considered to be the most remarkable fossil discovery of the Jehol biota, which in the next decades would reveal the most important record of Mesozoic birds worldwide. In the late 1990s, Confuciusornis was thought to be both the oldest beaked bird as well as the earliest bird after Archaeopteryx. It was also considered to be only slightly younger than Archaeopteryx – the Yixian Formation, the rock unit where most Confuciusornis specimens have been found, was thought to be of Late Jurassic (Tithonian) age at the time. Although two bird genera, Sinornis and Cathayornis, had already described from the Jehol biota in 1992, these were only based on fragmentary remains and stem from the younger Jiufotang Formation, which was considered to be of Early Cretaceous age. Later, both formations have been dated to the Lower Cretaceous (Barremian to Aptian stages, 131–120 million years ago). In 1995, local farmers began digging for fossils near the village of Sihetun, Beipiao, in what would become one of the most productive localities of the Jehol biota. Large-scale professional excavations at this single locality have been carried out by the IVPP from 1997 onwards; recovered fossils include several hundred specimens of Confuciusornis. Many additional sites producing fossils of the Jehol biota have been recognized since, distributed over a large region including Liaoning, Hebei, and Inner Mongolia. Due to the great abundance, preservation, and commercial value of the fossils, excavations by local farmers produced an unusually high number of fossils. Although a portion of these fossils have been added to the collections of Chinese research institutions, more have probably been smuggled out of the country. In 1999, it was estimated that the National Geological Museum of China in Beijing housed nearly 100 specimens of Confuciusornis, and in 2010, the Shandong Tianyu Museum of Nature was reported to possess 536 specimens of the bird. The majority of specimens, however, are held privately and thus are not available for research. At one time forty individuals were discovered on a surface of about 100 m2. This has been explained as the result of entire flocks of birds being simultaneously killed by ash, heat or poisonous gas following the volcanic eruptions that caused the tuff stone in which the fossils were found to be deposited as lake sediments. Additional species and synonyms Since the description of Confuciusornis sanctus, five additional species have been formally named and described. As with many other