Abstract:

Ediacaran fossils mark a pivotal position in the evolution of life as it transitions between the predominantly microbial ecosystems of the Precambrian and the animal ecosystems of the Phanerozoic. Ediacaran communities were dominated by sessile epibenthos and their ecological structures were relatively simple. Many modern feeding modes such as macropredation, epibiosis, and interspecific competition or mutualism were absent or poorly represented. It is generally accepted that the soft-bodied Ediacara biota is part of a “failed” evolutionary experiment and it has no clear genetic relationship with Phanerozoic organisms. However, corresponding to the evolution of typical Ediacaran soft-bodied biota, there was a great proliferation of tubular animals at the end of the late Neoproterozoic, represented by the Gaojiashan biota, where Lagerstätte fossil deposit hosts a variety of soft-bodied or lightly biomineralized tubular fossils (Shaanxilithes, Gaojiashania, Sinotubulites, Conotubus, and Cloudina), calcareous microfossils (Protolagena), and calcareous cyanobacteria.
Unlike other Ediacara fossil assemblages, the Gaojiashan biota is dominated by benthic sessile suspension feeders or detritus feeders. Of them, Cloudina is a millimetre-scale conical tube tapering adapically from an aperture to a rounded apex. The bulk of the tube is constructed by a series of successively stacked, repetitive, unevenly spaced, and funnel-shaped tube wall units. Cloudina occupied an epibenthic suspension-feeding life-mode, with the apex anchoring to microbially bound muddy substrate and the aperture extending upwards into the water column. Conotubus is a centimetre-sized conical tube consisting of a series of nested cylindrical-to-funnel-shaped tube walls (cylinders hereafter). This conotubular construction of nested cylinders is similar to that of the late Ediacaran fossil Cloudina. Integrated morphological, taphonomic and palaeoecological data suggest that it likely employed an epibenthic life-mode, with the apex anchoring to the muddy substrate and the aperture extending upwards into the water column and it was probably a suspension-feeding organism. Gaojiashania is a centimetre-size tube that consists of a series of repeating units. Each unit consists of a rigid ring and a flexible bucket-shaped tube wall. Each tube maintains a constant diameter and the rigid ring elements typically range from 1 to 2 mm in length, whereas the bucket components are more varied at 0.5-5 mm in length and are ornamented with transverse annuli. Gaojiashania, however, may have been a procumbent epibenthos, possibly with the rings anchoring to microbially bound muddy substrate. Sinotubulites are characterized by millimetre-to centimetre-size multilayer tubes open at both ends. The tube consists of two morphologically different walls: a multilayer inner wall with weak ornamentation, and a multilayer outer wall with transverse or oblique corrugations and sometimes longitudinal ridges. Surface ornamentation and polygonal shape of the cross-section suggest that Sinotubulites probably lived as an epibenthos lying on the sea floor. Shaanxilithes is a ribbon-shaped impression with constant widths (1-6 mm or more) and is characterized by a series of closely spaced transverse annulations. Shaanxilithes is tentatively interpreted as a sessile epibenthos, with one end anchoring to the sandy or muddy substrate. Therefore, the Gaojiashan biota is characterized by complex morphological types and diverse body plans of variable size and lifestyle, representing a major biological innovation in Earth’s evolution.
Flourishing microbial mats on the Precambrian level-bottom forced the Gaojiashan biota to evolve some adaptive strategies, either through fixation to (such as Cloudina, Conotubus, and possibly Shaanxilithes) or free laying on (such as Gaojiashania and Sinotubilites) the substrate by adhering to or inserting into microbial mats. To cope with the frequent disturbance by storm events, ecological adaptations were manifested in many Gaojiashan organisms. The polygonal cross-section and longitudinal ridges in Sinotubulites may stabilize the tube lying on the substrate, preventing it from rolling in strong currents. After being subjected to sediment obrution, Cloudina and Conotubus employed specialised palaeoecological strategies to rejuvenate and self-right the tubes, while Gaojiashania and Sinotibulites could partially self-right through curving, extension and constriction, indicating strong burial-resistant capabilities.
Abundant meandering traces and possible burrows in the Gaojiashan biota and in the comparable Shibanta biota imply that bilaterians were an essential part of later Ediacaran ecosystems and indicate the possible presence of epibenthic and semibenthic free-moving and detritus-feeding organisms. The complexity of predator-prey dynamics at the beginning of metazoan diversification is evidenced by borings on Cloudina tubes.

Key words: Gaojiashan biota, terminal Neoproterozoic, ecological diversification, biological innovation, microbial-mat substrate, storm event deposit