The earliest euantiarch from the Pragian (Early Devonian) of Guangnan, Yunnan Province, China.

Antiarchs represent a highly specialized clade of early jawed vertebrates adapted to benthic habitats and form a major component of Devonian aquatic ecosystems. Euantiarchs, the most diverse antiarch clade, previously had their earliest record from the Emsian of the Early Devonian in China. They achieved a nearly cosmopolitan distribution during the Middle to Late Devonian. Here, we describe a new euantiarch, Yangfanyu anachoresis gen. et sp. nov., based on articulated specimens from the Pragian (Early Devonian, ~ 410 Ma) of Guangnan, Wenshan Prefecture, Yunnan Province, southwestern China. Using X-ray computed tomography and three-dimensional digital visualization, we reconstructed the complete dermal head and trunk shields. In addition to diagnostic euantiarch characters, Yangfanyu retains several plesiomorphic features, notably the cavity for the cranio-spinal process and the oblique ridge on the lateral wall of the trunk shield. Phylogenetic analysis placed Yangfanyu at the most basal position among euantiarchs, forming the sister taxon to all other euantiarchs. This placement extends the fossil record of Euantiarcha by approximately 10 million years, refines character polarity at the base of the clade, and indicates that the characteristic euantiarch morphology was assembled progressively through the sequential acquisition of derived characters during the earliest stages of euantiarch evolution.