北京西山杏石口组的鱼化石

A NEW PALAEONISCOID FISH FROM XINGSHIKOU FORMATION OF WESTERN HILLS OF BEIJING

本文记述了北京西山杏石口组的—占鳕——Xingshikous xishanensis gen. et sp. nov.,其形态与延长群的 Triassodus 和石拐群召沟组的 Daqingshaniscus 有所相似,但更近似后者.根据古鳕类的进化水平,推证含鱼化石沉积杏石口组的时代为早侏罗世.文中还讨论了我国北方古鳕类基干支的某些分子如 Turfania、Triassodus、Xingshikous、Daqingshaniscus 和 Uighuroniscus 的分布及其生物地层意义.

A new palaeoniscoid fish, Xingshikous xishanensis gen. et sp. nov., collected from Badachu, of the Xingshikou Formation, the lower part of Mentougou coal-series, is represented by 26 specimens, which are mostly preserved as impressions. The fish-bearing bed is overlain, by the basalt flow, succeeded below by Shuangquan Formation or Hongmiaoling Formation. As we know, both Xingshikou and Shuangquan Formations are questionable in age. Therefore the discovery of these fish fossils is of interesting significance in stratigraphy. Order Palaeonisciformes Family Palaeoniscidae Cenus Xingshikous gen. nov. Type species Xingshikous xishanensis sp. nov., V8754.1, IVPP, (Plate I, 1-4). Diagnosis A palaeoniscoid fish of moderate size, the dorsal and anal fins nearly equal in size. The dorsal fin just opposite the space between ventral and anal fins. Caudal fin heterocercal, with scale-lobe extending to the terminal. Fin rays completely jointed and branched distally. Unpaired fins with slender fringing fulcra. Ridge scales present in front, of unpaired fins. Flank scales small, rhomboidal, more than 70 rows in number. Comparison and Discussion In general, the fish of Badachu is basically palaeoniscoid in characteristic. Although the skull is not well preserved in our specimens, the outline of some bones can be traced out. The Badachu form resembles Triassodus (T_3; Su, 1984) and a specimen CD107, collected from Yanchang Group (T_3) by Prof. Li Chunyu and now at my disposal, but can be distinguished from them in several points (see Table 1). The Badachu form is very similar to Daqingshaniscus (J_2) (Chen, 1988) from Zhaogou Formation of Shiguai Group, in body form, situation of dorsal fin, scale-lobe of caudal fin and the shape and size of scales, as well as the number of scale rows. Anyhow it is clearly different from the latter by the approach of dorsal, anal and ventral fins in size (Plate I, 1, 3, & 4). Therefore the author considers that the Badachu fish (V8754) represents a new form of Palaeoniscidae. As the description shown above is incompletely known, especially in the bones of skull and mandibles. Therefore searching for better materials of Xingshikous is desirable. As the characters show (see Table 1), Xingshikous is close to Daqingshaniscus. And according to the stratigraphical correlation, Xingshikou Formation is lower than Zhaogou For- mation (see Table 2). Thus, the age of the fish-bearing bed, Xingshikou Formation, would be considered rather as Early Jurassic than as Late Triassic. Up-today, there are three fish-bearing beds discovered from Western Hills of Beijing, i.e. 1) Xingshikou Formation (Early Jurassic, Xingshikous); 2) Dahuichang Formation (Late Jurassic, Lycoptera and Holostei indet.); 3) Tianzhu Formation (Pliocene, Matsya and Barbus spp.) (Table 2).