Rusophycus trace fossil amongst a bioturbated sandstone slab (trilobite nest). This specimen is of Ordovician age and was collected in Erfoud, Morocco. Rusophycus is a trace fossil typically associated with trilobites. They are defined as 'resting traces' and form when benthonic trilobites stop to rest, compressing and partially displacing the sediment beneath them, leaving behind their stationary impression. Often Rusophycus fossils are found at the end of a Cruziana trail; Cruziana being the 'walking trace' of the trilobite. This specimen is referred to as a trilobite 'nest' because its displays an abundance of trilobite activity. The nest measures 20 x 19 cm and is approximately 2 cm thick. Supplied with an information label.