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The Early Jurassic (late Hettangian to early Toarcian) bivalve fauna of the Sierra de Santa Rosa Formation of the Antimonio terrane (Sonora, NW Mexico) is analyzed taxonomically and biogeographically. Fifty taxa are recognized, representing 36 genera and subgenera. Thirty-four of these taxa have not been mentioned from the Jurassic of this region previously. This fauna is of great biogeographical interest, because Early Jurassic bivalves from low paleolatitudes of the tectonically complex western margin of North America are still poorly documented. About half of the described species are also known from other localities along the eastern Pacific margin. The second largest group is composed of widespread taxa, which, in addition to eastern Pacific occurrences, are also reported from other regions, particularly from Europe. The smallest group is endemic taxa that appear to be limited to Sonora during the analyzed time intervals. Geological evidence indicates that the Antimonio terrane was tectonically transported southeastward between the Middle and Late Jurassic from an original position at the southwestern margin of the United States by the Mojave-Sonora megashear. We calculated similarities of contemporaneous pectinoid bivalve faunas from seven eastern Pacific regions to independently constrain Early Jurassic paleolatitudinal positions of this terrane. Cluster analyses and similarity coefficients tentatively suggest that tectonic displacement of the Antimonio terrane toward lower paleolatitudes may already have started in Early Jurassic (Pliensbachian) time.

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