Proximal development,systematic taxonomy,and dispersal pattern of the Early-Middle Ordovician graptolite Acrograptus from South China  

作　　者：LI LiXia FENG HongZhen WANG WenHui CHEN WenJian 

机构地区：[1]School of Earth Sciences and Engineering,Nanjing University,Nanjing 210093,China

出　　处：《Science China Earth Sciences》2012年第7期1110-1122,共13页中国科学（地球科学英文版）

基　　金：supported by National Natural ScienceFoundation of China (Grant No. 40972009)

摘　　要：Based on the well-preserved specimens from the Nanba section in Yiyang, Hunan Province, the definition of the graptolite ge- nus Acrograptus is revised here. The revised Acrograptus is a dichograptid form with stipes subborizontal to deeply declined, sicula long and slender; th 11 budding from the top of the metasicula, proximal development of isograptid type with th 12 as the dicalycal theca; asymmetric proximal end resulting from the divergence of thl1 and thl2 from the sicula at different levels; slender crossing canals composed of extremely slender Acrograptus is assigned to the subfamily Sigmagraptinae prothecae of thlt and thl2. According to the revised definition, of the family Dichograptidae, including six species, i.e., A. affinis （Nicholson）, A. filiformis （Tullberg）, A. gracilis （Tornquist）, A. nicholsoni （Lapworth）, A. pusillus （Tullberg）, and A. saukros （Ni）. During the Early-Middle Ordovician, these six species were widely distributed in South China, especially in the Upper Yangtze Region as well as the central and eastern Jiangnan Region. This study further indicates that Acrograptus first appeared in a deep-water setting on the Jiangnan Slope in South China during early Floian （Tetragraptus approximatus Biozone）, and subsequently expanded into the shallow-water region on the Yangtze Platform due to a remarkable dispersion event in middle Floian （Pendeograptus fruticosus Biozone）. It is here indicated that the dispersal of Acrograptus may be directly controlled by the large-scale transgression event occurring in middle Floian.Based on the well-preserved specimens from the Nanba section in Yiyang,Hunan Province,the definition of the graptolite genus Acrograptus is revised here.The revised Acrograptus is a dichograptid form with stipes subhorizontal to deeply declined,sicula long and slender;th1 1 budding from the top of the metasicula,proximal development of isograptid type with th1 2 as the dicalycal theca;asymmetric proximal end resulting from the divergence of th1 1 and th1 2 from the sicula at different levels;slender crossing canals composed of extremely slender prothecae of th1 1 and th1 2.According to the revised definition,Acrograptus is assigned to the subfamily Sigmagraptinae of the family Dichograptidae,including six species,i.e.,A.affinis (Nicholson),A.filiformis (Tullberg),A.gracilis (Trnquist),A.nicholsoni (Lapworth),A.pusillus (Tullberg),and A.saukros (Ni).During the Early-Middle Ordovician,these six species were widely distributed in South China,especially in the Upper Yangtze Region as well as the central and eastern Jiangnan Region.This study further indicates that Acrograptus first appeared in a deep-water setting on the Jiangnan Slope in South China during early Floian (Tetragraptus approximatus Biozone),and subsequently expanded into the shallow-water region on the Yangtze Platform due to a remarkable dispersion event in middle Floian (Pendeograptus fruticosus Biozone).It is here indicated that the dispersal of Acrograptus may be directly controlled by the large-scale transgression event occurring in middle Floian.

关 键 词：proximal development systematic taxonomy dispersal pattern Acrograptus Ordovician South China 

分 类 号：P532[天文地球—古生物学与地层学] P534.42[天文地球—地质学]