Quantitative decoding of Ediacaran locomotory trace fossil morphologies: Evidence for the emergence of slender anterior-posterior body profiles

Trace fossils are vital for studying early animals and their co-evolution with paleoenvironments during the terminal Ediacaran, a period with sparse body fossil records. Thus, patterns of morphologic evolution are difficult to untangle for Ediacaran trace-makers and quantitatively unexplored. In this study, we use the integral scale, which reflects the distance within which a trajectory (i.e., force and displacement) is self-correlated, as a potential indicator for the characteristic length of trace-maker’s locomotion. By analyzing modern and fossilized animal-trace-correlated trajectories, a proportionality between the characteristic locomotory length and the trajectory integral scale is found. Since the length of the structure producing locomotion is no larger than that of the body, the characteristic locomotory length also reflects the minimal body length. Applying this scaling law to Ediacaran−Cambrian locomotory trace fossils (e.g., Archaeonassa, Gordia, Helminthopsis, Parapsammichnites), we identify clear evidence of slender anterior-posterior body axes after around 545 Ma, with gradually increasing minimal body length-to-width ratios to up to 4−12. The trace-makers probably had relatively rigid bodies with robust hydrostatic nerve-muscle systems enhancing directional sensation and movement, enabling them to thrive in dynamically complex, heterogeneous, and shifting habitats. These adaptations likely drove niche partitioning and cascading diversification, underpinning the evolutionary roots of the Cambrian Explosion and more familiar animals of the Phanerozoic. Our findings establish a novel quantitative approach to studying deep-time locomotory trace fossils, offering robust insights into early animal anatomy and paleoecological dynamics.