Abstract

The Bathonian–Oxfordian ostreid fauna from the main ridge of the Tanggula Mountains of the Qinghai-Xizang Plateau, China, consists of six taxa: Actinostreon gregareum (J. Sowerby, 1815), Actinostreon sp. A, Liostrea birmanicaReed, 1936, Gryphaea (Bilobissa) bilobata (J. de C. Sowerby, 1835), Nanogyra nana (J. Sowerby, 1822) and Eligmus rollandiDouvillé, 1907. Liostrea birmanica is only known from the eastern Tethys and south Xizang area, Eligmus rollandi is limited to the Tethys, G. (B.) bilobata occurs in northwest Europe and the northern Tethys, whereas A. gregareum and possibly N. nana have a complex global distribution between paleo-latitudes 60° north and south.

Actinosteon gregareum first occurs in the Sinemurian of northern Chile, and during the Toarcian it underwent trans-Pacific dispersal to arrive in east Africa. During the Bajocian it dispersed rapidly along the southern and northwestern margins of the Tethys, northwestern Europe, and western Canada (Stikine Terrane), but it disappeared from South America in the Aalenian. It occupied Kachchh, southern Xizang, and the northern and northeastern Tethys as early as the Bathonian but it did not reach the northwestern Pacific until the Late Jurassic. The species declined after the Kimmeridgian, being limited to northern Africa (southern Tunisia) and the northwestern Pacific (Japan) during the Tithonian. By the end of the Jurassic it was extinct.

Actinostreon gregareum apparently possessed very high fertility typical of opportunists that rapidly colonize new habitats. As a result of ocean current dispersal, presumably by both planktotrophic larvae and postlarval pseudoplankton, it rapidly spread along continental margins and island chains. Occasionally, either directly or by island hopping, it crossed the vast Tethys and Pacific oceans, colonizing all warm and temperate waters at low and intermediate paleolatitudes. It may also have used the Hispanic Corridor as a means of dispersal between the Tethys and Pacific oceans as early as the Toarcian.

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