Rebuilding Earth's first skeletal animals: the original morphology of Corumbella (Ediacaran, Brazil).

The evolutionary onset of animal biomineralization in the late Ediacaran (ca 555-538 Ma) is marked by the global appearance of enigmatic tubular fossils with unresolved phylogenetic relationships. Among these, Corumbella werneri from the Tamengo Formation (Corumbá Group, Brazil) has been variously interpreted as affiliated with cnidarians or bilaterians. Using synchrotron imaging and machine learning, we analysed new specimens of C. werneri to reconstruct their original skeletal organization. Our findings reveal that Corumbella's tubes were originally conico-cylindrical. Large individuals of Corumbella, including less compacted specimens, and compression experiments with modern annelid tubes all indicate that previous reconstructions of a quadrate outline and midline features were misled by taphonomic artefacts. We also show that the wall of Corumbella is composed of a single layer of ring-shaped elements. Unlike the fourfold symmetry of scyphozoans or the complex cataphract-like structures of Cambrian bilaterians (e.g. halkieriids, tommotiids and wiwaxiids), Corumbella displays structural similarities with other late Ediacaran corumbellomorphs, such as Costatubus. These taxa exhibit a distinctive barrel-on-barrel tube construction, with modular elements stacked on each other rather than nested. Our findings redefine Corumbella's morphology and phylogenetic affinities, contributing to a broader understanding of early biomineralizing metazoans and their ecological roles in the Ediacaran biosphere.