Marine Ostracod Distributions during the Early Breakup of Southern Gondwanaland

Abstract

Plate tectonic and palaeo-oceanographical development of southern Gondwanaland, in particular the South Atlantic and Indian oceans, is documented in its marine Jurassic and Cretaceous ostracod faunas. During breakup, which commenced in mid-Jurassic times, seaways developed across the supercontinent via the Falkland Plateau and southern Africa, to Madagascar, India, and Australia. These were connected (probably via the SE Pacific) to the Neuquen Basin of west Argentina, but were isolated from the Equatorial South Atlantic by the Walvis-Rio barrier. The southern Gondwanaland oceans fostered a distinctive ostracod fauna that can be recognized in Bajocian to Cenomanian strata: South Gondwana Fauna (SGF). It contains numerous endemic forms, with the most characteristic species belonging to the Progonocytheridae. A major faunal change occurred between late Cenomanian and early Turonian times, when new taxa migrated from the Equatorial South Atlantic, via the Temperate South Atlantic, and ousted the extant SGF. Vigorous colonisation and rapid speciation was probably encouraged by radical oceanographical changes resulting from modifications in palaeogeography, especially the flooding of the Walvis-Rio barrier between the Equatorial and Temperate South Atlantic oceans. By Coniacian time at the latest, a cosmopolitan Pan Gondwana Fauna (PGF) was established, characterized by species belonging to the Trachyleberididae and Brachycytheridae. Subtle alterations in composition of the SGF and PGF may also be related to palaeogeographical changes.