延吉日本蚌的个体变异并初论日本蚌类的演化趋势

ONTOGENETIC VARIATION OF NIPPONON AIA YANJIENSIS GU (LOWER CRETACEOUS BIVALVIA) WITH DISCUSSION ON EVOLUTIONARY TRENDS OF NIPPONONAIIDS

一、前言1986年夏,陈丕基等在辽宁西部进行白垩系调查时,于孙家湾组采获一批双壳类化石;同年9月和11月,常征路、蒋福兴和张平安等两次去该化石点补充采集,得到更丰富的标本。这些材料交与笔者研究,鉴定为Nippononaia yanjiensis Gu和Unio yanbianensis Gu,这两个种都曾在吉林东部延吉盆地的铜佛寺组发现过(《中国的瓣鳃类化石》,1976)。在我国的早白垩世地层中,珠蚌类与类三角蚌类(包括日本蚌类)大量共生的现象十分罕见。通常认为,

Here discussed are nippononaiid specimens, more than 200 in number, all from the non-marine Cretaceous Sunjiawan Formation of western Liaoning, Northeast China. They are monotonous in aspects for specific classification, though there are some variations from our observation, such as in shell body size, surface ornamentation and hinge structure. At this fossil locality the species Nippononaia yanjiensis Gu is associated with Unto yanbianensis Gu and the gastropod Viviparus cf. ogonoensis Kob. et Suz. The present species and the same association have also been discovered from the Tongfushi Formation in eastern Jilin, Northeast China, from which the type specimens of this species were repofted by Gu (in Gii et al., 1976). The non-marine Upper Jurassic and Lower Cretaceous stratigraphic . sequence in western Liaoning has been established and currently accepted by most geological workers, which can be simplified as follows:Sunjiawan Formation——Nippononaia yanjiensis Gu——disconformity——Haizhou Formation——conformity——Shahai Formation——Eonippononcia sinensis (Nieh)——disconformity——Jiufutang Formation——Megyinaia Liaoningensis Chen——conformity——Yixian FormationThe Jurassic-Cretaceous boundary in western Liaoning is recognized as being placed between the Jiufutang and Shahai Formations.The species Nippononaia yanjiensis Gu is described as follows (Pls. Ⅰ—Ⅵ):Shell elliptical to cuneate in outline, modera tely inflated with a length varying from 10 mm to 55 mm and a height about half the length. Adult specimens sculptured with V-ribs on the umbonal area and with chevron ribs on both anterior and postero-dorsal areas. Angles of V-ribs about 30°— 40°. Antero-median surface almost smooth, except for some obscure radial lines occurring on anteroventral surface. Marginal interior crenulation present.Usually there are two pseudocardinal (anterior lamellar) teeth and two posterior lamellar teeth on each valve, although a median tooth is variably appearing on each valve. Adductor scars and pedal scars are all smooth.In immature stage the specimens have ornamentation similar to that of Eonippononaia sinensis (Nieh) from the Shahai Formation, western Liaoning. Its median V-ribs and antero- and posterodorsal chevron ribs are distributed almost on the whole surface of the shell, and there is no marginal interior crenulation occurring in this stage. But the juvenile specimens show the important characters of Nippononaia tetoriensis Maeda from the Kuwajima Formation, Japan, such as the surface sculpture and marginal crenulation. Therefore, the evolutionary lineage is assumed here as E. sinensis-N. tetoriensis-N. yanjiensis.According to. Matsumoto et al. (1982) and Tamura (1980, 1986), in Japan the Kuwajima fauna of the inner, zone is corresponding to the Ryoseki fauna of the outer zone; it may be dated Valanginian in age based on ammonites. It seems that the present species, N. yanjiensis Gu, mav be of the Hauterivian in age based on the study of its evolutionary relationships with N. tetoriensis Maeda, N. zhejiangensis Gu et Ma and N. ryosekiana Suzuki.After a comparison of the nippononaiid and trigonioidid muscle scars (including the adductor and pedal scars) as well as ornamentations and hinges with those of unionids, the authors are inclined to consider that the non-marine trigonioidids (s. 1.) including nippononaiids have much closer phylogenetic relations to marine trigonioidids than to non-marine unionids, although most palaeontological.workers referred them to unionids as everyone knows.The Chinese nippononaiids have been found in abundance not only from the Cretaceous but also from the Middle arid Upper Jurassic formations. This paper gives a review on the published Chinese materials in the literatures from 1976 up to now. The authors find that the family Nippononaiidae Kobayashi, 1968 are composed of at least seven genera (or subgenera), namely, Nippononaia Suzu ki, 1941, Nakamuranaia Suzuki, 1943 (=Nagdongia Yang, 1978; =Yunnanoconcha Ma, in Ma et ah, 1976), Martinsonella Hong, in Martinson, 1965, Danlengiconcha Liu, in Cai and Liu, 1978, Eonip pononaia Guo, 1981, and Cyotrigonioides Guo, 1981. The geographical distribution of Nippononaiidae extends from the Tethyan eastern margin (eastern Xizang and western Yunnan) in the west to the Circum-Pacific margin in the east, and its vertical distribution ranges from the Middle Jurassic up to, the lower Upper Cretaceous.Based upon the reexamination of the abovementioned materials the authors put forth a presumption on the nippononaiid evolutionary trends at the end of this paper (Text-fig. 17), with the nippononaiid lineage divided into four evolutionary stages including several phases: (1) Cyotrigonioides-Danlengiconcha (Middle—Upper Jurassic) stage with V-ribs only on the umbonal area (V-s angles about 40°—70°) and with postero-dorsal chevron ribs in the later phase (Danlengiconcha); (2) Eonippononaia (upper Upper Jurassic—lower Lower Cretaceous) stage, sculptured with V-ribs on the whole surface, with angles of median V-ribs about 30°—45° in the later phase and greater than 50°in the early phase, and with no marginal interior crenulation; (3) Nippononaia (s. s.) (Lower Cretaceous) stage, with strong marginal interior crenulation, including two evolutionary branches, namely, the N. tetoriensis Maeda-iV. yanjiensis Gn-N.? jilinensis. Gu et Yu branch, with the median-anterior smooth surface area expanding gradually, and the N. tetoriensis (or N. yanjiensis)-N: zhejiangensis: Gu et Ma-N. ryosekiana Suzuki branch, with the trend toward development of the V-sculpture and. the narrowing of the V-s angles; (4) Martinsonella (lower Upper Cretaceous) stage, sculptured only with V-ribs on the umbonal area which are similar to those of the first stage, but with smaller angles (5°—10°) of V-s than the first stage and without chevron ribs on both the antero-and postero-dorsal areas.