The Early Cambrian trace fossil Plagiogmus in its type area

Plagiogmus from the Kalmarsund area in Sweden is an Early Cambrian trace fossil, not Middle Cambrian as originally dated. It is an endichnial burrow, however showing clear affinity to the surfaces of some laminae within the preserving sandstone beds. The previously described ladder-like structure, giving the false impression of an epichnial trail, is the floor of the burrow. The roof of the burrow is preserved as a smooth, almost flat or slightly concave structure, formed by collapse of the roof during compaction, and giving the false impression of a hypichnial trail cast. The animal moved forward by extension and contraction in a strongly mucus-lined burrow, and the transversal ridges across its floor often indicate the direction of movement. Only traces of a very thin infilling of the burrow have been found, in contrast to the thick, laminated backfill found in specimens from Australia, the other area of known common occurrence. In a number of other respects the present material confirms observations made on the Australian material.