Comparative osteology of the hynobiid complex Liua‐Protohynobius‐Pseudohynobius (Amphibia, Urodela): Ⅰ. Cranial anatomy of Pseudohynobius

Abstract

Hynobiidae are a clade of salamanders that diverged early within the crown radiation and that retain a considerable number of features plesiomorphic for the group. Their evolutionary history is informed by a fossil record that extends to the Middle Jurassic Bathonian time. Our understanding of the evolution within the total group of Hynobiidae has benefited considerably from recent discoveries of stem hynobiids but is constrained by inadequate anatomical knowledge of some extant forms. Pseudohynobius is a derived hynobiid clade consisting of five to seven extant species living endemic to southwestern China. Although this clade has been recognized for over 37 years, osteological details of these extant hynobiids remain elusive, which undoubtedly has contributed to taxonomic controversies over the hynobiid complex Liua‐Protohynobius‐Pseudohynobius. Here we provide a bone‐by‐bone study of the cranium in the five extant species of Pseudohynobius (Ps. flavomaculatus, Ps. guizhouensis, Ps. jinfo, Ps. kuankuoshuiensis and Ps. shuichengensis) based on x‐ray computer tomography data for 18 specimens. Our results indicate that the cranium in each of these species has a combination of differences in morphology, proportions and articulation patterns in both dermal and endochondral bones. Our study establishes a range of intraspecific differences that will serve as organizing hypotheses for future studies as more extensive collections of these species become available. Morphological features in the cranium for terrestrial ecological adaptation in Hynobiidae are summarized. Based on the results, we also discuss the evolution and development of several potential synapomorphies of Hynobiidae, including features of the orbitosphenoid and articular.

Our bone‐by‐bone study on the cranium of five species in a rare hynobiiid salamander genus, Pseudohynobius, not only reveal several morphological features for terrestrial adaptation in hynobiids, but also found several potential synapomorphies uniting crown or stem and crown Hynobiidae. Our findings are essential in understanding the palaeoecology and character evolution in hynobiid salamanders along with accumulating fossil discoveries of this clade in the Mesozoic Era.

1. INTRODUCTION

Salamanders, frogs and caecilians are the three radiations that define the crown clade of Amphibia (Lissamphibia of many authors) and which collectively form the extant sister taxon to all other extant tetrapods (Schoch, 2014). With well‐developed limbs and tails, salamanders stand out among extant amphibians in having the most conservative body plan that closely resembles the basal tetrapod condition and which commonly serve as the modern analogue when investigating the fish‐tetrapod transition (e.g. Wagner & Larsson, 2007; Brainerd, 2015). A comprehensive understanding of the evolution of salamanders benefits from the establishment of broadly based evolutionary patterns across deep time, which demands not only from discoveries of fossil taxa, but also a thorough understanding of deeply nested clades.

Hynobiidae have long been considered as a primitive clade of crown group salamanders (or Urodela; Dunn, 1923; Hecht & Edwards, 1977), because they have several plesiomorphic features of urodeles (see Jia & Gao, 2016a, and references therein), and therefore bear the potential in understanding the evolution of salamanders in a broad sense. The Hynobiidae have 81–82 extant species in 9–10 genera (Fei & Ye, 2016; AmphibiaWeb, 2020; Frost, 2020), and are closely related to the Cryptobranchidae, the giant salamanders. The Hynobiidae have an extremely poor fossil record in the Cenozoic (66 Mya to date), providing limited information on the evolution of the family because the known fossils were incompletely preserved (see Jia & Gao, 2016a; Vasilyan et al., 2017, and references therein). During the last two decades, several stem hynobiid taxa were known from the Jurassic and Cretaceous beds (166–110 Mya) in northern China (e.g. Gao et al., 2013; Jia & Gao, 2016a), extending the fossil record of Hynobiidae to the Middle Jurassic Bathonian (168.3–166.1 Mya; Jia & Gao, 2019). These stem hynobiids are known with complete articulated skeletons and are morphologically diversified with a mosaic of both primitive and derived osteological features (see Jia & Gao, 2019). However, inadequate knowledge of the osteology of the extant relatives of these extinct hynobiids hampers our understanding of the character evolution within the Hynobiidae (e.g. Dong & Wang, 1998; Gao et al., 1998; Gao & Shubin, 2001; Wang, 2004; Wang & Evans, 2006; Zhang et al., 2009; Rong, 2018; Jia & Gao, 2019), and further obstructs our understanding of the paleobiology of these early salamanders (e.g. life history, ecological preferences). This situation is gradually improving as morphological investigations accumulate for several primitive extant hynobiids (e.g. Onychodactylus in Smirnov & Vassilieva, 2002; Vassilieva et al., 2013; Salamandrella and Ranodon in Lebedkina, 2004); but, more detailed osteological studies are needed for derived taxa (Batrachuperus, Liua, Pachyhynobius, Pseudohynobius, Hynobius; e.g. Zhang, 1985; Zhao & Zhang, 1985; Xiong et al., 2016; Jiang et al., 2018; Jia et al., 2019).

