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

Abstract

The evolutionary onset of animal biomineralization in the late Ediacaran (<i>ca</i> 555-538 Ma) is marked by the global appearance of enigmatic tubular fossils with unresolved phylogenetic relationships. Among these, <i>Corumbella werneri</i> 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 <i>C. werneri</i> to reconstruct their original skeletal organization. Our findings reveal that <i>Corumbella'</i>s tubes were originally conico-cylindrical. Large individuals of <i>Corumbella</i>, 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 <i>Corumbella</i> 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), <i>Corumbella</i> displays structural similarities with other late Ediacaran corumbellomorphs, such as <i>Costatubus</i>. These taxa exhibit a distinctive barrel-on-barrel tube construction, with modular elements stacked on each other rather than nested. Our findings redefine <i>Corumbella</i>'s morphology and phylogenetic affinities, contributing to a broader understanding of early biomineralizing metazoans and their ecological roles in the Ediacaran biosphere.