Three schools of classical Chinese astronomy in the Han period are attributed to astronomers of the earlier Warring States period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to that period and was preserved as part of the Dunhuang Manuscripts. Native Chinese astronomy flourished during the Song dynasty, and during the Yuan dynasty became increasingly influenced by medieval Islamic astronomy (see Treatise on Astrology of the Kaiyuan Era). As maps were prepared during this period on more scientific lines, they were considered as more reliable.