鄂西下寒武统甲壳类化石新发现

NEW DISCOVERIES OF LOWER CAMBRIAN CRUSTACEAN FOSSILS FROM WESTERN HUBEI

本文描述的标本全部采自鄂西宜昌莲花沱黄鳝洞和长阳鸭子口钟鼓湾(插图1)下寒武统水井沱组。采自宜昌的标本包括2新属、2新种:Liantuoia inflata gen. et sp. nov., Huangshandongia yichangensis gen. et sp. nov.。有的与北美布吉斯页岩(Burgess Shale)产的叶虾类Proboscicaris Rolfe, Fieldia Walcott等相似,有的与角虾类Caryocaris

This paper describes two new genera, three new species and one new subspecies, among which the bradoriids Sunella grandis similis subsp, nov. and S. bispinata sp. nov. are preserved at the bottom of the Lower Cambrian Shuijingtuo Formation in Zhongguwan, Changyang, West Hubei; the phyllocarids Liantuoia inflata gen. et sp. nov. and Huangshandongia yichangensis gen. et sp. nov. are come from the Shuijingtuo Formation in Huangshandong, Yichang, West Hubei (text-fig. 1). The Shuijingtuo Formation named by Chang Wen-tang (1957) is composed of bioclastic limestone in the uppermost part; yellow, yellowish-green to greyish-green shale and sandy shale in the middle part, and black shale intercalated with thin-bedded limestone with abundant eodiscids, bradoriids and phyllocarids in the lower part. Based on Liangshanella, which is restricted to the Chiungchussu Stage in East Yunnan and South Shaanxi, in coexistence with Liantuoia inflata and Huangshandongia yichangensis in the Huangshandong section, it can be inferred that the age of the two phyllocarid species might fall within the Chiungchussu Stage. Since the trilobites and bradoriids as seen in the lower part of the Shuijingtuo Formation in Yichang occur above Sunella bispinata sp. nov. and S. grandis similis subsp, nov. in the Zhongguwan section, the two Sunella species are older than the two phyllocarid species of Yichang and represent the Chiungchussu Stage chronostratigraphi cally. As S. bispinata and S. grandis similis, have not been found to occur together in the same stratigraphic horizon (p1. Ⅰ, figs. 5,7), it is supposed that they cannot represent the male and female (or female and male) carapaces of the same species respectively. In addition, the anterior and posterior spines which stretch out from anterodorsal and posterodorsal angles separately are hollow, approximately equivalent in length and branching at the base, indicating that they performed the function of hinge-structure, strengthening, sensation, defence and balance. Cambrian crustacean carapaces characterized by anterior and posterior spines are common, such as those of Sunella, Isoxys, Tuzoia, Dioxycaris, Dabashanella, etc., also including the ostracod Vltavina bohemica. This might indicate a convergence related to the palaeo-environment. Both Liantuoia inflata gen. et sp. nov. and Huangshandongia yichangensis gen. et sp. nov. are established only based on the carapaces without soft parts and appendages, especially without known valve or bivalve, but they resemble Proboscicaris Rolfe, Fieldia and Caryocaris Salter in outline. The authors agree with Rolfe (1969) to classify them temporarily in Phyllocarida with question. Description of new genus and species Sunella grandis similis subsp, nov. (P1. Ⅰ, fig. 7; P1. Ⅱ, figs. 1—5; text-figs. 2, 3) Diagnosis: Valves semicircular; test very thin; dorsal edge straight; anterior margin curved. A shallow furrow in front of the valve forming an angle of 50°with dorsal edge. Velum border observable. Remarks: S. grandis similis is distinguished from S. grandis Huo by the curved anterior margin and the very thin test. Sunella bispinata sp. nov. (P1. Ⅰ, figs. 1—6; P1. Ⅱ, figs. 13, 14; text-fig. 4) Diagnosis: Valves with a slightly retral swing; dorsal edge straight and very long. Anterodorsal and posterodorsal angles produced into a long spine separately. A shallow furrow in anterior area of the valve forming an angle of 40°to 50°with the dorsal edge. Velum border observable. Remarks: The new species resembles some species of Isoxys Walcott in anterior and posterior spines, but differs in having a furrow in the anterior area, which is an important character in Sunella and may be related to the soft body, without an upwards arching dorsal edge and no branching in the base of spines as seen in the latter. Liantuoia gen. nov. Type species: Liantuoia inflate gen. et sp. nov. Diagnosis: Carapace in medium size, clavate in outline. Dorsal edge straight and long. Anterodorsal angle produced into a handle-shaped rostrum. Marginal rim clear. Remarks: The new genus is distinguished from Proboscicaris Rolfe by the smaller, thinner and smooth carapace, and from Fieldia Walcott in the absence of five narrow longitudinal bands in the carapace. Distribution: West Hubei, China; Lower Cambrian. Liantuoia inflata gen. et sp. nov. (P1. Ⅱ, figs. 6—10; text-fig. 5) Diagnosis and Remarks: See the correspoding parts of this genus. Huangshandongia gen. nov. Type species: Huangshandongia yichangensis gen. et sp, nov. Diagnosis: Carapace in medium size, shuttle-shaped in outline. Anterodorsal edge extending downward; posterodorsal edge straight; anterior margin forming a gentle arc with ventral margin; posterior margin straight. Remarks: The new genus is similar to Caryocaris Salter in size and outline, but the latter is distinguished by its shorter anterodorsal edge, posterior margin with spines, midventral ridge and network on the surface of carapace. Distribution: West Hubei, China; Lower Cambrian. Huangshandongia yichangensis gen. et sp. nov. (P1. Ⅱ, figs. 11, 12; text-fig. 6) Diagnosis and Remarks: As for the genus.