian biotas share less in common with Australia than with parts of China. Knowledge of the Himalayan Cambrian biota serves a critical role in constraining the Caenozoic uplift and erosional history of the orogen.