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.

Abstract The Hispanic Corridor is a postulated marine seaway linking the eastern Pacific and western Tethyan oceans as early as Early Jurassic times. Two existing hypotheses relate the Pliensbachian-Toarcian bivalve extinction and recovery to immigration of bivalve species through the Hispanic Corridor. The extinction hypothesis implies that, in South America, the Pliensbachian-Toarcian extinction can be partly explained by the immigration of bivalves through the Hispanic Corridor and subsequent competitive replacement. The recovery hypothesis states that, in NW Europe, the renewed rise in diversity in the late Toarcian/Aalenian was largely a consequence of immigration of taxa from Andean South America via the Hispanic Corridor. To test these hypotheses, I calculated immigration and origination rates of bivalves per million years. In both regions, early Pliensbachian to Aalenian immigration rates remained at low levels, thus disproving both hypotheses. By comparison, the origination of new species generally played a much more important role than immigration in controlling overall diversity of both regions. Future research should investigate if this is a more general pattern in the recovery of post-extinction biotas. The apparently global Pliensbachian-Toarcian diversity crisis may be best explained by a combination of physicochemical factors, invoking intense volcanism, sea-level highstand and widespread anoxia, as well as biological factors. Recovery from this mass extinction commenced when origination rates increased again, which, in the Andean basins, was in the Aalenian and in NW Europe, the late Toarcian.