Taphonomy And Paleoecology Of Ediacaran Tubular Fossil Conotubus Hemiannulatus And Gaojiashania Cyclus From The Gaojiashan Lagerst(?)tte In Southern Shaanxi Province, South China

The upper Ediacaran Gaojiashan Lagerstatte (551-541 Ma) in southern Shaanxi Province of South China hosts a variety of soft-bodied or lightly biomineralized tubular fossils (Shaanxilithes, Sinotubulites, and Cloudina) and calcareous microfossils (Protolagena), among which Conotubus and Gaojiashania are only known in this Lagerstatte worldwide. The paleobiological significance of the Gaojiashan biota has been discussed extensively, mostly in the Chinese literature. However, the taphonomy and paleoecology of this biota has not been studied in detail, limiting the interpretative power of these exceptionally preserved fossils.This study focuses on the taphonomy and paleoecology of Conotubus hemiannulatus and Gaojiashania cyclus. Integrated paleoecological, sedimentological, and taphonomic analysis shows that event deposition played an important role in the biostratinomy of the Gaojiashan Lagerstatte. Gaojiashan fossils, particularly pyritized fossils, preferentially occur in mm-thick normally-graded calcisiltite/siltstone layers, which are interpreted as. distal event deposits, but are rarely present in calcilutite and mudstone beds. Two taphofacies are recognized in the middle Gaojiashan calcisiltites/siltstones, on the basis of biostratinomic data (e.g., fossil fragmentation and disarticulation). Both taphofacies contain fossils (Conotubus, Gaojiashania, and Protolagena) that were buried in-situ by distal event deposits below the average storm wave base (SWB); evidence for in-situ burial includes oblique orientation, rejuvenation, growth interruption, and reorientation. This research represents one of the few biostratinomic studies of Precambrian Lagerstatten. The results show that the Gaojiashan Lagerstatte, like many Phanerozoic Konservat-Lagerstatten, was strongly influenced by event deposition which provides an obrution mechanism for quick burial.Early Cambrian Chengjiang-type Konservat-Lagerstatten owe their exceptional preservation to two primary modes of preservation:pyritization and carbonaceous compression with associated aluminosilicate clay coats. This study indicates that the late Ediacaran Gaojiashan Lagerstatte of southern Shaanxi Province, South China, shares a similar taphonomic array. Environmental scanning electron microscopic (ESEM) and energy dispersive X-ray spectroscopic (EDS) analyses of preserved soft tissues of representative Gaojiashan fossils, Conotubus hemiannulatus and Gaojiashania cyclus, indicate that authigenic pyritization is the primary taphonomic pathway within this deposit, with ca.80% of the collected fossils preserved through pyritization. Burgess Shale-type (BST) carbonaceous compression preservation and corresponding, yet unsettled, relationships with aluminosilicate clay coats are also present in this Lagerstatte, but may have played an auxiliary role in their exceptional preservation. Our taphonomic study documents that, like those in the Chengjiang-type deposits, authigenic pyritization tends to preserve more labile tissues than those preserved as carbonaceous compressions. We predict further discoveries of siliciclastic Lagerstatten sharing similar taphonomic modes to the early Cambrian Chengjiang and the late Ediacaran Gaojiashan deposits in which authigenic pyritization is dominantly responsible for exceptional preservation.Conotubus hemiannulatus is a cm-sized conical tube that tapers adapically from an aperture to a rounded apex. The tube consists of a series of nested cylindrical-to-funnel-shaped tube walls. The nested cylindrical tubes are broadly similar to the construction of the late Ediacaran fossil Cloudina. Integrated morphological, taphonomic, and paleoecological data suggest that Conotubus hemiannulatus occupied an epibenthic life-mode, with the apex anchoring to microbially-bound muddy substrate and the aperture extending upwards into the water column. It was probably a suspension-feeding organism. This soft bottom dweller employed specialized paleoecological strategies to rejuvenate and self-right the tubes after being subjected to sediment obrution, indicating strong burial-resistant capabilities. The tube likely provided space for the living organism, with additional larger cylinders added episodically to accommodate growth. Currently available evidence suggests that Conotubus hemiannulatus had non-biomineralized or weakly biomineralized tubes, and its phylogenetic affinity remains unresolved.Gaojiashania cyclus is a phylogenetically problematic tubular fossil that is only known from the late Ediacaran Gaojiashan Lagerstatte in southern Shaanxi Province, South China. It is a cm-sized tube that consists of a series of rigid and flexible couplets of tube walls that are edge-to-edge contacted with each other. The rigid wall is thicker (1-3 mm in thickness) and relatively shorter (1-2 mm in length), forming a circular ring; whereas the flexible wall is thinner (0.5-1 mm in thickness), elongated (0.5-5 mm in flexible and thus variable length) and ornamented with transverse annuli. The flexibility of the tubes allows the creature to curve, extend, and constrict at free angles. The tube construction of G. cyclus is unique in both fossil and modern tubular organisms hitherto known, and strikingly differs from that of other Ediacaran and Cambrian tubular fossils. Integrated morphological, taphonomic, and paleoecological data suggest that G. cyclus may have been a procumbent epibenthos, possibly with the rings anchoring to microbially-bound muddy substrate. The preserved tube is interpreted as the exterior part of the animal rather than a dwelling tube. Currently available evidence suggests that G. cyclus had non-mineralized or weakly mineralized tubes, and its phylogenetic affinity remains unresolved.