Early Pliocene tracer of North Atlantic and South Pacific sea surface currents: Janthina typica (Bronn, 1860) (Mollusca: Gastropoda)
Janthina typica is an extinct, rare, floating species of gastropod from the early Pliocene whose fossils have an unusual geographic distribution, appearing in the eastern North Atlantic archipelagos (Canary, Azores, Madeira and Selvagen Islands), Morocco, and Pacific (New Zealand, Australia and Japan). This study examines the origin of this biogeography and how the species may have dispersed via sea surface currents. We have considered the published ecological aspects of the genus Janthina, the Janthina typica fossil localities, and ocean palaeocurrents. Abundant specimens of J. typica are found in marine deposits on Gran Canaria island (northeast Atlantic), 40Ar/39Ar dated at 4.2 Ma. These deposits therefore accumulated just before the end of the early Pliocene warm climate and closely predate the start of global changes that gave rise to the world’s present climate. In the early Pliocene, the cold Canary Current did not yet exist. The subtropical northeastern Atlantic Ocean was warmer than today and its waters would have met the Circumtropical Current that crossed the Central American Seaway from the Caribbean to the Pacific. From there, the South Equatorial Current flowed towards the eastern coast of Indonesia before splitting north towards Japan and south as the East Australian Current. The latter must also have extended along the southern coast of Australia, crossing the Bass Strait before reaching the area of modern-day Perth in southwestern Australia. The reverse journey (from Australia to the eastern Atlantic Ocean) would have posed far more obstacles, and is considered improbable. J. typica therefore likely originated in the East Atlantic. The main causes for its extraordinary geographic distribution are its ecology as a floating animal in warm water, tectonic plate movements that permitted an open Central American Seaway and a restricted Indonesian Seaway, and Earth’s rotation and its influence on marine currents.