陕西洛南中元古代的具壳动物化石

SHELLY FOSSILS FROM MIDDLE PROTEROZOIC IN LUONAN.SHAANXI PROVINCE

文中所描述的化石是目前已知的最古老的具壳动物化石并首次发现于中元古代洛南群巡检司组的底部。根据洛南地区前寒武系的地层层序、同位素年龄以及叠层石的组合特征等,与天津蓟县剖面对比,巡检司组与蓟县的雾迷山组相当,已为国内外学者熟知,其推测年龄约为1200Ma。文中着重于对化石材料的形态描述,共描述了4属(其中新属2个),6个新种,1个相似种。这些材料说明1200Ma前已存在着具壳的后生动物。

Some shelly fossils occurred in a rock unit about 1200 million years ago were first discovered recently in Luonan County, Shaanxi Province. Four genera and six species are described here. All of them were found in the basal portion of Xunjiansi Formation(Pt_2x)which is the middle unit of Luonan Group. The fossiliferous horizon in Xunjiansi Formation is composed of gray-black dolomitic shale. The Upper Precambrian sequence of Northern Luonan can be subdivided into four unit in descending order as follows (see Fig.1 ): The Luoquan Formation(Zl), disconformably underlying the Lower Cambrian ((?)_1x)bearing trilobite Bergeroniellus lonanensis Hsiang is tillitic(making the lower part) and ice-aqueous deposit (making the upper part). Dazhuang Formation(Pt_3d), unconformably underlying the Luoquan Formation consists predominatly of dark gray to black carbon-siliceous shale. Luonan Group, disconformably underlying the Dazhuang. Formation is mainly composed of stromatolitic dolomite. Gaoshanhe Group, overlying on the metamorphic volcanic rocks of Xiong-er Group is made up of quartzite, slate and a few dolomite. The radiogenic, U-Pb age of the Xiaohe Granite, intrusived in the Gaoshanhe Group and Luonan Group is determinded as 999 Ma, and Rb-Sr age of whole rock samples from the Dazhuang Formation is 924 Ma. The biostratigraphic correlations based on stromatolite assemblages suggest that the Xunjiansi Formation may be comparable with the Wumishan Formation in the Jixian section. According to the geologic relationships, radiometric and biostratigraphic evidences stated above an age in excess of 1200 million years appears to be reasonable for the shelly fossils. Description of the shelly fossils: Genus Xunjiansiella Hua et Jian, gen. nov. Etymology: Xunjiansi-a village name in Luonan County, Shaanxi Province. Type species: Xunjiansiella retroswinge Hua et Hu gen. et sp. nov. Diagnosis: Carapace valve ear-shaped to rounded quadrangular and present the differentiations of dorsal and venter; dorsal edge slightly, ventral border retral swinging; possesses marginal structure. The surface of valves is smooth. Discussion. The Carapace pattern in the new genus resembles to that in bradoriids, such as in having the differentiations of dorsal edge, the retral swing venteral edge and that marginal structure. The terms used to describe bradoriida are cited in systematic description of the new genus. But it is difficult to disscus the texonomic position so far for the specimens we collected are a few. Xunjiansiella retroswinga Hua et Hu gen. et sp. nov. (pl. I, Fig.1) Etymology: retro and swing-Latin, backward and swing in reference to the ventral swing backwards. Material. Two carapaces preserved on bedding plane. Holotype: 91 LnSF-1. Description. Carapace semiovate, valves with a strong retral swing; the ratio between height and length rather large; surface smooth. Dorsal edge straight, the posterior and the anterior angle obtuse; marginal rim well defined. Remaks: This new species with smooth surface valve and a flatform border may be refered it to Xunjiansiella but differs from other species of the genus in having a strong retral swing and a platform border. Horizon: Xunjiansi Formation Xunjiansiella longa Jian et Hua gen. et sp. nov.