Charophytes from the Lower Cretaceous Xiagou Formation in the Jiuquan Basin (northwestern China) and their palaeogeographical significance

Abstract

The charophyte flora from the Lower Cretaceous Xiagou Formation in the Jiuquan Basin, Gansu Province (Northwest China) was investigated, and a total of four species were recognized: Aclistochara huihuibaoensis, Mesochara stipitata, Sphaerochara verticillata and Clavator grovesii var. jiuquanensis. A synonymy is here proposed, i.e., Charites xiagouensis is newly combined to S. verticillata, which is dominant in the flora assemblage. The here-described charophyte assemblage has a high biostratigraphic interest suggesting an early Aptian age for the Xiagou Formation. A sedimentological analysis of the facies indicates that the Xiagou charophyte flora thrived in a shallow freshwater lake. As noticed by previous authors, the late Early Cretaceous flora from the Jiuquan Basin shows a reduced diversity comparing with those from the Central Tethyan Archipelago, where clavatoraceans originated and radiated. As previous studies showed, one of the five clavatoraceans with the greatest global biogeographical extension, C. grovesii var. jiuquanensis spread to the Jiuquan Basin in the early Aptian probably dispersed by the aquatic bird Gansus yumenensis.