Resurgence of cloudinomorph fossils with possible cnidarian affinity at the peak of the Cambrian Explosion (Cambrian Series 2, Stage 3) in southern Shaanxi, China

During the Precambrian-Cambrian transition, tubicolous animals were prevalent among small shelly fossils. These include the oldest-known skeletal fossil Cloudina from the terminal Ediacaran, and anabarites and hyoliths from the early Cambrian; considered to be possible annelids, cnidarians, and molluscs, respectively. Tubular fossils have important bio-stratigraphic significance during this period, and their diversity and evolutionary radiation trend herald the Cambrian explosion. However, the relative morphological simplicity of these fossils and lack of preserved soft-body information has resulted in poor taxonomic constraints. Here, numerous tubular fossils exhibiting multi-stage cylindrical or conical cups were recovered from the middle-upper Guojiaba Formation of Cambrian Series 2, Stage 3, in Mianxian, South China. One new genus (Mianxiantubus) and three new species (M. cyathiformis, M. obconicus, and M. varius) were established to accommodate the morphological diversity of the fossils. However, their affinities remain contentious and undetermined and have been tentatively ascribed to Cnidaria. The possible morphology and growth patterns of Mianxiantubus suggest a close relationship with cloudinomorph fossils, although it is notably distinct from the quintessential cloudinids owing to the presence of transverse septa. Moreover, many other tubular fossils have been recovered from the same level, which may extend to the overlying Xiannvdong Formation (Stage 3), co-occurring with archaeocyathids. This suggests that tubicolous organisms continued to diversify and occupied specific ecological niches even during the peak phase of the Cambrian explosion, dominated by bilateral epifaunal vagrants.