ALT Two photographs at the top show experimental tank containing sand and priapulid worms, which have created burrows seen here in cross-section. Arrows indicate direction of movement. Diameter of coin at the bottom of each image is 2.4 cm. In the lower row are two field photographs of trace fossils. On the left, a treptichnid from the terminal Ediacaran of Namibia (Scale bar at top right is 0.5 cm). In the middle, Treptichnus pedum from the early Cambrian of Namibia (scale bar at top right is 5 cm). On the right, a schematic diagram of the T. pedum 3D structure, with solid line numbered arrows denoting individual probe order offset by 20°–40° and a longer dotted line showing general direction of movement.