Hyolithellus

Abstract: The abrupt appearance of the diverse small skeletal fossils in the Early Cambrian is a major piece of evidence for the early Cambrian metazoan radiation event. In terms of skeletal composition, the early skeletal fossils could be subdivided into three groups: calcareous, phosphatic, and siliceous (Runnegar and Bengtson, 1995). The early metazoans, which utilized calcium phosphates for constructing hardparts (skeletons or sclerites), include brachiopods (Linguliformea)(Holmer, 2001), tubular fossils (hyolithelminths and Sphenothallus)(Li et al., 2004 b), bradoriids (Shu, 1990), lobopodian (e.g. Microdictyon)(Chen et al., 1995), protoconodonts and conodont-like fossils (Li et al., 2003), tommotiids (Qian and Bengtson, 1989; Bengtson et al., 1990), protoconulariids (Qian, 1999), etc. Among them, the elongated tubular fossils have been worldwide recovered from the Lower Cambrian, and are usually referred to hyolithelminths, which mainly includes two genera, Hyolithellus with circular cross-sections and Torellella with lenticular cross-sections (Fisher, 1962). Although hyolithelminths have been previously assigned to (or compared with) hyoliths (Syssoiev, 1959; Chen, 1984), annelids (serpulid polychaetes)(Qian, 1989), pogonophorans or cnidarians since these animals commonly bear or live in a tube, the biological affinities of hyolithelminths have not been resolved yet because of the simple tubular morphology and the lack of soft body preservation. The purpose of this paper is mainly to describe hyolithelminth fossils collected from the basal Cambrian Xihaoping Member in the Xiaoyang section. All specimens were obtained through acetic acid etching of carbonate samples. Based on the specimens from the Xihaoping Member at the Xiaoyang section, Torellella bisulcata sp. nov., which is mainly characterized by bearing a narrow and shallow sulcus on each narrow side, is erected. The Xiaoyang section is located at Xiaoyang, Zhenba County, Shaanxi Province, and is palaeogeographically located in the northern margin of the Yangtze Platform. The upper Neoproterozoic and Lower Cambrian stratigraphic successions at this section are well exposed. In ascending order, the stratigraphic successions consist of the Dengying Formation, the Shuijingtuo and Shipai formations. The basal Cambrian Xihaoping Member (the upper part of the Dengying Formation) consists predominantly of dolomites with abundant skeletal fossils (Text-fig.1), including phosphatic brachiopods (Li and Holmer, in Press), sponge spicules, molluscs, cambroclaviids, hyolithelminths (Li et al., 2004), etc. The Xihaoping Member (8.37 m thick) disconformably overlies the upper dolomite member of the Dengying Formation, and is disconformably overlain by the Shuijingtuo Formation. The 31 m thick lower member of the Shuijingtuo Formation consists of thin-bedded siltstones, mudstones and limestones, and is rich in skeletal fossils, including trilobites, brachiopods, protoconodonts and conodont-like fossils, microdictyoniid plates, sponge spicules, etc. Based on the small shelly fossils, the Xihaoping Member has been considered as the middle Qiongzhusian Stage (Qian, 1999). Although no complete trilobite specimens have been found, the author has recovered the librigenal spines of trilobites through etching of the carbonate samples with acetic acid. The occurrence of ligrigenal spines further indicates that the Xihaoping Member could be correlated with the lower or middle Qiongzhusian, consequently, the correlation of the lower Xihaoping Member with the Shiyantou Member in eastern Yunnan (Wang and Xu, 1987) is rejected (Li et al., 2004).