Fossil shrinkage cracks occurring on bedding surfaces of fine-grained siliciclastic rocks are characterized by sinuously curved, subcircular and subordinately sigmoidal cracks and networks of cracks. They are described from shallow marine deposits of Proterozoic age but not from the Recent. The crack infillings are usually easily separated from both lower and upper bedding surfaces. Such cracks, preserved in fine-grained quartzitic rocks of the Duruchaus Formation (Nosib Group) of the Damara Supergroup in central Namibia, are described. A relationship to shrinkage and cracking of a microbial mat that underwent partial desiccation during temporary aerial exposure is suggested and the name "microbial shrinkage cracks" is proposed. Evidence favouring microbial participation in the development of the cracks includes: (a) the complete absence of a shrinkable argillaceous or otherwise suitable interlayer; (b) the ready separability of the crack infillings from adjoining lower and upper bedding surfaces and accordingly their independance from the beds above and below; (c) the presence of pseudomorphs after pyrite and possibly ferroan carbonate on and close to the adjoining bedding surfaces; (d) the predominance of curved and sub-circular cracks, sometimes forming loops and 8-shaped marks; (e) the smoothened ripple morphology of the upper bedding surface below the cracks. The genetic model suggests the following sequence of events: (a) biostabilization of the siliciclastic sediment surface and successive growth of a microbial mat in a shallow marine, tidal environment; (b) aerial exposure of the mat leading to progressive dewatering and to crack initiation of the surface layer; (c) infilling of the cracks with siliciclastic detritus during renewed flooding; (d) recovery and continued growth of the mat, overgrowing the crack infillings; (e) burial of the mat under sediments with a subsequent aquatic event.