The genus Pseudohynobius is a derived hynobiid taxon deficient in detailed morphological studies. The species belonging to this genus are moderate‐sized salamanders (total length: 138.0–213.2 mm) that live endemically to southwestern China (Chongqing Municipality, Hubei Province, Hunan Province and Guizhou Province; Fei & Ye, 2016). According to AmphibiaWeb (2020) and Frost (2020), the genus Pseudohynobius consists of six extant species, including Ps.flavomaculatus Fei & Ye, 1983; Ps.shuichengensis Tian et al., 1998; Ps.kuankuoshuiensis Xu et al., 2007; Ps.jinfo Wei et al., 2009; Ps.guizhouensis Li et al., 2010; and the problematic ‘Ps.puxiongensis’ (e.g. Peng et al., 2010; Xiong et al., 2011; see below). Members of this clade are all metamorphosed salamanders at adult stage, and live primarily on land, except going back to water for breeding as in several other extant hynobiids. Phylogenetically, Pseudohynobius is consistently found as the sister group to Liua in molecular studies based on mitochondrial DNA (Zeng et al., 2006; Weisrock et al., 2013), nuclear DNA (Chen et al., 2015) or a combination of both types of molecules (Weisrock et al., 2005; Peng et al., 2010; Pyron & Wiens, 2011; Zheng et al., 2011). Nevertheless, interspecific relationships of Pseudohynobius were in dispute: a recent study based on three mitochondrial genes (cytochrome c oxidase I, cytochrome b and 16S rRNA; Zhao et al., 2016) support that Ps. flavomaculatus and Ps. jinfo group into a subclade, which forms the sister taxon to another subclade containing Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis, with ‘Ps. puxiongensis’ being the most basal species (Figure 1). Such a topology concurs with the cladistic analysis based solely on cytochrome b (Yang et al., 2013); but differs substantially from studies based either on cytochrome c oxidase I or 16S rRNA (Xia et al., 2012), in which Ps. shuichengensis was found instead as the most basal taxon of Pseudohynobius. The genus has no fossil record known so far, but previous dating analyses indicate that the splitting event between Pseudohynobius and Liua occurred in the Miocene (Tortonian–Burdigalian) (16–8 Mya of Zheng et al., 2011; 18.8–11.4 Mya of Chen et al., 2015) or Oligocene‐to‐early Miocene (33.34–22.59 Mya of Zhao et al., 2016).

FIGURE 1.

