Two new probable feeding traces of Rusophycus from the Cambrian of China: tracemaker’s behavior and formation mode

Five Rusophycus specimens were collected from the Cambrian strata exposed near Wuhai, Inner Mongolia, China. They are associated with upward-fining laminasets of silt to clay and one with an intercalated calcareous layer. Two specimens, α and β preserved as convex hyporelief are found surrounded with a furrow of elliptical outline. Their serial thin sections show intricate features of disturbance and deformation such as distinct rounded grooves in Rusophycus α and fragmentary hyolith skeletons in Rusophycus β. Rusophycus α and associated structures are interpreted to have formed by a tracemaker that disturbed sediments to create a sinkhole-shaped depression and then formed Rusophycus within the depression. The positional correspondence of the structures in hyporelief and features in cross sections suggests that the structures on the bottom layer might have affected the formation of Rusophycus α and depression. Rusophycus β is interpreted to have formed by a tracemaker that disturbed the calcareous layer to mostly feed on transported hyoliths. Rusophycus α is a first-reported case that Rusophycus is enclosed within a larger depression probably made by the same individual, and Rusophycus β another first-reported case where a tracemaker likely fed on hard-shelled organisms. Rusophycus α is a furrow formed at the sediment–water interface and almost immediately filled with another upward-fining laminaset, and Rusophycus β is a burrow formed slightly below sediment–water interface during the deposition of the overlying layer and filled with processed sediments including hyoliths. These formation modes do not fully accord with the widely accepted notion of the formation of Rusophycus as a furrow or burrow, and their deviations are considered to have resulted from continued sedimentation with local sediment starvation and/or scouring in a tidal environment.