After several decades of intense research, the topic of the Cambrian explosion has fed an abundant literature focusing on its biodiversity and phylogenetic patterns. Our understanding of this is, however, limited by the lack of accurate dating of the fossil radiation and abiotic changes. Although efforts have been made to develop a refined timescale, the Cambrian remains one of the last systems for which series and stage boundaries have not yet been accurately defined. This can be ascribed in part to the paucity of comprehensive litho-, bio-, and chemostratigraphic studies of early-mid Cambrian successions from key regions, such as Sonora (Mexico) and Iran. Data from Sonora are essential for establishing the still incomplete although fundamental Cambrian chronostratigraphy of Laurentia, whereas Iranian Cambrian successions represent the best opportunity for correlations between the western Gondwana margin and the Yangtze Platform in South China. This study presents preliminary stratigraphic data from Sonora and Iran. Special attention has been given to the range of the abundant and widelydistributed polyphyletic small shelly fossils (SSFs) due to their high potential for the subdivision of the early Cambrian.

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