Redescription of Wushaichthys exquisitus and phylogenetic revision of Thoracopteridae

Abstract

Wushaichthys exquisitus, a small fish from the Middle Triassic of Guizhou Province, China, was named and studied by Xu et al. (2015), and Xu et al. interpreted it as the most primitive species in the family Thoracopteridae because of the presence of the following features: the skull roof is formed by the frontal bone, which is laterally expanded whereas the dermopterotic is expanded posteriorly; the parietal bone is fused with the dermopterotic; the supraorbital sensory canal ends in the frontal; presence of a triangular extrascapular, which is separated from its counterpart medially by the posttemporals; and presence of a narrow and vertical preopercle with an anterior process contacting the maxilla. However, this hypothesis was questioned because W. exquisitus also shares some features with Peltopleuridae, such as the deepened-flank scales, which means W. exquisitus should be a peltopleurid fish. A redescription of Wushaichthys exquisitus is conducted here based on five new specimens. Several new characters are described here, including the vertical preopercle sensory canal, which extends from ventral to dorsal margin of preopercle with a branch to the maxillary process; a small hyomandibular process on the antero-dorsal margin of opercle; and the presence of two lateral lines. A phylogenetic analysis based on 101 characters and 23 taxa was conducted. The results of the phylogenetic analysis suggest that W. exquisitus is the most basal taxon of the extinct family Thoracopteridae, confirming previous studies. The new results also indicate that Thoracopteridae is the sister group of Peltopleuridae, and these two families form the order Peltopleuriformes. This conclusion also agrees with several previous studies. Based on the detailed redescription, the diagnosis of the genus Wushaichthys is emended as well as the diagnoses of Wushaichthy exquisitus and of the family Thoracopteridae.