摘要: The Qaidam Basin is a key area for understanding the paleoenvironmental and
faunal history of the Tibetan Plateau. The fossil schizothoracine fish, Hsianwenia wui, evolved
extraordinarily thickened bones to adapt to the aridification of the Qaidam Basin during the
Pliocene. However, the nature of the bone thickening itself remains elusive. To promote the
further investigation of the physiological mechanism of the pachyostosis and the phylogenetic
interrelationships of Hsianwenia and all relevant cyprinids, here we present a comprehensive
morphological study of Hsianwenia. We have new information on the anterior part of the cranial
cavity, a large supraneural 3 in the Weberian apparatus, numerous procurrent caudal fin rays
supported by the preural centrum (Pu) 5, and a neural arch on Pu2. We also find the differentiated
pattern of the bone-thickening: the pachyostosis exists in the endoskeleton but not in the dermal
skeleton; it is more obvious in ventral bones than in dorsal ones, when the thickening is present
in the dorsally and ventrally grouped endoskeletal bones (e.g., the epineural and epipleural
intermuscular bones). Considering the integrity of musculoskeletal system manipulating the
chewing activities, we suspect that the thickened pharyngeal jaws and the hard food processing
might be associated with the unique hind protrusion (cleithral “humeral” process) of the dermal
pectoral girdle of Hsianwenia.