Interrelationship of species of Pseudohynobius and the Liua‐Protohynobius‐Pseudohynobius complex based on Peng et al. (2010) and Zhao et al. (2016). Note that photographs of representative formaldehyde‐preserved specimens of each species are in dorsal view: Ps. flavomaculatus (the paratype CIB 17343); Ps. jinfo (CIB 85290, the holotype); Ps. guizhouensis (LTHC 0711005, the holotype); Ps. kuankuoshuiensis (ZMC 7604025); Ps. shuichengensis (the paratype LTHC 9460085); Pr. puxiongensis (CIB 98264, the holotype); Liua tsinpaensis (CIB 18349, the holotype); Liua shihi (CIB 19910401); Batrachuperus yenyuanensis (CIB 17005). Ps, Pseudohynobius; Pr, Protohynobius

The Pseudohynobius is a problematic taxon, in part because of the ongoing arguments over its taxonomy (i.e. the number of valid species; Xiong et al., 2011; Fei & Ye, 2016). It was argued that Liua tsinpaensis should be transferred into Pseudohynobius as ‘Ps. tsinpaensis’, because L. tsinpaensis shares similar traits with Pseudohynobius, including ‘skull and hyobranchial apparatus, absence of labial folds, absence of horny sheath on palms and soles, and primary terrestrial habits’ (Fei & Ye, 2016: 228). However, L. tsinpaensis is consistently found in molecular studies to be more closely related to Liua shihi than Pseudohynobius, and accordingly was retained in the genus Liua (e.g. Zeng et al., 2006; Zheng et al., 2011; Weisrock et al., 2013). Several studies have been conducted on the osteology of species of Liua (Zhang, 1985; Zhao & Zhang, 1985; Xiong et al, 2016), but unfortunately, made no morphological comparison between L. tsinpaensis and any species of Pseudohynobius.

Another ongoing controversy surrounds the validity of ‘Pseudohynobius puxiongensis’. The taxon ‘Ps. puxiongensis’ was originally described as Protohynobius puxiongensis (Fei & Ye, 2000). Later, it was found to be the sister taxon of the clade containing two species of Pseudohynobius (Ps. flavomaculatus and Ps. shuichengensis) in a molecular study based on complete mitochondrial genomes (Peng et al., 2010), and therefore was merged into Pseudohynobius as ‘Ps. puxiongensis’. Xiong et al. (2011) supported this transfer based on the same karyotype (2n = 52) shared between ‘Ps. puxiongensis’ and Ps. flavomaculatus, even though they realized that Ps. shuichengensis has a karyotype (2n = 50) different from that of ‘Ps. puxiongensis’, and the karyotypes of other Pseudohynobius species remain unknown. A morphological comparison between ‘Ps. puxiongensis’ and the five species of Pseudohynobius was briefly conducted by Xiong et al. (2011), but focused only on measurements of external morphology and the number of vomerine teeth, rather than detailed osteological differences. The validity of ‘Pseudohynobius puxiongensis’ and its relationships with other species of Pseudohynobius need to be scrutinized, but these are subjects obviously beyond the scope of this study, and will be published in separate papers elsewhere.

The above taxonomic disputes in the hynobiid complex Liua‐Protohynobius‐Pseudohynobius cannot be helped by the fact that no detailed morphological studies have been undertaken in any of the known species of Pseudohynobius. That is despite the type species of this genus, Ps.flavomaculatus, being reported 37 years ago (Fei & Ye, 1983). The endemic nature of this rare clade has long limited its availability to herpetologists. In this context, our study represents a unique opportunity to introduce the morphological details of this elusive corner of hynobiid diversity. We provide the description of the cranial anatomy of five species of Pseudohynobius (Ps. flavomaculatus,Ps. guizhouensis,Ps. jinfo,Ps. kuankuoshuiensis and Ps. shuichengensis) based a bone‐by‐bone investigation using high‐resolution x‐ray computer tomography (μCT) of wet specimens. Based on the results, we discuss morphological features for terrestrial adaptation in Pseudohynobius and other hynobiids, and provide a discussion on the evolutionary implications of several morphological characters that are taxonomically and phylogenetically informative in hynobiids. We hope that our data and descriptions will serve as a useful platform for future studies delineating new phylogenetically informative characters applicable to the analysis of both fossil and extant hynobiids and that will untangle the taxonomic controversies regarding the complex Liua‐Protohynobius‐Pseudohynobius.

