Bivalve palaeobiogeography and the Hispanic Corridor: time of opening and effectiveness of a proto-Atlantic seaway

The Hispanic Corridor is an embryonic seaway between the eastern Pacific and western Tethyan oceans that has been postulated to have preceded Middle Jurassic drifting and the birth of the Atlantic Ocean by many millions of years. In this study the distribution of pectinoid bivalve morphotypes is analysed to examine the origin of the Hispanic Corridor and its effectiveness during Early Jurassic times. A comparison of pectinoid faunal similarities, based on similarity coefficients, between various regions at opposite ends of the Corridor suggests that it opened progressively during Early Jurassic times. However, with this approach it is difficult to exclude alternative dispersal routes, and to determine when faunal interchange began. Analysis of the percentage of pectinoid morphotypes that were (1) present at opposite sides of the Corridor, (2) simultaneously absent in the western Pacific/eastern Tethys and (3) confined to relatively low palaeolatitudes provides evidence that the Hispanic Corridor developed from an effective barrier in earlier Early Jurassic (Hettangian to Sinemurian) times into a filter, allowing the passage of a few morphotypes, during later Early Jurassic (Pliensbachian and Toarcian) times. The apparently two-way faunal exchange through the Hispanic Corridor is consistent with the establishment of a megamonsoonal circulation for Pangaea, which may have caused seasonally changing directions of oceanic surface currents through the Corridor. (C) 2001 Elsevier Science B.V. All rights reserved.