New Material of Wenlock Graptolite Cyrtograptus murchisoni from Ziyang County, Shaanxi Province, China and Its Stratigraphic Significance

Several well-preserved, complete specimens of Cyrtograptus murchisoni Caruthers, 1867 are discovered in the Bajiaokou-A section in Ziyang County, southern Shaanxi Province. In this paper, we describe these specimens, especially the characteristics of the proximal end, main stipe, the primary cladium, secondary cladium and theca, by a comparison with those specimens from elsewhere in the world. The species is characterized by its thick main stipe and cladia. The secondary cladia are common, with spiral circles in the proximal end, bird beak-shaped theca with short apertural spines. The first occurrence of the index species indicate precisely the base boundary of the Wenlock Series of Silurian in China.