Biomechanical analyses point to dense collagen fibre architecture in an Ediacaran frond.

Ediacaran macrofossils record the earliest large and complex organisms on Earth. Despite extensive study, fundamental aspects of their internal anatomy and tissue composition remain poorly known, limiting understanding of how these organisms were constructed and functioned, as well as how their complex anatomies evolved. Here, we establish a new quantitative biomechanical framework to infer tissue properties in the Ediacaran frond Charniodiscus. Analyses of fossil morphology, numerical simulations and dimensional analysis indicate that the commonly preserved twisted forms reflect three-dimensional torsional deformation driven by turbidity flow-induced instability. By mapping deformation modes in dimensionless phase space, we calculate a Young's modulus consistent with densely aligned collagen fibres, under a eumetazoan framework. These results point to the presence of a load-bearing stem and substantial functional specialization. The inferred mechanical properties are largely incompatible with non-animal, sponge or placozoan tissues, and independently strengthen the proposal that Charniodiscus was a total-group eumetazoan. The presence of these tissues marked a foundational evolutionary step, providing the scaffolding for increased size, complexity and innovation in early animals.