The biotic “Lacustrine Mesozoic Revolution”. New data from the Barremian of Iberia
Résumé
Key evolutionary events in the Mesozoic evolution of lacustrine systems include (1) a worldwide expansion of wetlands, (2) diversification of aquatic macrophytes and (3) diversification of aquatic insects. These events were considered a "Mesozoic Lacustrine Revolution" by Cohen (2003) and were dated as "Middle Mesozoic". New data from the Barremian of Spain precise tempos and taxa involved in such "Revolution". (a) Aquatic Macrophytes were dominated by charophytes during Late Jurassic and Early Cretaceous. A significant diversification occurred worldwide with the substitution of Triassic and Mid Jurassic porocharacean-dominated floras by characean- and clavatoracean floras, the latter being diverse only in the Tethyan realm. In the Iberian Plate these floras became enriched with the first aquatic angiosperms, starting from the Early Barremian. Aquatic angiosperms include very abundant Montsechia vidali bearing Spermatites seeds, and less-abundant "Ranunculus" ferreri. Other taxa, like the floating Proteaephyllum reniforme and the hydrophytic lycophytes Thomsonia sp., were very rare. These aquatic plants appear to be limited to permanent oligotrophic lakeshores. (b) The dominant families of aquatic insects were not the same in the Early Cretaceous as in the Jurassic. Ephemeropterans, aquatic heteropterans, dipterans and coleopterans were common and diverse in the Early Cretaceous. In comparison with the Jurassic, practically no extinction occurred at the family level, but some recent families appeared, which are unknown or rare from older deposits (i.e. mayfly Euthyplocidae, Potamantidae; beetle Dytiscidae, Gyrinidae, Hydrophyllidae). Other families have their last representatives (beetles Coptoclavidae and Schizophoridae). In odonatans modern anisopteran lineages radiated, whereas some groups such as "anisozygopterans" decreased. During the latest Early Cretaceous a large-scale extinction occurred, mainly of lentic insects. In the Late Cretaceous only the running waters supported rich insect faunas, and aquatic taxa evolved in lotic environments.