The restudy of Haopterus gracilis from the Yixian Formation, Liaoning, China

Abundant and diverse pterosaur records have been reported from the Jehol Biota since the end of the last century, including an early reported member, Haopterus gracilis, from the Yixian Formation. The referral of Haopterus gracilis has been a subject of debate since its discovery. Various phylogenetic analyses have suggested different positions for Haopterus gracilis, including the sister taxon of the Ornithocheiroidea or placed differently in the Pteranodontoidea. Recent research has proposed that Haopterus gracilis is the sister-taxon of the Lebanese istiodactyliform Mimodactylus libanensis. Here we redescribe the holotype and only specimen of Haopterus gracilis, a relatively complete skeleton of a juvenile individual in detail using Micro-CL scanning. Haopterus possesses several pteranodontoid features including a stout scapula shorter than the coracoid, a sternum with a constriction posterior to the sternocoracoid articulations, and humeri with trapezoidal deltopectoral crests and constricted mid-shafts. A close affinity between Haopterus and istiodactylids has been suggested by several researchers, whereas some features of Haopterus revealed here distinguish Haopterus from istiodactylids, including the low skull with a long rostrum occupying more than half the skull length anterior to the jaw articulation, and a long tooth row with different tooth morphologies. The phylogenetic position of Haopterus was reevaluated by utilizing a more comprehensive character matrix derived from three previous matrices. The new analysis revealed Haopterus in the Pteranodontoidea and, within this clade, it was an istiodactyliform. The study of Haopterus holds potential significance in enhancing our understanding of the morphology and taxonomy of short-toothed pteranodontoids.