2. MATERIALS AND METHODS

2.1. Specimens

All specimens are preserved in 10% formalin solution, and these are reposited in three different institutions: Chengdu Institute of Biology (CIB), Chinese Academy of Sciences, Sichuan Province; Department of Biology, Liupanshui Normal University (LTHC), Liupanshui City, Guizhou Province; and Zunyi Medical University (ZMC), Zunyi City, Guizhou Province. A total of 18 specimens were used in this study (Table 1), including 10 specimens of Ps. flavomaculatus (including the holotype CIB 17342), one specimen of Ps. guizhouensis (LTHC 0711005 [holotype]), two specimens of Ps. jinfo (CIB 85290 [holotype] and paratype CIB 85291), three specimens of Ps. kuankuoshuiensis (ZMC 7504023 [holotype] and paratypes ZMC 7604025 and 8208010), and two specimens of Ps. shuichengensis (paratypes LTHC 9460084, 9460085). All selected specimens are adult individuals, as evidenced by the remodeled pterygoid and complete ossification of the mesopodium in the limb. Specimens of Ps. flavomaculatus were catalogued with a field number in the first place, but was later assigned with an official catalogue number (Table 1). The cranial morphology is described based on holotypes of Ps. flavomaculatus (CIB 17342), Ps. guizhouensis (LTHC 0711005), Ps. jinfo (CIB 85290) and Ps. kuankuoshuiensis (ZMC 7504023), and on the paratype of Ps. shuichengensis (LTHC 9460084), supplemented with variations observed in other specimens of each species.

TABLE 1.

Information about the adult specimens (N = 18) of the five species of Pseudohynobius investigated in this study

Taxon	Catalogue #	Field catalogue #	Total length (mm)	Snout‐pelvic length (mm)	Skull length (mm)	Skull width (mm)	Date of collection	Locality	Coordinates	Elevation (m)
Ps. flavomaculatus	CIB 17342	770041	172.41	86.19	15.42	12.85	1977	Hubei	N 30°18′, E 108°56′	1830
	CIB 17343	79I0103	178.79	88.84	15.20	12.27	1979			1850
	CIB 17344	79I0107	171.46	87.27	15.01	12.58	1979			1845
	CIB 17592	79I0106	175.74	90.56	15.20	12.55	1979			1845
	CIB 17593	79I0087	160.94	80.54	14.69	12.20	1979			1845
	CIB 17594	79I0078	146.96+	83.46	14.95	11.54	1979			1840
	CIB 17595	79I0109	171.22	85.74	14.26	12.36	1979			1845
	CIB 17597	79I0088	135.52	75.9	14.65	11.71	1979			1840
	CIB 17598	79I0104	151.7	82.18	15.69	12.97	1979			1845
	CIB 17599	79I0089	163.34	84.72	14.25	11.89	1979			1845
Ps. jinfo	CIB 85290	—	196.19	90.82	15.38	13.28	2006	Chongqing	N 28°50′, E 107°20′	2150
	CIB 85291	—	165.68	80.5	13.05	11.56	2006
Ps. guizhouensis	LTHC 0711005	—	162.28	82.94	16.48	12.86	2008	Guizhou	N 26°12′, E 107°30′	1650
Ps. kuankuoshuiensis	ZMC 7504023	—	156.3+	93.83	15.68	12.40	1975	Guizhou	N 28°12′, E 107°10′	1350
	ZMC 7604025	—	157.91+	89.59	16.45	13.68	1976
	ZMC 8208010	—	—	—	16.7	14.13	1976
Ps. shuichengensis	LTHC 9460084	—	195.38	100.69	16.93	14.64	1994	Guizhou	N 26°34′, E 104°48′	1970
	LTHC 9460085	—	196.87	101.51	18.08	14.92	1994

