Prior to the advent of widespread bioturbation during Cambro - Ordovician times, microbial mats may have covered large expanses of the continental shelf Evidence of matgrounds in shallow-marine settings is provided by abundant wrinkle structures in Lower Cambrian strata of the Great Basin, United States. Wrinkle structures from the Lower Cambrian Harkless Formation commonly co-occur with a distinctive assemblage of invertebrate fossils, providing evidence for the possibility of selective metazoan colonization of matground substrates. Molds of linguliform brachiopods are abundant on many wrinkle surfaces. The agglutinated problematicum, Volborthella tenuis, also is found on wrinkle surfaces and in laminations beneath wrinkle-structure surfaces. Bedding-parallel trace fossils, such as Planolites, Diplichnites, and Taphrhelminthopsis, commonly crosscut wrinkle structures, while vertically oriented trace fossils are absent. Microbial mats containing layered microbial communities would have considerably compressed stratified redox zones beneath the sediment-water interface in marine-shelf settings. Sulfidic and anoxic conditions within and beneath microbial mats would have precluded habitation by many metazoans, while those that adapted to such conditions may have found matgrounds a unique, though temporally fleeting, ecological niche. The distinctive, low-diversity fossil assemblage found in association with the wrinkle structures in the Great Basin suggests that some early animals may have been adapted to hypoxic and sulfidic conditions found in matground substrates, while others may have been physiologically excluded from these environments.