广西六景剖面艾菲尔-吉维特阶界线层的腕足动物化石

FOSSIL BRACHIOPODS FROM EIFELIAN-GIVETIAN BOUNDARY BED OF LIUJING SECTION, GUANGXI, CHINA

广西六景剖面民塘组底部艾菲尔-吉维特阶界线层的腕足类,是一个台地边缘相和盆地相之间过渡环境中的非近岸底栖型腕足动物群.该动物群以贝体小,属种分异度较高,缺乏明显的地方性分子为特征,石燕贝类占有明显的优势,具背中隔板类型的正形贝类发育,并有鸮头贝科的分子出现.Echinocoelia, Davidsonia 和 Pentamerella 是本腕足动物群的优势属群;其他重要分子有 Skenidioides, Vallomyonia, Leptathyris, Rhynchospirifer, Warrenella 和 Stringocephalus.文中描述腕足类17属19种,其中4新种: Vallomyonia sinensis sp. nov., Pentamerella nanningensis sp. nov., Davidsonia guangsiensis sp. nov., Echinocoelia guangsiensis sp. nov..

The fossil brachiopods dealt with in this paper are collected from Bed 3a and the base of Bed 4 in the lower part of the Mingtang Formation in the Liujing Section, Guangxi, South China, with their horizon situated about 2.2m above the base of the formation. Here described are 19 species (including 4 new species) in 17 genera, namely Skenidioides sp., Schizophoria sp. l, Schizophoria sp. 2, Vallomyonia sinensis sp. nov., Kayserella? sp., Hypsomyonia sp., Phragmophora sp., Pentarnerclla nanningensis sp. nov Davidsonia uangsiensis sp. nov., Prodavidsonia sp., Spinatrypa sp., Leptathyris sp., llmenispina sp., Rhynchospirifer sp., Rhynchospirifer sp., Changtangella sp., Ambocoelia sp., Echinocoelia guangsiensis sp. nov., and Warrenella sp. Among them, the genera Skenidioides, Kayserella, Hypiomyonia, Phragmophora, Pentamerella, Davidsonia, Prodavidsonia, Echinocoelia and Warrenella are reported for the first time from South China. These brachiopods represent a faunule that might lived in a transitional environment between platform facies and basinal facies. This faunule is characterized by the high generic diversity and the abundant thin shelled, small-sized brachiopods. It is obviously different from the neritic brachiopod faunas of Eifelian and Givetian in South China. Within the faunule, the dominant species are Echinocoelia guangsiensis sp. nov., Davidsonia guangsiensis sp. nov., and Pentarnerella nanningensis sp. nov., with Skenidioides, Vallomyonia, Leptathris, Rhynchospirlfer. Warrenella, Stringocephalus, etc. as important elements. A conodont analysis suggests that the horizon containing these brachiopods is at the base of the conodont ensensis Zone, which assigns the brachiopods to late Eifelian, at least to the .earliest Givetian in age. The typical Givetian elements have appeared in the faunule, such as Davidsonian and Stringocephalus. The brachiopods Vallomyonia, Pentamerella, Leptathyris, Echinocoelia and Warrenella are the main genera of both faunas in Liujing area and in central Nevada (the Lepiahyr?lar Zone and the Warrenella kirki Zone). The corresponding species of the genera in both faunas show great similarities to each other in external shapes and internal structures. In addition, the faunule in the Liujing Section shares Prodavidsonia, Rhynchospirifer and Kaplex with those faunas in western Europe. Another important character of the faunule lies in the less endemic genera. Almost all of the genera recognized in the faunule are cosmopolitan elements, or common in the Old World Realm, suggesting that the faunule in Liujing has a close affinity to those of the Old World Realm. DESCRIPTION OF NEW SPECIES Vallomyonia sinensis sp. nov. (pl. l. figs. 20, 22-30) Diagnosis Size small; outline transversely suboval with obtuse cardinal angles; ventral part biconvex in lateral profile. Ventral interarea high-apsaclinal, slightly curved, cleft by open triangular delthyrium. Dorsal interarea ribbon-like, anaclinal. Dorsal shallow. Valves costellate. Growth lines inconspicuous. Ventral muscle field broad, confined to the delthyrial cavity, and defined by a low step along its anterior edge. Dental plates obsolescent. Cardinalia schiziphorid. Cardidal process bilobed. Brachiophore supporting plates obsolescent. Median septum high-triangular. Adductor muscle bounding ridges raised from the valve floor, plate-like, not connecting with brachiophore supporting plates, but converging to meet the median septum anteriorly. Discussion The new species differs from other species of the genus in having obsolescent brachiophore supporting plates and plate-like adductor muscle bounding ridges on brachial valve. Occurrence Bed 3a, lower part of Mingtang Formation. Pentamerella nanningensis sp. nov. (Pl. II, figs. 14-19) Diagnosis Outline approximately transversely oval. Ventral part biconvex in lateral profile. Ventral beak low and weakly curved. Exterior plicated with weak ventral sulcus and corsal fold. Oval spondylium present in pedicle valve, apically supported by a short median ridge. Brachial valve interior: inner plates short; outer plates lyre-shaped, converging toward midline at valve floor. Discription The new species closely resembles P. wintereri Johnson and P. cf. davidsoni from the Eifelian and Givetian faunas of central Nevada, but the latter two species may be distinguished by having ventral median septum to support the spondylium in pedicle valve. Occurrence Bed 3a, lower part of Mingtang Formation. Davidsonia guangsiensis sp. nov. (Pl. II, figs. 1-13) Diagnosis Pedicle valve strongly convex. Ventral interarea apsaclinal. Delthyrium large, covered by weakly convex pseudodeltidium. Brachial valve flat. No radial and concentric ornaments on suface of valves except the cement trace on pedicle valve. Hinge teeth strong knoblike; dental plates absent. Two distinct impr of spirally coiled lophophore separated by median ridge. Socket plates recurved posteriorly, connected with each other through a massive transverse ridge. Median ridge present. Cardinal process small. Discussion The new species differs from the species of the genus from Europe and Nevada in having a relatively small cardinal process. Occurrence Bed 3a, lower part of Mingtang Formation. Echinocoelia guangsiensis sp. nov. (Pl. III, figs. 1-9, 16, 18, 22, 26) Diagnosis Size small; outline triangular to semicircular. Ventral part biconvex in lateral profile. Pedicle valve subsemiconic; ventral interarea flat and broad, subcataclinal. Delthyrium apically closed by a triangular plate. Hinge teeth small and thin. Dental plates absent. Median septum apically supporting the triangular plate. Cardinal process small, bilobed. Crural plates and median myophragm present. Crura clavae, extending to central part of valve. Discussion The new species closely resembles Echinocaelia denayensis Johnson from central Nevada, but the latter differs in having a relatively large cardinal process and the prismatical crura. The new species is different from E. caneocamera Johnson in having a ventral median septurn. E. septata Johnson differs from the new species in having a higher ventral interarea and a relatively developed ventral median septum. Occurrence Bed 3a, lower part of Mingtang Formation.