The miniature fish Habroichthys orientalis (Su, 1959) (Peltopleuriformes) from the Ladinian Xingyi Fauna (China): new data on ontogeny, sexual dimorphism, and CT-scanned otoliths

After more than 65 years, new well-preserved specimens of the Ladinian (Middle Triassic) Xingyi Fauna allow the redescription of the miniature peltopleuriform fish Habroichthys orientalis (Su 1959). This taxon was previously identified as Peltopleurus, but this revision highlights the diagnostic characters typical of Habroichthys. Namely, the single deep flank scale row with a final semicircular scale, the often completely fused skull roof, the thin maxilla not sutured to the stick-shaped preopercle, and the lack of teeth. For the first time in a Triassic fish, a µ-CT scan has been used to study the 3D shape of the in situ otoliths. Some peculiar features of the genus are confirmed, such as the stiff shield-like skull roof frequently found isolated and the sexually dimorphic anal hooklets likely used during mating. Juveniles and growth lines on adult scales show the presence of an ontogenetic series, with implications also for the paleoenvironmental restoration of the area. Morphological comparison revealed H. orientalis to be most similar to H. dincae from the almost coeval Pelsa/Vazzoler Lagerstätte in the Dolomites. Their presence on opposite sides of the Paleo-Tethys suggests multiple West–East population interchanges (the so-called “Triassic Silk Road”) rather than a single migration event that could previously have been hypothesized based on the Anisian appearance of the genus in Slovenia and in southern China.