2.2. Micro‐CT scan and 3D reconstruction

Most specimens except the holotype of Ps. guizhouensis (LTHC 0711005) were whole‐body scanned along the sagittal axis by using the Quantum GX microCT Imaging System (PerkinElmer^®) at CIB. The scan was performed at a voltage of 90 kV and a current range of 60–88 μA at the high‐resolution scan mode (57 min/section). Each scan of the cranial part produced 512 16‐bit tiff files with an image resolution of 512 × 512 pixels. Voxel size of the generated numerical models ranges between 50–90 μm; but higher resolution of the cranial part (21.5–41.3 μm) was achieved using the subvolume reconstruction function implanted in the Quantum GX microCT Imaging System (Table 2). The holotype of Ps. guizhouensis was whole‐body scanned with a Nikon XT H 320 LC scanner at China University of Geosciences, at a voltage of 145 kV and a current range of 57 μA. The resulting files have an image resolution of 2000 × 2000 pixels for the image stacks and a voxel size of 39.9 μm for the volume file (Table 2). Image stacks were imported into the software VG Studio Max 2.2 (Volume Graphics) for visualization, segmentation, color rendering and measurements. Linear measurements conducted by us (Table 1) include: the skull length (SKL; length from the anterior edge of the premaxilla to the posterior margin of the exoccipital), skull width (SKW; maximum distance between the two lateral sides of the skull), snout‐pelvic length (length from the snout tip to the posterior extremity of the pelvic girdle) and total length (TL; length from the snout tip to the posterior end of the last caudal vertebra). To facilitate description, we followed the terminology of Francis (1934) and supplemented with that for hyobranchial apparatus by Rose (2003) and that for braincase by Villa et al. (2014), except for others that are noted otherwise. For comparative studies, wet specimens of several other hynobiid species deposited at both CIB and the Field Museum of Natural History (FMNH), Chicago, USA, were used (Table 3). We used single quotation marks around names of taxa that we regard as invalid (e.g. ‘Pseudohynobius puxiongensis’).

TABLE 2.

Micro‐CT scan parameters of specimens of the five species of Pseudohynobius investigated in this study

Taxon	Catalogue #	Current (μA)	Voltage (kV)	Slices	Sections #	Image resolution (pixel)	Voxel size (μm)	Holotype	Date of scan
Ps. flavomaculatus	CIB 17342	88	90	512/section	9	512	40.8–72	√	2017/05/23
	CIB 17343	88			9		38.3–72		2017/05/24
	CIB 17344	88			9		38.8–72		2017/05/24
	CIB 17592	60			6		50–90		2017/07/17
	CIB 17593	60			6		37.2–90		2017/07/18
	CIB 17594	60			6		38.8–90		2017/07/18
	CIB 17595	60			6		39.6–90		2017/07/17
	CIB 17597	60			6		38.6–90		2017/07/18
	CIB 17598	60			6		38–90		2017/07/18
	CIB 17599	60			6		36.6–90		2017/07/18
Ps. jinfo	CIB 85290	60	90	512/section	7	512	21.5–90	√	2017/05/26
	CIB 85291	80			6		41.3–90		2017/05/27
Ps. guizhouensis	LTHC 0711005	57	145	3905	1	2000	39.9	√	2018/11/18
Ps. kuankuoshuiensis	ZMC 7504023	60	90	512/section	6	512	39.4–90	√	2017/07/14
	ZMC 7604025				6		40.7–90		2017/07/14
	ZMC 8208010				5		40.6–90		2017/07/14
Ps. shuichengensis	LTHC 9460084	60	90	512/section	9	512	40.4–90		2017/07/14
	LTHC 9460085				8		42–90		2017/07/15

TABLE 3.

Specimens of other extant salamander species used for comparative studies

Taxon name	Catalogue number
Amphiuma tridactylum	FMNH 7181702
Andrias japonicus	FMNH 60644
Batrachuperus karlschmidti	FMNH 49380
Batrachuperus yenyuanensis	FMNH 49371
Cryptobranchus alleganiensis	FMNH 33838
Dicamptodon tenebrosus	FMNH 59246
Liua shihi	CIB 17600, 19910401
Liua tsinpaensis	CIB 18349
Onychodactylus japonicus	FMNH 16095, 285321
Pachyhynobius shangchengensis	CIB72887, 72891
Paradactylodon mustersi	FMNH 211936
Ranodon sibiricus	FMNH 83050, 83051
Salamandrella keyserlingii	FMNH 83525, 83526

2.3. Institutional abbreviations

CIB, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, Sichuan Province, China; FMNH, Field Museum of Natural History, Chicago, Illinois, USA; LTHC, Liupanshui Normal University, Liupanshui, Guizhou Province, China; PKUP, Peking University Paleontological Collections; ZMC, Zunyi Medical University, Zunyi, Guizhou Province, China.

