The T. Pedum Tracemaker at the Ediacaran-Cambrian Boundary

Treptichnus pedum defines the base of the Cambrian, yet the identity of its tracemaker and the reliability of this ichnotaxon as a biostratigraphic marker remain debated. We document an extensive assemblage of exceptionally preserved T. pedum from the basal Fish River Subgroup at Farm Sonntagsbrunn, Namibia, which reveals the full three-dimensional architecture of the burrows, including a segmented master tunnel, oblique and vertical probes, and a finely structured tri-lobate lower surface. Weathered specimens show paired, bilaterally symmetrical lobes arranged in consistent series, features that enable quantitative reconstruction of the morphology and locomotor organization of the tracemaker. Curvature-based integral scale analysis of meandering trajectories indicates that coordinated locomotor units were about three body widths long and that full body length approached ten body widths. Combined with the tri-lobate basal structure and repeated bilateral lobes, these data indicate that the tracemaker possessed multiple pairs of lateral appendages restricted largely to the anterior third of the body, and moved using peristaltic waves supplemented by limb-driven probing. This anatomy and gait rule out priapulids or other scalidophorans in this locality, despite their established production of Treptichnus-like traces elsewhere, and instead point to a segmented, vermiform bilaterian with parapodia-like limbs. The resulting pattern of frequent vertical probing and incremental redirection indicates limited posterior maneuverability and a behaviorally constrained mode of sediment exploration. Importantly, similar probing-dominated strategies evolved independently in multiple bilaterian clades across the Ediacaran–Cambrian transition, suggesting a broader ecological response to rapidly changing substrate conditions. Our findings refine interpretations of the T. pedum tracemaker, constrain the anatomy of one of the earliest complex burrowers, and clarify the evolutionary dynamics underlying its use as the primary marker for the base of the Cambrian System.