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

STRATIGRAPHY

The Jurassic of the Tanggula area has been subdivided into six units, they are, in ascending order, the Quemoco, Matuo, Tuotuohe, Xiali, Suowa, and Zhaworong Formations (Sha et al., 1998) (Table 1). Boundaries between the formations are all conformable but, in the absence of ammonites or microfossils, age control is not precise. According to the concurrent ranges of palaeotaxodont, pteriomorph, and isofilibranch bivalves, the Quemoco Formation is Bajocian-Bathonian in age, the Matuo Formation is Bathonian, the Tuotuohe, Xiali, and Suowa Formations are Callovian-Oxfordian, and the Zhaworong Formation is pre-Cretaceous and possibly as old as Oxfordian (Sha et al., 1998).

Ostreids only occur in the Matuo, Tuotuohe, Xiali, and Suowa Formations (Table 1). According to known geological ranges in other parts of the world, the ostreids' concurrent ranges in the Matuo Formation indicate a Bathonian age, and a Bathonian-Oxfordian age for the Tuotuohe, Xiali, and Suowa Formations. However, the Tuotuohe, Xiali, and Suowa Formations overlie the Matuo Formation. These three formations are, therefore, mainly or even all Callovian-Oxfordian in age, as is also indicated by other bivalves present (Sha et al., 1998). There are no ostreids in either the Quemoco and Zhaworong Formations. As there are no Cretaceous marine deposits recognized in the Tanggula and surrounding areas, the Zhaworong Formation overlying the Callovian-Oxfordian Suowa Formation is probably no younger than Tithonian and its lower limit is possibly near the Oxfordian/Kimmeridgian boundary. These conclusions based on ostreids are consistent with those of Sha et al. (1998), based on other bivalves.

BIOGEOGRAPHY

All elements of the Bathonian-Oxfordian ostreid fauna described below are restricted to warm and temperate zones of low and intermediate paleolatitudes. Liostrea birmanica is only recorded from the eastern Tethys and south Xizang areas, Eligmus rollandi is limited to the Tethyan region as a whole, Gryphaea (Bilobissa) bilobata is restricted to northwest Europe and the northern Tethys, and Actinostreon gregareum has a global distribution between paleolatidudes 60 deg north and south (Fig. 2).

Such distribution patterns indicate that 1) during the Jurassic all the oceans and seas were connected to each other, allowing the dispersal of some cosmopolitan species such as A. gregareum and N. nana; 2) the Tethys and the northwestern European epicontinental sea produced some endemic taxa, e.g., F. rollan& (Hallam, 1977); and 3) the distribution of these ostreids was most likely controlled by environmental parameters linked to latitude.

The distribution of Actinostreon gregareum.-Actinostreon gregareum occurred first in the Sinemurian of northern Chile, spreading to east Africa, Madagascar, and possibly Spain during the Toarcian. In southern and northwestern areas of the Tethys (e.g., eastern Iran), northwestern Europe, and western North America (e.g., the Stikine terrane of western Canada), this species appeared in the Bajocian. However, its earliest occurrence in the northeastern Tethys (including the areas of Tanggula, western Yunnan and Burma) is Bathonian, whereas in the northwestern Pacific (Japan) it is Late Jurassic (Fig. 3). This distribution pattern is related to the migration history of this bivalve.