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

Abstract

The tommotiids are an important group of Cambrian small shelly fossils, primarily retrieved from the carbonate rock by acid process. Herein, the 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 occurrences in a variety of sedimentary lithofacies, but also provides the opportunity to look for the sclerites or scleritomes (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 & Bengtson, 1989. The large mitral and sellate sclerites (about 1 cm) illustrated herein manifest a relatively consistent morphology during the size increasement. The sellate sclerites may contain two sub-types, the larger sellate sclerite with sella on the sellate side and the smaller convex sellate side without sella on either sides. 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 has been recovered from the Indian subcontinent, previously unknown outside South China. 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 palaeontological finds not only signifies the utility of T. zhangwentangi for intercontinental biostratigraphical correlation, but also suggests that the Terreneuvian SSF biostratigraphy between the Indian subcontinent and South China are quite consistent, comprising of three (at least two) correlative SSF Assemblage Zone (Zone Ⅰ, possible Ⅲ, and Ⅳ of South China). Additionally, our result may also support a relatively close palaeogeographical linkage between these two regions in early Cambrian. Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/topic/collections/advances-in-the-cambrian-explosion