A new stem hynobiid salamander (Urodela, Cryptobranchoidea) from the Upper Jurassic (Oxfordian) of Liaoning Province, China

Hynobiids are a group of small- to moderate-sized salamanders living primarily in Asia. They are a primitive crown-group clade, with a poor fossil record. Several hynobiid-like taxa have been discovered from the Lower Cretaceous strata of northern China during the last 20 years, with <i>Liaoxitriton zhongjiani</i> and <i>Nuominerpeton aquilonaris</i> identified as the oldest known stem hynobiids. However, the record of pre-Cretaceous hynobiid-like taxa is only known by <i>Liaoxitriton daohugouensis</i>, of which both the morphology and the congeneric status with <i>L</i>. <i>zhongjiani</i> remain problematic. Here, we report on a new hynobiid-like salamander, <i>Linglongtriton daxishanensis</i>, gen. et sp. nov., on the basis of two specimens from the Upper Jurassic Lanqi/Tiaojishan Formation (∼160 Ma) of Liaoning Province, China. <i>Linglongtriton</i> is diagnosed by a unique combination of features revealed by both observation under microscope and micro-computed tomography (μCT) scan of the holotype, including nasals separated from each other at the midline; prootic, opisthotic, and exoccipital retained as discrete elements; dentary with a lateral groove; articular not ossified; metacarpal III enlarged; a single centrale; and distal tarsals 4 and 5 fused into a single element. Phylogenetic analysis identified <i>Linglongtriton</i> and several other hynobiid-like taxa, including <i>Liaoxitriton daohugouensis</i>, as stem hynobiids, thereby extending the temporal range of the stem by at least 40 Ma—from the Early Cretaceous (Aptian–Barremian) to the Middle Jurassic (Bathonian). Comparative study of <i>Linglongtriton</i> with living and fossil hynobiids sheds new lights on the evolution and developmental mechanisms of several characters, including nasal separation and tarsal elements.