西藏早白垩世腕足类Peregrinella的发现

<正> 早白垩世地中海海域内,分布着一个大型的、引人注目的腕足类化石Peregrinellamulticarinata(Lamarck),阿格(Ager,1964,1973)曾把它和三叠纪的Halorella比较,同称为“昙花一现的化石”。认为它们具有广泛、断续的地理分布,在地史上忽然出现,突然消失,既不了解它的祖先,又未见其后代。这种现象决不能单纯用“相化石”解释,而是和生命环境的瞬息改变相联系。

THE DISCOVERY OF EARLY CRETACEOUS Peregrinella (BRACHIOPODA) IN XIZANG (TIBET)

The genus Peregrinella has been recorded from southeast France, Musen-alp, Carpathian Mts, North Caucasus to California of America with a wide, separate geographical distribution, being known as an important index fossil of the Valanginian to Hauterivian in the Tethyan sea. G. Biernat (1957) presumed that Peregrinella migrated to Europe from the California by the northern boreal sea. D.Ager(1973, 1981)considered it as a "fleeting fossil" that just suddenly appears and disappears, having no apparent direct ancestors or descendants. Now, this"litter stranger"has also been found in Xizang, situated on the northern slope of the Shelongre Hill, 2 km south from Gongboxue village, southeast to Yamzho Yumco lake. The Peregrinella-bearing strata were named as Ruemowa Formation, the upper part of Yulongbaijia Group by Wang Naiwen et al. (1982). It is characterized by the grey, grey-yellowish calcareous sandstone with lenses of argillaceous limestone, about 50 m thick. The specimens of these brachiopods were collected from the argillaceous limestone associated with belemnites and ammonoides: Criocerafites (Criocera-tites)cf. loryi(Sarkar). Below the Peregrinella bed some fragments of ammonoides Olcostephanus sp. were collected. According to the faunal feature, the Ruemowa Formation can be exactly correlated with that of classic Peregrinella beds in Rottier, France beloging to the Lower Hauterivian.Like those in western Europe, the Tibetan Peregrinella fauna was low diversity both in the genus and species, mainly including two forms:one is the type species P. multicarinata(Lamarck)with regular, simple, distinct costae, the other with bifurcate costae is quite similar to Rhynchonella silesica Ascher(1906), which not only existed in Silesia, but had also been observed by Biernat(1957, p. 38)in French specimens. In this paper the writers describe it as a new subgenus Peregrinellina(type species: P. xizangensis sp. nov.) on the basis of the bifurcate costae. The study of serial sections indicates that the crura in Peregrinella are radulifer in form, and arise on the distal ends of the hinge plates, sometimes directly in contact with median septum. The Peregrinella probably inhabited shallow water, muddy bottom conditions. This is reflected in its large size, strong costae and the sediments with arenaceous and argillaceous materials.