A new dayongaspid galeaspid from the Silurian of the Lower Yangtze Region: Implications for biogeography and the evolution of key adaptations in galeaspids

Galeaspids are an extinct group of jawless armored fishes that are integral to understanding the origin of the anatomical innovations of jawed vertebrates. Resolving the nature of the jawless ancestor of jawed vertebrates requires resolution of the anatomy of the earliest galeaspids, which is currently poorly known. Here we describe a new dayongaspid, Xihaiaspis wuningensis gen. et sp. nov., from the early Telychian (early Silurian) Qingshui Formation of Lixi Town, Wuning County, Jiujiang City, Jiangxi Province, China. This first report of dayongaspids from the Lower Yangtze region supports the relative proximity of, and faunal exchange between, South China and Tarim during the early Telychian. It also informs on the monophyly of Dayongaspidae and the early branching position of this family within Galeaspida. In particular, Xihaiaspis wuningensis gen. et sp. nov. provides insights into the nature of the last common ancestor of Dayongaspiformes, Eugaleaspidiformes and Polybrachiaspiformes. These include the nature of the median dorsal opening, which was transversely oriented and slit-shaped, as well as on the presence of ventro-lateral fin-folds and dorsal spines, which were likely present in the ancestral galeaspid.