New findings of Dunyu (Eugaleaspiformes, Galeaspida) from the Xiaoxi Formation in South China and their biostratigraphic significance

New discoveries of the late Silurian fossil fish Dunyu (Eugaleaspidae, Eugaleaspiformes, Galeaspida), Dunyu tianlu sp. nov. and Dunyu sp., are described from the Xiaoxi Formation in Xiushan of Chongqing and Xiushui of Jiangxi, China respectively. D. tianlu sp. nov. can be distinguished from D. longiforus and D. xiushanensis in its nearly equal preorbital and postorbital regions of the headshield. As the currently only known genus of Eugaleaspiformes during the late Silurian, Dunyu not only displays a large morphological difference with galeaspids from both the early Silurian and Early Devonian but also occupies a phylogenetic position that is far from the root of Eugaleaspiformes, which indicates that the lineages nested between Yongdongaspidae and Eugaleaspidae should have diversified before the early Ludlow, even during the Telychian. Discovery of new specimens of Dunyu provides direct evidence on the genus level for the correlation of the late Ludlow strata between the margin and interior of the Yangtze Platform, further supporting that the central part of the Yangtze Platform suffered from widespread transgression in the late Silurian.