Occurrence of Tannuolina in the early Cambrian siltstones of the Hazara Basin, North Pakistan, and its biostratigraphical significance

The tommotiids are an important group of Cambrian small shelly fossils, primarily retrieved from carbonate rock by acid dissolution. In this study, abundant isolated sclerites of Tannuolina are recovered from the siltstone-dominated upper part of the Hazira Formation in the eastern Hazara Basin, North Pakistan. This discovery of tommotiids preserved in the siltstones not only suggests their occurrence in a variety of sedimentary lithofacies, but also provides the opportunity to look for the sclerites or scleritomes (or even soft tissue) of tommotiids in much wider taphonomic windows. The sclerites include two morphs (i.e. mitral and sellate types). Through morphological comparison, they can be identified as Tannuolina zhangwentangi Qian and Bengtson, 1989. The large mitral and sellate sclerites (about 1 cm) illustrated in the present study manifest a relatively consistent morphology during size increase. The sellate sclerites may contain two sub-types, a larger sellate sclerite with sella on the sellate side and a smaller convex triangular sclerite without sella on either side. In the original scleritome, the two sub-types of sellate sclerites probably combine as a composite with the duplicatural side of the smaller one attached on the sella area of the large one. For the first time, T. zhangwentangi, previously unknown outside South China, has been recovered from the Indian subcontinent. The middle and upper part of the Hazira Formation exclusively bearing T. zhangwentangi can directly invite correlation with the S. flabelliformis-T. zhangwentangi Assemblage Zone of South China representing the uppermost Cambrian Stage 2. This new find not only signifies the utility of T. zhangwentangi for intercontinental biostratigraphical correlation, but also suggests that the Terreneuvian small shelly fossil (SSF) biostratigraphy between the Indian subcontinent and South China is consistent, comprising three (at least two) correlative SSF Assemblage Zones (Zone I, possibly Zone III, and Zone IV of South China). Additionally, our result may also support a relatively close palaeogeographical linkage between these two regions in the early Cambrian.