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

ABSTRACT

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 Liaoxitriton zhongjiani and Nuominerpeton aquilonaris identified as the oldest known stem hynobiids. However, the record of pre-Cretaceous hynobiid-like taxa is only known by Liaoxitriton daohugouensis, of which both the morphology and the congeneric status with L. zhongjiani remain problematic. Here, we report on a new hynobiid-like salamander, Linglongtriton daxishanensis, gen. et sp. nov., on the basis of two specimens from the Upper Jurassic Lanqi/Tiaojishan Formation (∼160 Ma) of Liaoning Province, China. Linglongtriton 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 Linglongtriton and several other hynobiid-like taxa, including Liaoxitriton daohugouensis, 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 Linglongtriton with living and fossil hynobiids sheds new lights on the evolution and developmental mechanisms of several characters, including nasal separation and tarsal elements.

ORCID

Jia Jia http://orcid.org/0000-0002-8243-0156

Ke-Qin Gao http://orcid.org/0000-0002-5765-4596

ACKNOWLEDGMENTS

We are grateful to C.-F. Zhou (Shandong University of Science and Technology) for the help in field work and collecting data for the stratigraphy at the fossil locality. We thank J. P. Jiang (CIB) and A. Resetar (FMNH) for access to specimens of living salamanders. We also thank Z.-X. Luo, J. Lemberg, and A. I. Neander (The University of Chicago) for their assistance in scanning specimens loaned from FMNH. We are grateful to the handling editor, A. Huttenlocker, and production editor, J. Jacobs, for editorial work, and the two reviewers, P. Skutschas and D. DeMar, for their extremely helpful comments and suggestions. We thank the Willi Hennig Society for providing free access to the TNT software. This study was supported by grants from Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral (Shandong University of Science and Technology) (no. DMSM2017002), Key Laboratory of Economic Stratigraphy and Palaeogeography, Chinese Academy of Sciences (Nanjing Institute of Geology and Palaeontology) (no. 2017KF03), and National Natural Science Foundation of China (NSFC 41702002, 41872008). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.