The Lower–Middle Ordovician (Ibexian, Whiterockian) sedimentary rocks exposed at Rancho Las Norias includes the informally named Las Norias formation, which consists of an intercalation of carbonate and clastic sediments with abundant marine fauna. These deposits occur as the most austral sedimentary rocks of Ordovician age for Laurentia, providing a critical link to understand the distribution of Ordovician marine faunas of North America. An investigation of the gastropod fauna from the upper portion of the Las Norias formation, Sonora, Mexico, is undertaken for the first time. The gastropod assemblage includesMaclurites acuminatus, ?Monitorellasp.,Lecanospirasp.,Malayaspiraaff.M.rugosa,Lophospira perangulata, andHormotoma? sp. This assemblage indicates a paleogeographic relationship with Laurentia, including the USA (Nevada), Canada (British Columbia, Newfoundland), Greenland, and the Argentine Precordillera.

In Mexico, Ordovician sedimentary rocks are exposed in the states of Baja California, Sonora, Chihuahua and Oaxaca, comprising approximately 30 stratigraphic successions ranging from Lower to Upper Ordovician. The ages of the sequences have been established primarily by utilizing conodonts and graptolites, which have also allowed us to differentiate between platform and oceanic basin environments. The State of Sonora has the most complete Ordovician stratigraphic sequences, ranging from Tremadocian to Hirnantian. The deposits in Baja California are Floian in age, while the sequences of Chihuahua range from Sandbian to Katian, and the deposits in Oaxaca are Tremadocian. The Ordovician deposits of northern Mexico (Baja California, Sonora, and Chihuahua) present a palaeogeographic relationship to the North American craton, mainly owing to faunal interspecific affinities, while the southern deposits (Oaxaca) are controversial owing to the high degree of endemism of the faunas; however, they show affinity with Gondwana, Baltica and Avalonia, with a possible insular origin. The biotic assemblages of the Ordovician of Mexico include a variety of taxa, including algae, poriferans, corals, bryozoans, brachiopods, molluscs, trilobites, echinoderms, graptolites and conodonts as predominant elements. Despite many years of field studies in Mexican Ordovician localities, biostratigraphic correlations are as yet insufficient and incomplete or are based on limited interpretations. Thus, the Ordovician biostratigraphic data from Mexico compiled in the present paper have great potential and significant value. The advancement in the knowledge of the Ordovician biostratigraphy of Mexico will contribute to a major understanding of the relationships with the Ordovician System to a continental scale. Future advances will come mainly through increasing the amount and quality of data as well as improving biocorrelations among the Ordovician sequences of Mexico.

En la región de Arivechi, estado de Sonora, aflora una secuencia sedimentaria del Paleozoico inferior (Cámbrico), representada por las formaciones La Sata, El Mogallón, La Huerta y Milpillas, con una litología predominante de caliza, caliza arcillosa y lutita. Esta secuencia está caracterizada por una abundante y diversa biota fósil, la cual consiste en algas oncolíticas, icnofósiles, esponjas, braquiópodos y trilobites. En este trabajo se describe e ilustra por primera vez para México, la especie Ptychagnostus atavus, trilobite agnóstido colectado en capas del Drumiano (Cámbrico-Miaolingiano) de la Formación El Mogallón. Las rocas cámbricas de la región de Arivechi, se depositaron como parte de una potente secuencia de sedimentos del margen occidental del cratón de Laurentia, asociadas a un ambiente de plataforma exterior. La distribución cosmopolita de esta especie sugiere la existencia de una amplia provincia faunística, que comprende localidades de Europa (Suecia), Asia (Corea), Canadá (Columbia Británica), Estados Unidos de América (California, Nevada, Utah), México (Sonora) y Argentina (Precordillera Argentina). Su amplia distribución confirma su utilidad relevante como índice estratigráfico y de correlación global para el Cámbrico medio.