陕西洛南下寒武统辛集组疑难管状化石桶螺(Cupitheca)

本文对采自陕西洛南上湾剖面下寒武统辛集组疑难管状化石Cupitheca进行了研究,系统厘定和描述了两个种,分别为Cupitheca holocyclata和Cupitheca costellata,并对Cupithecidae科进行了重新修订。本文所描述标本的壳体多为次生磷酸盐化保存,壳体表面纹饰保存较好;C.holocyclata管体表面为横向纹饰,C.costellata管体表面为纵向纹饰。C.holocyclata全球广布,如南澳大利亚、南极乔治王岛、格陵兰东北部、加拿大纽芬兰等地皆有报道,产出于下寒武统中上部地层,具有一定的洲际地层对比意义。本文为C.holocyclata在中国首次发现。C.costellata仅发现于华北,即本文所研究的洛南辛集组及其同期地层安徽淮南与霍邱的雨台山组。

ENIGMATIC TUBULAR FOSSIL CUPITHECAFROM THE LOWER CAMBRIAN XINJI FORMATION OF LUONAN,SHAANXI PROVINCE,CHINA

Cupithecais an enigmatic tubular fossil among the Small Shelly Fossils(SSFs)and occurs worldwide in the SSFs-bearing rocks of the Lower Cambrian,such as South China(Qian,1977;Xing et al.,1984;Duan,1984;Qian,1989),North China(Xiao &Zhou,1984;Pei & Feng,2005),South Australia(Bengtson et al.,1990;Gravestock et al.,2001),Antarctica(Wrona,2003),Greenland(Malinky & Skovsted,2004;Skovsted,2006)and Canada(Skovsted & Peel,2007).This tube-like fossil is characterized by a straight or curved conch with circular to oval cross-section,low angle of divergence of conch.Its growth pattern is very special:older parts of the calcareous conch successively aborted during ontogeny in connection with formation of secondary transverse wall sealing off apical end(Bengtson et al.,1990).In China,it has a wide distribution and long stratigraphic range(Table.1).However,most of the Cupithecaspecimens recovered from China are generally preserved as steinkerns with various forms of initial part and tube outline,and were described as several genera(Duan,1984;Xiao &Zhou,1984).But through detailed taxonomic emendation,Bengtson et al.(1990)and Demidenko(in Gravestock et al.,2001)found that Actinotheca Zhou et Xiao,1984,Arcitheca Duan,1984,Emeitheca Duan,1984,Ensithcea Duan,1984 and Varitheca Duan,1984 all belong to junior synonyms of Cupitheca Duan,1984.Especially,based on the abundant specimens with external shells of Cupithecafrom South Australia,Bengtson et al.(1990)demonstrated that the ornamentation of the shell can be useful character for differentiation of species.The tubular specimens of Cupithecadescribed herein were collected from the basal part of the Lower Cambrian Xinji Formation at Shangwan Section of Luonan County,Shaanxi Province,North China Platform(Figs.1,2).Based on trilobite fossils,the Xinji Formation can be correlated with the Drepanuloides Biozone of the middle Tsanglangpuan stage(Cambrian Stage 4)on the Yangtze platform(Yu et al.,1984;He & Pei,1985).The co-occurring small shelly fossils include chancelloriid sclerites,hyoliths,Pojetaia runnegari,Mackinnonia rostrata,Stenotheca drepanoida,Pelagiella subangulata and Apistoconchacf.apheles.The SSF assemblage of the basal Xinji Formation suggests an approximate correlation between the basal Xinji Formation and the Stenotheca drepanoida Zone in South Australia(Gravestock et al.,2001),and also indicates a middle to late Botomian age(Cambrian Series 2,Stage 4)for the Xinji Formation(Li et al.,2014).The Cupithecaspecimens from the Xinji Formation include phosphatized tubes and steinkerns.Based on the shell outer sculpture,C.holocyclata with transverse ridges and C.costellata with longitudinal ridges are systematically described in detail.The conch of C.holocyclatais short and small,straight or slightly curved with paralleled transverse ridges on the surface.The angle of divergence of conch is low with circular to oval transverse section.Each conch is sealed off apically by a transverse convex wall.The diameter of the convex wall is apparently shorter than the upper conch.The periphery of the convex wall consists of several short and fine rods which are vertical to the surface.The outline of C.costellatais similar to C.holocyclata,but the outer surface of C.costellata bears approximately paralleled longitudinal ridges along the conch's vertical axis.C.holocyclata has a wide distribution,such as South Australia(Bengtson et al.,1990;Gravestock et al.,2001),King George Island of Antarctica(Wrona,2003),Northeast Greenland(Malinky & Skovsted,2004;Skovsted,2006)and Newfoundland of Canada(Skovsted & Peel,2007)ranging from the middle to late Early Cambrian.This is the first time to report C.holocyclata with exact external conch discovered in China.C.costellata was only discovered from North China,in the Xinji Formation of Luonan,Shaanxi Province(this paper)and the Yutaishan Formation of Huainan and Huoqiu,Anhui Province(Xiao & Zhou,1984).