Trace fossils are crucial for understanding early animals and their co-evolution with paleoenvironments, especially during the Proterozoic–Cambrian transition. While the emergence of eumetazoans with defined body axes likely triggered the Cambrian Explosion, the fragmented body fossil record often makes locomotory trace fossils the primary source of anatomical insights into trace-makers. However, evidence of a slender anterior–posterior (A–P) axis in Proterozoic organisms has been scarce and underexplored. This study introduces the concept of integral scale—representing the self-correlation of trajectory forces and displacements at the organism–substrate interface—along with smoothness criteria, as proxies for the locomotory structures of trace-makers. By analyzing both fossil and modern locomotory trajectories, we demonstrate that slender trace-makers exhibit unusually high trajectory smoothness and a proportional relationship between normalized characteristic locomotory length and integral scale across time. Applying this scaling law to terminal Proterozoic trace fossils, we find normalized characteristic lengths of 5–8, indicating that body lengths likely did not exceed this range. These findings provide quantitative evidence for slender A–P axes in early bilaterians dating back to as early as 545Ma, likely supported by hydrostatic bodies and robust nerve-muscle systems. Such adaptations would have enhanced directional sensing, locomotion, and environmental exploration, allowing these organisms to thrive in complex and dynamic habitats. Our results shed light on the evolutionary roots of the Cambrian Explosion and the Cambrian Substrate Revolution, offering a quantitative framework for studying deep-time trace fossils and early animal paleoecology.

Trace fossil,Integral scale,Cambrian,Ediacaran