3. RESULTS

The morphological description below applies to all five species of Pseudohynobius unless otherwise noted when differences were observed in any particular species. Intraspecific variations in the cranial morphology among the five species of the genus are listed whenever noticed. However, due to our limited specimen sampling, especially in Ps. jinfo (n = 2), Ps. guizhouensis (n = 1) and Ps. shuichengensis (n = 2), these variations may need further investigation with more extensive survey of specimens in future studies.

3.1. General skull morphology

The skull in all five species in the genus Pseudohynobius is ovoid in dorsal view, being slightly longer than wide (Figure 2): the ratios of SKW to SKL are 0.83 in Ps. flavomaculatus (CIB 17342), 0.78 in Ps. guizhouensis (LTHC 0711005), 0.86 in Ps. jinfo (CIB 85290), 0.79 in Ps. kuankuoshuiensis (ZMC 7504023) and 0.86 in Ps. shuichengensis (LTHC 9460084). The maximum width between the posterior ends of the maxillae is slightly narrower than the dimension spanning the craniomandibular joints in Ps. jinfo, but wider than that in the other four species (Figure 2). The snout is short and rounded. Each pair of the dermal roof elements (nasals, frontals, and parietals) are not tightly articulated, but are separated by a narrow fissure filled with connective tissues along the midline suture.

FIGURE 2.

Micro‐CT rendered reconstruction of the skull of the five species of Pseudohynobius in dorsal view: (a) Ps. flavomaculatus (CIB 17342, the holotype), (b) Ps. jinfo (CIB 85290, the holotype), (c) Ps. guizhouensis (LTHC 0711005, the holotype), (d) Ps. kuankuoshuiensis (ZMC 7504023, the holotype), (e) Ps. shuichengensis (LTHC 9460084, a paratype). adf, anterodorsal fenestra; app, ascending process of palatoquadrate; fpf, frontoparietal fontanelle; fr, frontal; lac, lacrimal; mx, maxilla; na, nasal; op‐ex, opisthotic‐exoccipital complex; pa, parietal; pm, premaxilla; prf, prefrontal; pro, prootic; pt, pterygoid; qu, quadrate; sm, septomaxilla; sq, squamosal; st, stapes

3.2. Dermal skull roof

The paired premaxillae articulate with each other medially to form the anterior margin of the snout (Figures 2, 3, 4; Figures S1 and S2). The pars dorsalis (dorsal process or alary process) is triangular, with a wide base and a sharply pointed dorsal apex setting in a deep notch of the nasal (see below). The pars dorsalis is proportionally the shortest in Ps. kuankuoshuiensis in comparison to other species. The base of the pars dorsalis is penetrated by one to three foramina, through which pass the terminal branches of the mesial ramus of the deep ophthalmic nerve (CN V₁) and the associated blood vessels nurturing the skin of the upper lip (Figure 5a; Figure S1). In several specimens of Ps. flavomaculatus (CIB 17344, 17592), Ps. jinfo (CIB 85290), and Ps. shuichengensis (LTHC 9460084), the pars dorsalis is expanded medially to articulate with the nasals and to form the border of the anterodorsal fenestra (Figure 2; Figures S1 and S2). The pars dentalis of the premaxilla is slightly curved along the contour of the snout, lingually bearing a row of closely packed teeth (see below). The pars palatina develops as a narrow ledge along the lingual aspect of the pars dentalis. In ventral view, the inner margin of this bony ledge runs parallel to the curvature of the tooth row of the premaxilla. The pars palatina articulates extensively with the vomer to contribute to the anterior part of the palate.

