上扬子地台湘北—鄂西层序地层研究表明，奥陶纪有两次二级海平面升降旋回，形成两个二级复合层序（即中层序）．其界面介于第８与第９个三级层序之间，基本与Ｎｅｍａｇｒａｐ－ｔｕｓｇｒａｃｉｌｉｓ带的首次出现相吻合，并可与澳洲、美洲、欧洲等地进行对比．古地磁、古气候及古生物等资料也反映出该界面是重要的转折点．因此，对应这两个中层序，可将奥陶系分为两个亚系 The Upper Yangtze platform The study of the northern Hunan-western Hubei sequence stratigraphy shows that there are two second-order sea-level rise-and-fall cycles in Ordovician, forming two second-level composite sequences (ie, middle-sequence). The interface is between the 8th and the 9th tertiary sequences and basically coincides with the first appearance of the Nemagrap-tusgracilis band and can be compared with Australia, the Americas, and Europe. Paleomagnetic, palaeoclimate and palaeontology data also reflect the interface is an important turning point. Therefore, corresponding to these two middle-level sequences, Ordovician can be divided into two subfamilies