Jurassic ostreoida (bivalvia) from China (Tanggula Mountains, Qinghai-Xizang Plateau) and their paleobiogeographic context

Journal of Paleontology, May 2002 by Sha, Jingeng, Smith, Paul L, Fursich, Franz T

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 birmanica Reed, 1936, Gryphaea (Bilobissa) bilobata (J. de C. Sowerby, 1835), Nanogyra nana (J. Sowerby, 1822) and Eligmus rollandi Douville, 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 600 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.

INTRODUCTION

THE JURASSIC was a time of great paleogeographic change. Rifting of the Pangean supercontinent, which began in the Triassic, led to the opening of the North Atlantic and to the north-- westward drift of North America, resulting in a narrowing of the Pacific (Smith, 1999; Smith and Briden, 1977). At the same time, sea level was undergoing eustatic oscillation superimposed upon an overall significant sea-level rise from the Hettangian to the Tithonian (Hallam, 1981, 1988, 1992). This complex interplay of geographic reconstruction and eustatic sea-level change presumably imprinted itself upon Jurassic biogeography by influencing the dispersal of organisms. For example, one outstanding question concerning Jurassic dispersal between the Pacific and Tethyan oceans is the relative importance of 1) nascent marine links across a fragmenting Pangea (the Hispanic Corridor); 2) direct, Pantropical dispersal across the Pacific Ocean; and 3) dispersal around the margins of Pangea (Newton, 1988; Smith et al., 1990).

In order to address these complex issues, it is necessary to document the spatial and temporal distribution of as many different groups as possible, preferably at the species level, paying particular attention to the poorly studied but critically important eastern Tethyan region, which was the portal between the Pacific and Tethys oceans. In this paper we contribute to this effort by describing new collections of Jurassic ostreids from the mountains of Tibet.

The Tanggula Mountains, whose main ridge has an average height of more than 5,000 m, lie in the hinterland of the Qinghai-- Xizang (Tibetan) Plateau. They are now sandwiched geologically between the Longmoco-Yushu Paleotethys suture and Bangongco-Nu Mesotethys suture (Fig. 1), but during the Jurassic they were located at the northeastern margin of the Tethys, linking the northern Tethys with the northwestern Paleo-Pacific. Middle and Late Jurassic marine deposits, occasionally intercalated with nonmarine rocks, are widely distributed in the area, and Jurassic bivalves are common (Sha et al., 1998).

The ostreid fauna from the main ridge of the Tanggula Mountains is composed of only five identifiable and one indeterminate species. Biogeographically, this fauna consists of three groups: 1) a Tethyan group; 2) a northwest European-northern Tethyan group; and 3) a global group. It is difficult to identify ostreids to the species level because individuals vary greatly in outline due to their cemented mode of life and clustered habit. However, in this study this difficulty was lessened by the fact that the ostreids are well preserved and some species, such as Liostrea birmanica Reed, are very abundant.

There is some disagreement about stratigraphic nomenclature and correlations between the Boreal and Tethyan faunal realms across the Jurassic-Cretaceous boundary. The present authors follow Sha and Fursich (1993, 1994), using the Berriasian as the global initial stage of the Cretaceous, and the Volgian, which is equivalent to the global terminal Jurassic stage of Tithonian, as the Boreal Jurassic terminal stage. Following Gradstein and Ogg (1996), the Portlandian corresponds to the upper part of the Volgian.