FIGURE 3.

Micro‐CT rendered reconstruction of the skull of the five species of Pseudohynobius in ventral view: (a) Ps. flavomaculatus (CIB 17342, the holotype), (b) Ps. jinfo (CIB 85290, the holotype), (c) Ps. guizhouensis (LTHC 0711005, the holotype), (d) Ps. kuankuoshuiensis (ZMC 7504023, the holotype), (e) Ps. shuichengensis (LTHC 9460084, a paratype). amf, anteromedial fenestra; app, ascending process of palatoquadrate; mx, maxilla; obs, orbitosphenoid; op‐ex, opisthotic‐exoccipital complex; pm, premaxilla; pro‐op‐ex, prootic‐opisthotic‐exoccipital complex; ps, parasphenoid; pt, pterygoid; qu, quadrate; st, stapes; vo, vomer

FIGURE 4.

Micro‐CT rendered reconstruction of the skull of the five species of Pseudohynobius in left lateral view: (a) Ps. flavomaculatus (CIB 17342, the holotype), (b) Ps. jinfo (CIB 85290, the holotype), (c) Ps. guizhouensis (LTHC 0711005, the holotype), (d) Ps. kuankuoshuiensis (ZMC 7504023, the holotype), (e) Ps. shuichengensis (LTHC 9460084, a paratype). fr, frontal; mx, maxilla; na, nasal; obs, orbitosphenoid; op‐ex, opisthotic‐exoccipital complex; pa, parietal; pm, premaxilla; prf, prefrontal; pro‐op‐ex, prootic‐opisthotic‐exoccipital complex; ps, parasphenoid; pt, pterygoid; qu, quadrate; sm, septomaxilla; sq, squamosal; st, stapes

FIGURE 5.

Micro‐CT rendered reconstruction of premaxilla, maxilla, nasal and lacrimal of the five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a) Left premaxilla in lateral (upper) and medial (lower) views, (b) left maxilla in lateral (upper) and medial (lower) views, (c) left nasal in dorsal (upper) and ventral (lower) views, (d) right lacrimal in dorsal (upper) and ventral (lower) views. lpn, lateral process of nasal; mpn, medial process of nasal; pdp, pars dorsalis of premaxilla; pfm, pars facialis of maxilla; plm, pars palatina of maxilla; plp, pars palatina of premaxilla

The maxilla is more than twice the length of the premaxilla (Figures 2 and 3). It bears a short anterior process, with a blunt anterior end in articulation with the premaxilla; the maxilla and premaxilla together constitute the floor of the external naris. Among extant hynobiids, the anterior process of the maxilla is short and blunt as found in Batrachuperus londongensis (Jiang et al., 2018), Batrachuperus yenyuanensis (Jia et al., 2019), Batrachuperus karlschmidti (FMNH 49380), Hynobius formosanus (Vassilieva et al., 2015), Liua shihi (CIB 17600; Zhang, 1985; Zhao & Zhang, 1985; Xiong et al., 2016), Liua tsinpaensis (CIB18349; Xiong et al., 2016), Pachyhynobius shangchengensis (CIB 72887, 72891; Clemen & Greven, 2009), Paradactylodon mustersi (FMNH 211936), Ranodon sibiricus (FMNH 83050, 83051; Lebedkina, 2004) and Salamandrella keyserlingii (FMNH 83525, 83526; Ma, 1964). However, the process is elongated to overlap the premaxilla in Onychodactylus japonicus (FMNH 285321; Smirnov & Vassilieva, 2002). The anterior process of the maxilla remains unknown for Cenozoic fossil hynobiids, but is known as an even longer and tapering process that extensively overlaps the premaxilla in the Mesozoic stem hynobiids, for example, Nuominerpeton aquilonaris (Jia & Gao, 2016a) and Linglongtriton daxishanensis (Jia & Gao, 2019).

