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

Abstract

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; th11 budding from the top of the metasicula, proximal development of isograptid type with th12 as the dicalycal theca; asymmetric proximal end resulting from the divergence of th11 and th12 from the sicula at different levels; slender crossing canals composed of extremely slender prothecae of th11 and th12. 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 (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.