Sedimentary structures produced by microbial activity are well known from ancient carbonate facies, but little is known about equivalent microbial structures in ancient siliciclastic facies. Wrinkle marks and Kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics, and they may represent microbial activity. To evaluate this possibility, these structures were studied in Vendian–Cambrian strata of North America and compared to structures formed by modern microbial mat communities in Redfish Bay, Texas. Striking similarities in sedimentologic, petrographic, and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes. The relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the Ordovician, microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea-floor environments.