Asioaspis, a new genus of Polybranchiaspiformes (Galeaspida, stem-gnathostomes) from the Lower Devonian of Yunnan, China

A new genus and species, Asioaspis brachyotus gen. et sp. nov. of Polybranchiaspiformes, is described from the Lower Devonian Lochkovian Xishancun Formation near Miandian Reservoir, Qujing City, Yunnan Province, China. The new genus is characterized by an elongated ovoid headshield with serrated lateral margins, a small subcircular median dorsal opening, short and laterally projecting cornual processes, an exceptionally broad ventral rim, 12 pairs of branchial fossae, and coarse granular tubercles (~3/mm²). It displays a mosaic of characters of the most primitive polybranchiaspiform (Platylomaspis), derived Polybranchiaspiformes, and even the basal members of the galeaspid family Dayongaspidae. Phylogenetic analysis resolves A. brachyotus as an intermediate taxon between the most primitive Gumuaspidae and other derived Polybranchiaspiformes. Although A. brachyotus also shares the broad ham-brim-like ventral rim with the Gumuaspidae, its other characters are more like other derived Polybranchiaspiformes. In particular, its median dorsal opening and paired orbital openings are not highly close-set on the top of the headshield as in the Gumuaspidae, but rather significantly separated from each other and close to the margin of the ventral rim as in the Pentathyraspidae, Duyunolepididae, and Polybranchiaspidae. These proportions indicate that it was unlikely to have lived a semi-infaunal lifestyle and are more consistent with the epibenthic habit typical of most galeaspids, representing an evolutionary transition from the semi-infaunal benthic to epibenthic lifestyle within Polybranchiaspiformes.