Cambrian oncolites from San José de Gracia, Sonora, Mexico

Recently discovered oncolites from a Lower Cambrian locality in San Jose de Gracia (SJG), Sonora (Mexico), illustrate the wide geographic distribution that they had in this part of Laurentia some 515–510 Ma ago. They are exceptionally large (~44 mm) in comparison to other marine forms of the North American Cambrian Carbonate Bank, but similar in their overall morphology and depositional environment. Their association with abundant biomorphs, especially indigenous filamentous structures with the curvature and spatial arrangement that fit the minimum criteria for biogenicity, supports their biological origin. Key morphological features include laminated growth in at least a portion of the oncolite, subspherical shape with internal porosity, and a nucleated habit on skeletal debris (mollusk shells) and mud concretions. The similarity of SJG oncolites with others from Cambrian outcrops separated today by hundreds of kilometers, suggests that massive microbially-driven, shallow marine ecosystems existed in western Laurentia during this time. This speaks of the impact that microbes had in shaping marine sediments, and the global primary productivity and carbon cycle during early stages of the Phanerozoic.