评“论湖南海扇”——兼评大隆相地层的深水成因论

REMARKS ABOUT "ON HUNANOPECTEN" WITH A REVIEW ON DEEP-WATER ORIGIN OF TALUNG FORMATION

湖南海扇(Hunanopecten)常见于我国南方晚二叠世“大隆相”地层(张仁杰等,1977;方宗杰,1985,1987;丁保良等,1982;丁伟明,1982;殷鸿福,1982,1985)。1977年,张仁杰创立了这一属名,并定义:“壳小至中等,卵圆形,不斜或略前斜,近等壳,左壳较右壳膨凸,右壳足丝凹口深;左右两壳及两耳均具同心线。每壳壳顶前后各具两枚片状齿,近背缘的一枚与铰缘平行,其下的一枚与铰缘略斜交,壳顶下具一个三角形弹体窝,肌痕不详。”这一描述基本上是正确的。但是对铰部特征的描述过于简略。

Yin (1985b) and the writer (1981) proposed the same family name Hunanopectinidae separately, based on quite different understandings of the hinge of Hunanopecten from each other. It is necessary for us to clarify the hinge of Hunanopeten in detail with a discussion on the taxonomic position and autecology of this genus. Results from restudy of related materials have confirmed the writer's view-point on the ligament type of Hunanopecten which actually belongs to the typical aviculopectinid-type instead of the pectinidtype (Pl. 1, fig. 11c; Text-fig. 1A). The lamellar teeth of Hunanopecten differ from the true cardinal crura of the Pectinidae because they are below the ligament area radiating from the base instead of from the apex of the ligament pit. It may be better to call these lamellar teeth primitive carlinal crura (Newell and Boyd, 1985, p. 4). The most striking feature of the family Hunanopectinidae is the coexistence of aviculopectinid-type ligament and strong lamellar teeth, by which it may be distinguished from all other comparable scallop families. It seems best to place the Hunanopectinidae in the Aviculopectinacea (Waller, 1978). The family can be diagnosed as: Small, commonly prosoclinal or nearly aclinal pectiniform shells, subequivalve, with left valve more convex than right one; deep byssal notch observable in right anterior auricle; surface marked with concentric fila or undulations.; radial ornaments obsolescent; ligament as in Aviculopecten; bellow the ligamental area hinge with strong, lamellar isodont teeth, subparallel to the dorsal margin; monomyarian with posterior adductor only (Fang, 1981). Yin(1985b) must have confused cardinal crura with auricular crura because no cardinal crura can be seen in the Leiopectinidae but auriculal crura. It must be pointed out that Yin's opinion on the Leiopectinidae-Hunanopectinidae-Pecrinidae lineage did not hold water. Contrary to Yin's opinion that Hunanopecten lived a pseudoplanktonic life (Yin, 1982, p. 342; 1985b, p. 638), the characters of Hunanopecten suggest that it was a byssate epibenthic bivalve. It seems that Hunanopecten displays the combined shell characteristics of two different morphological groups, swimming and attaching. On the one hand, Hunanopecten is provided with auricle-notch stabilization mechanism (Stanley, 1970, 1972), indicating its epibyssate attaching habit; on the other hand, it has small, thin, nearly smooth shells with a low convexity, indicating its swirmning habit (Kauffman, 1969; Stanley, 1970). Hunanopecten may be included in the habitat group of free-living epifaurial bivalves (Kauffmen, 1969). Other bivalves, ammonoids and brachiopods, which are commonly associated with Hunanopecten, are also regarded as shallow benthos (Liao, 1979; Zhou, 1985; Fang, 1981). Most of them are only found in siliceous and argillaceous facies of the Talung (Dalong) Formation, indicating their dependence on the substrate environment. In short, all these factors mentioned above, point to the same conclusion that Hunanopecten is an epibyssate bivalve rather than a pseudoplanktonic, including the functional interpretation of the shell form, Paleoecologi. cal analysis of associated fossils and their distribution. Some Chinese geologists hold that the Talung Formation was deposited in deep marine environments, but there is considerable doubt about the plausibility of this view based on the following evidences: 1. All the macrofossils (bivalves, ammonoids, brachiopods, gastropods) in the Talung siliceous facies are regarded as shallow benthos. 2. It is imprudent to presume a deep-water origin for radiolarians and all deposits rich in radiolarians, some of which must have been laid down in distinctly shallow epicontinental seas. 3. In fact, no irrefutable deep-water facies evidence have yet been described from Talung siliceous facies. On the contrary, the theory of deepwater origin is in contradiction to some shallowwater facies evidences, such as the occurrence of Lingula, carbonaceous shale and so on.