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The revision of Early Jurassic monotacean, pectinacean and plicatulacean bivalves is based on museum material and own collections from 50 localities in the provinces of San Juan, Mendoza and Neuqué in western Argentina. Twenty-four species belonging to 17 genera of the families Monotidae, Oxytomidae, Asoellidae, Propeamussiidae, Entoliidae, Pectinidae, Terquemidae, Plicatulidae, Placunopsidae and Dimyidae are systematically described. The new pectinid genus Agerchlamys and new species of Kolymonectes, Entolium and Terquemia are proposed. All other species are referred to previously known taxa. Placement of genus Kolymonectes in the family Propeamussiidae is supported by evidence of an outer prismatic calcitic shell layer and lack of ctenolium. Further, a close relationship of Radulonectites to Camptonectes is revealed. The subgenus Harpax is re-validated and distinguished from Plicatula s.s. by differences in hinge, ligament and relative convexity of valves. Ten of the taxa are referred to species which have an almost cosmopolitan distribution. The others belong to distinct South Pacific stocks, some of which were previously thought to be typical to Boreal regions, but which have a bipolar distribution. Palmoxytoma, Kolymonectes, Radulonectites, Agerchlamys and the group of Plicatula (Harpax) raps Bayle & Coquand characterize an Early Jurassic Austral Realm. Along the East Pacific, the boundaries between Austral and Tethyan Realms lie in the Andean region. A biostratigraphical zonation of the Argentinian Lower Jurassic based on pectinaceans is discussed, including five Assemblage Zones which can be recognized in Argentina and Chile. These are named after local species of mainly circum-Pacific genera, such as Otapiria, Radulonectites and Posidonotis. Their equivalence with other local biostratigraphic units is analysed and their correlation with other biostratigraphical schemes based on bivalves from other circum-Pacific regions is attempted. |
