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

Left valves of ostreids in our Chinese material show attachment areas varying from tiny to as large as half the shell surface, demonstrating that these bivalves had a cemented mode of life during the postlarval period. Ostreids were also able to fix themselves to movable objects such as driftwood or cephalopods, thereby further facilitating their dispersal (Seilacher, 1960).

Fursich (1977) recognized that N. nana was an opportunistic species (Levinton, 1970). Actinostreon gregareum also exhibits the characteristics of opportunistic taxa, particularly with respect to distribution pattern and relative abundance. Its only requirements are a hard substrate and more or less fully marine conditions. It occurs in most marine environments between latitudes 60 north and south. Communities dominated by such ostreids are very low in diversity, the other elements being eurytopic taxa. One of the special features of opportunists is their ability to spread rapidly and occupy new adverse environments where other organisms cannot withstand the stress (Levinton, 1970). Actinostreon gregareum also demonstrated this special dispersal capacity.

DISCUSSION

Jurassic climate was warm and equable (Hallam, 1985; Crowley and North, 1991), however, there is no record of A. gregareum in the areas north and south of 60 deg. This biogeographic pattern implies that the distribution of this bivalve was temperature-controlled. Consequently, this Jurassic thermophilous oyster was likely unable to migrate between the Tethys and Pacific oceans via polar areas. Both planktonic larval dispersal and pseudoplanktonic postlarval dispersal are strongly related to oceanic currents (Heinze, 1996; Sha et al., 1998). The early Toarcian, Bajocian, Callovian, and Oxfordian were times of global eustatic sea level rise (e.g., Hallam, 1981, 1983, 1988, 1992), which possibly facilitated the dispersal of ostreids as habitable areas were enlarged.

Westward trans-Pacific dispersal of A. gregareum and Nanogyra nana from the southeastern margin of the Pacific to the southern margin of the Tethys probably occurred during the Toarcian when these species appeared in the Malagasy Gulf. They could not have arrived there from the south given their absence from high latitudes and the fact that there was no seaway between Africa and Antarctica joining the southern Andes and east Africa until the Tithonian (Hallam, 1994). The best means of trans-Pacific dispersal is via island hopping of planktonic larvae. Recent oysters can have a life span of more than 25 yr and Miocene ones more than 47 yr (e.g., Bjerkan, 1918; Haranghy et al., 1965; Stenzel, 1971). If such long-lived oysters fixed themselves to floating objects, they could have been carried considerable distances in a single generation. It is, therefore, possible for long-lived individuals attached to large driftwood or ammonites, which in exceptional cases could serve as floats for a few years (Oschmann, 1994), to complete a trans-Pacific journey. Furthermore, after metamorphosis, oysters reach sexual maturity in 20-30 days in the Tropics and more than a year in cold climates (Stenzel, 1971). This implies that oysters must have been able to reproduce during their pseudoplanktonic phase, which would have been very advantageous for crossing the wide Pacific.