Taxonomic revision of Sinoeugnathus kueichowensis from the Middle Triassic of Guizhou and Yunnan, China

Abstract: The previously alleged ‘eugnathid amiiform’ Sinoeugnathus kueichowensis is a small-sized halecomorph from the Middle Triassic (Ladinian) marine deposits of Guizhou and Yunnan, China. A morphological redescription and taxonomic revision of this taxon are provided based on a detailed examination of 15 new specimens. Among them, IVPP V24315 (standard length = 64 mm) is appointed as the neotype, given that the holotype is missing. Studies of these specimens revealed some morphological details previously undescribed or misidentified for this taxon, including a hatchet-shaped antorbital, two broad suborbitals, a sensory canal in the maxilla, and three pairs of extrascapulars. For the first time, Sinoeugnathus was incorporated into an analysis of halecomorph phylogeny, and the results recover it as the sister taxon of the Anisian Subortichthys from Luoping, Yunnan, and both are grouped with two Ladinian genera Allolepidotus and Eoeugnathus from the Monte San Giorgio area into a monophyletic group (namely Subortichthyidae fam. nov. herein) at the base of Ionoscopiformes. This taxonomic reassessment of Subortichthys provides new insights into the phylogeny and paleogeographic evolution of Ionoscopiformes.