A new ionoscopiform fish (Holostei: Halecomorphi) from the Middle Triassic (Anisian) of Yunnan, China

摘要: Ionoscopiformes, sister to Amiiformes, are an extinct group of halecomorph fishes previously known mainly from the late Middle Triassic (Ladinian) and Late Jurassic of Europe and the Early Cretaceous of the New World. They have not been known in China until recently two ionoscopiforms (Robustichthys and Panxianichthys) were found from the early Middle Triassic (Anisian) of Yunnan and Guizhou, respectively. Here, a new ionoscopiform, Subortichthys triassicus gen. et sp. nov., is described on the basis of six well-preserved specimens from the early Middle Triassic (Pelsonian, Anisian, ~244 Ma) marine deposits exposed in Luoping, eastern Yunnan, China. The discovery documents the second ionoscopiform from the Luoping fossil beds and one of the oldest ionoscopiforms in the world, providing important information for understanding the origin and early diversification of this group. Subortichthys is an unambiguous halecomorph as it possesses two synapomorphies of this clade: a symplectic articulating with the lower jaw and a notched posterior margin of the maxilla. Within the phylogenetic framework of the Halecomorphi, results of the phylogenetic analysis place Subortichthys at the base of the Ionoscopiformes because it bears a sensory canal in the maxilla (an ionoscopiform synapomorphy), but lacks derived features of other ionoscopiforms. Notably, Subortichthys is distinguished from other members of this order with a unique combination of features, such as presence of three or four pairs of extrascapulars and a much expanded third infraorbital posteriorly contacting the preopercle. The new material reveals that the earliest diversification of ionoscopiforms was well undertaken in South China (a part of east Paleotethys Ocean then) by the early Middle Triassic (Anisian).