(pl. I, Fig.2, 3) Etymology: Long-Latin, in reference to the Longer Length of the valve. Material. Two carapaces preserved on bedding plane. Holotype: 91 LnSF-3. Description. Carapace was discovered. Carapace semiellipse, slightly retral swing; the length 4ram, height 2.2mm; dorsal edge long and convex upward gently; anterior angle and posterior angle both obtuse. Surface of valve smooth; marginal structure ridge-like. Discussion: This specimen bears a striking similarity to Mononotella long Huo et Shu 1984(1991, pl.40, Fig.8) Horizon: Xunjiansi Formation. Xunjiansiella simplex Hua et Hu gen. et sp. nov. (pl. I, Fig. 4, 5) Etymology: Simplex-Latin, simple, in reference to the morphology of the valve. Marterial: Two carapaces preserved on bedding plane, H91otype: 91 LnSF-4. Description: Carapace valve rounded rectangular and weak retral swing, dorsal edge arching upward slightly. Posterdorsal angle obstuse and anterodorsal angle rectangular. Surface of valves smooth. Ratio between height and length larger, no border structure. Discussion. The valves of the two specimens resemble to bradoriids Mononotella chuanshaanensis Huo et Shu 1984(1991, pl.40, Fig.9). Horizon: Xunjiansi Formation. Xunjiansiella orthis Jian et Hua, gen. et sp. nov. (pl. I, Fig. 6 ) Etymology: Orth-Greek, orthochromatic, in reference to the valve morphology. Material. Only one carapace preserved on bedding plane. Holotype:91 Ln-SF-6. Description. Only one specimen is collected. The length of valve about 2.5mm, the height 1.5ram. Carapace semicircularis, dorsal edge short and arching upward. Both the anterodorsal and the posterodorsal angles are obtuse. The ratio between height and length is medium. Remarked. X. Orthis bears some analogy to Huashangouensis orthis Huo et Shu 1991 as shown in plate 16, Fig.10(1991)in a ridge border, orthomorphic valve. Horizon. Xunjiansi Formation. Genus Combinivalvula Hou 1987 Type species Combinivalvula cf. chengjiangensis Hoa(cf. Jian et Hua)pl. I , Fig. 9 Combinivalvula chengjiangensis Hou, 1987, p295, pl. ig, fig. 1—8 Description. Only one specimen was discovered. Carapace ankylose at anterodorsal area; posterodorsal groove, about 2/3 times as long as the carapa- ce length, the anterodorsal one is not observed. The anteroventral and posteroventral part stretching outward separately. Surface of carapace smooth; about 4 mm in the length and 1.5 mm in height. Discussion: The specimen described here resembles to C. chengjiangensis Hou, 1987(pl. Ⅳ, Fig, 1—7)in carapace fused in the anterior median portion but the anterior and the posterior rostrums can not be seen owing to the tearing broken of the specimen, and thus it is referred questionably to C. chengjiangensis. Genus Luonanconcha Jian et Hu gen. nov. Etymology: Luonan-county name in shaanxi concha-Latin, shell or carapace. Type species: Luonancocha sinosa Jian et Hu gen. et sp. nov. Diagnosis, Shell flat and nephroid in shape, and narrower anteriorly and broader posteriorly; bilateral symmetry anterior margin convex strongly, while the posterior margin convex gently. Shell surface smooth. A set of intercalating furrows and ridges arranged in the anterior area of the shell transversely. Discussion: There is no comparable objects at the moment. Luonanconcha sinosa Jian et Hu gen. et sp. nov. (pt. I, Fig.7, 8) Etymology: Sino, Latin, Chinese. Material: Only one specimen preserved on bedding plane. Holotype: 91 Ln-SF-7. Description: The same as the genus. The specimen is 4 mm in long axis and 1.6 mm in short one. Genus Olivooides Qian 1977 Type species: Olivooides multisculcata Qian 1977 Olivooides chenerensis Hu et Hua sp. nov. (pl. I ,Fig.10, 11 ) Etymology: Chen-er-a locality name in Luonan County, Shaanxi Province. Material: Several specimens preserved on bedding plane. Holotype:91 Ln-SF-9 Description: Individual very small hollow globular shell, ovally in outline. The long axis about 0.77ram, the short one 0.55ram.Surface of globular shell very smooth; the wall thin and only one layer. A circular opening about 30 lira penetrated through the wall. Discussion. This new species bears analogy to Olivooides pearlformis Li 1991 and O.blandes Jiang 1984 in outline but differs obviously from the latters in having only one thiner layer.