QUANTITATIVELY UNRAVELING THE A-P AXIS AND BODY LENGTHS FROM TRACE FOSSILS, AND INSIGHTS INTO EARLY ANIMAL EVOLUTION

Trace fossils are invaluable for studying early animals and their co-evolution with paleoenvironments, particularly during the Proterozoic–Cambrian transition. While the emergence of eumetazoans with defined body axes likely drove the Cambrian Explosion, the patchy 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 elusive and underexplored quantitatively. This study employs the integral scale, representing the self-correlation of trajectory forces and displacements at the organism–substrate interface, alongside 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 exceptionally high trajectory smoothness and a proportionality between normalized characteristic locomotory length and integral scale across time. Applying this scaling law to terminal Proterozoic trace fossils, we identify normalized characteristic lengths of 5–8, suggesting minimal body lengths exceeding this range. These findings provide quantitative evidence for slender A–P axes in early bilaterians, likely supported by hydrostatic bodies with robust nerve-muscle systems. These adaptations would have enhanced directional sensation, locomotion, and environmental exploration, enabling these organisms to thrive in complex and dynamic habitats. Our results illuminate the evolutionary roots of the Cambrian Explosion and Cambrian Substrate Revolution, offering a quantitative framework for studying deep-time trace fossils and early animal paleoecology.