Middle Jurassic stem hynobiids from China shed light on the evolution of basal salamanders

Jia Jia,Jason S Anderson,Ke-Qin Gao

doi:10.1016/j.isci.2021.102744

Abstract

SummaryThe Hynobiidae are an early-diverging clade of crown-group salamanders (urodeles) with an important bearing on the evolution of urodeles. Paleobiology and early-branching patterns of the Hynobiidae remain unclear owing to a poorly documented fossil record. We reported a newly referred specimen to the stem hynobiid, originally named as “Liaoxitriton daohugouensis,” but here as Neimengtriton daohugouensis comb. nov., and predates the previously estimated origination time of Hynobiidae for at least 8 Myr. We interpret N. daohugouensis as semiaquatic at the adult stage, a previously unknown paleoecological preference among Mesozoic salamanders. Phenotypic variations of N. daohugouensis enlighten an unrecognized association between caudosacral vertebrae and fertilization modes in the early evolution of urodeles. Our cladistic analyses based on morphological characters not only recognize several stem hynobiids and establish Panhynobia nomen cladinovum for the total-group hynobiids but also shed light on the sequential evolution of morphological features in this primitive urodele clade.

Highlights

None of the features that initially were considered diagnostic for Liaoxitriton sensu lato or features claimed as synapomorphies uniting ‘‘L. daohugouensis’’ and L. zhongjiani are broadly distributed among other fossil (e.g., Nuominerpeton in Jia and Gao, 2016a) and extant (e.g., Batrachuperus in Jia et al, 2019) hynobiids or are developmental artifacts
We recognized several autapomorphies of N. daohugouensis and two other diagnostic features that are plesiomorphic for Urodela and never have been reported in other stem hynobiids, namely the presence of a stapedial foramen and the presence of an
We agree with certain morphological differences between N. daohugouensis and L. zhongjiani as recognized in previous studies (Wang, 2004a; 2004b; Wang and Evans, 2006a: table 2; Wang et al, 2008; Sullivan et al, 2014), including rostrum wide and rounded in N. daohugouensis vs. narrower and subpointed in L. zhongjiani; vomerine tooth rows oriented anterolaterally in N. daohugouensis vs. posterolaterally in L. zhongjiani; and metacarpal II not anteroposteriorly expanded in N. daohugouensis vs. expanded in L. zhongjiani

Summary

Introduction

Hynobiidae, commonly known as Asiatic salamanders, encompass 83 or 85 extant species in nine or ten genera (validity of Protohynobius as pending) that live primarily in Asia, with a single species (Salamandrella keyserlingii) extending into the European part of Russia (e.g., Duellman and Trueb, 1986; Blackburn and Wake, 2011; Vitt and Caldwell, 2014; Fei and Ye, 2016; AmphibiaWeb, 2021; Frost, 2021). The Hynobiidae are united with Cryptobranchidae as sister groups within the suborder Cryptobranchoidea (e.g., Estes, 1981; Duellman and Trueb, 1986; Larson et al, 2003; Zhang et al, 2006; Zhang and Wake, 2009; Pyron and Wiens, 2011; Weisrock et al, 2013; Chen et al, 2015), which has long been regarded as a primitive clade among the crown salamanders, or Urodela (Dunn, 1922), because cryptobranchoids retained several plesiomorphic features of urodeles, including the retention of an angular bone in the mandible and breeding by external fertilization (e.g., Estes, 1981; Duellman and Trueb, 1986; Jia and Gao, 2016a; 2019).

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Conclusion