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Department of Geology, Xian Mining College, Xian, China
The paper deals with the late Llandovery and early Wenlock Cyrtograptus and M. spiralis group discovered in Ziyand, Shaanxi. Both the appearance of Cyrtograptus and the morphological characteristics of the spiralis group are stratigraphically significant. According to the mode of preservation of the spiralis group, the rhabdosome is thin or elongated along its proximal parts. Sometimes the proximal part is closely coiled into a perfectly regular or an irregular planospiral shape. Distally it is coiled, arcuate, straight or irregular. The proximal thecae are small, gradually becoming larger towards the distal end. All thecae are subtriangular in form. Associates of the spiralis group include Cyrtograptus lapworthi, C. cf. laqueus, C. shishkaticus and other genera such as Sinostomatograptus, Stomatograptus. C. centrifugus, C. sp. (cf. C. graphicus Golikov) occur at a higher horizon. According to the analysis of the specimens, speculation on the affinities between Cyrtograptus and the spiralis group is made and the possibility of evolution from spiralis to Cyrtograptus suggested. These evolutionary changes can be used to subdivide stratigraphic units. It is suggested that the first appearance of Cyrtograptus should be regarded as the lower boundary of the middle Silurian.
