A Reconsideration of the Relationship between the Terminal Ediacaran Conotubus and Cloudina in southern Shaanxi: New Evidence from Geometric Morphometrical Analysis

The late Ediacaran witnessed rapid turnover of organisms. As one of the earliest biomineralizing animal fossils，Cloudina has greatimplication on the evolution of complex macroscopic organisms . With a relatively stable and wide distribution, Cloudina has become one of the most important indicator fossils of the late Ediacaran. Located in the northwestern margin of the Yangtze Platform, the Gaojiashan Lagerstätte only contains a large number of well-preserved phosphatized Cloudina in the upper part of the Gaojiashan Member of the Dengying Formation, but also consists of many excellent 3-D preserved pyritized Conotubus in its lower part. Conotubus and Cloudina are believed to have some close affinities according to their conotubular forms and funnel-in-funnel structures. However, the relationship between them is still in doubt due to the great distinction in individual sizes and preservation modes. With the help of industrial computerized tomography (CT) technique, the internal structure and integrated feature of the pyritized Conotubus can be well distinguished and isolated from the home rock, which made it possible for the geometric morphological analysis like the isolated phosphatized Cloudina. The result is hopeful as it proved that there are no significant morphological difference between Conotubus and Cloudina in their embryonic tube indicating that the pyritized Conotubus may be the precursor of the biomineralized Cloudina. Given its near-completed sedimentary accumulation and abundant and continuous faunal records for the final interval of Ediacaran, the Gaojiashan section and wjthin which the faunal turnover from Conotubus to Cloudina is consequently a strong candidate for the Global Stratigraphic Section and Point (GSSP).