Taxis behaviour of burrowing organisms recorded in an Ediacaran trace fossil from Ukraine

The trace fossil Archaeonassa cf. fossulata Fenton and Fenton is a bilobate, slightly undulating, corrugated ridge on upper bedding plains in the Ediacaran shallow marine siltstones of Ukraine which are no younger than 557 Ma. It was produced partly under microbial mats. This trace fossil shows strong orientation within a sector of 20 degrees-40 degrees, which is approximately perpendicular to the expected shoreline. The orientation points to ability of some Ediacaran burrowing organisms to taxis during early evolutional stages of bilaterians in response to physical or chemical stimuli, probably according to direction of tides. Undulations of A. cf. fossulata can be regarded as a record of the in-and-out burrowing in response to diurnal cycles of oxygen production within microbial mats. Probably, this is one of the oldest examples of such behaviour. In general, Archaeonassa is considered as a relatively common trace fossil in Upper Ediacaran deposits worldwide, especially in shallow marine deposits in the time interval from 560 to 550 Ma.