The pars facialis (facial/dorsal process) arises from the anterior half of the maxilla to articulate with the lacrimal and prefrontal in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis, but only with the lacrimal in Ps. flavomaculatus and Ps. jinfo. The anterior edge of the process ascends steeply to form much part of the posterior border of the external naris; the posterior edge of the process slops at a slightly wider angle to form the anteroventral margin of the orbit (Figures 4 and 5). The dorsal edge of the pars facialis is relatively straight in Ps. flavomaculatus, and Ps. shuichengensis, but is concave between two small projections along the suture with the lacrimal in Ps. guizhouensis, Ps. jinfo and Ps. kuankuoshuiensis. The base of the pars facialis is perforated by the foramen lateral nasi, passage of the lateral branch of the deep ophthalmic nerve (CN V₁), in the right maxilla of Ps. kuankuoshuiensis (ZMC 7504023, 7604025), the left maxilla of Ps. jinfo (CIB 85290) and the left maxilla of Ps. shuichengensis (LTHC 9460085), but has no foramina in other specimens of these taxa or any specimens of Ps. flavomaculatus or Ps. guizhouensis.

The pars dentalis of the maxilla is slightly curved in ventral view, with the tooth row terminating close to the posterior extremity of the maxilla (see below). It is a narrow flange, extending along the lingual side of the maxilla (Figures 3 and 5). The pars palatina narrows posteriorly and eventually merges with the posterior process of the maxilla (Figure 3). The posterior process of the maxilla dorsally bears a low ridge where both the jugal (Buckley et al., 2010; ligamentum quadro‐maxillare of Lebedkina, 2004) and pterygomaxillary ligaments (Elwood & Cundall, 1994; ligamentum rostro‐pterygoideum of Lebedkina, 2004) attach in connecting with the quadrate and anterolateral process of the pterygoid, respectively.

The anterodorsal fenestra opens between the partes dorsalis of the premaxillae and the nasals. This fenestra varies in size and shape among species of Pseudohynobius (Figure 2; Figure S1). The anterodorsal fenestra in Ps. flavomaculatus is proportionally the longest, with the posterior border reaching close to the nasal‐frontal suture; whereas it is shorter in Ps. guizhouensis, Ps. jinfo and Ps. kuankuoshuiensis, of which the posterior edge of the fenestra terminating to the level corresponding to the septomaxilla. In anterodorsal view, the fenestra is irregular‐shaped in Ps. flavomaculatus (Figure S2), the holotype (LTHC 0711005) of Ps. guizhouensis and the paratype (CIB 85291) of Ps. jinfo, with a subtriangular opening between the pars dorsalis of the premaxillae merged posteriorly with a spindle‐shaped opening between the nasals. In Ps. kuankuoshuiensis, however, it is an oval‐shaped fenestra that widely opens between the paired premaxillae and paired nasals. The paratype of Ps. shuichengensis (LTHC 9460084) has the smallest anterodorsal fenestra, which is confined to a tiny, irregular opening by the nasal and premaxilla (Figure 2e). Certain variations in shape and size are found in Ps. jinfo and Ps. shuichengensis: in Ps. jinfo, the anterodorsal fenestra is spindle‐shaped, and smaller in the holotype (CIB 85290) than in the paratype; and in Ps. shuichengensis, it is also spindle‐shaped, but larger in LTHC 9460085 than LTHC 9460084. In all species of Pseudohynobius, the anterodorsal fenestra confluents with a narrow fissure running along the midline of the dermal skull roof. The anterodorsal fenestra is open in all extant hynobiids, but is closed in cryptobranchids (Reese, 1906; Wu, 1982; Fei et al., 2006).

The nasals are paired, irregular and are slightly longer than wide as commonly seen in other hynobiids, except Liua tsinpaensis, in which the nasal is slightly wider than long as observed in the holotype of the latter taxon (CIB 18349; but see figure 3 in Xiong et al., 2016). In dorsal view, the nasals are slightly wider than frontals and articulate with the lacrimal but are separated from the maxilla by the latter. Posterolaterally, the nasal has a loose contact with the prefrontal, leaving discrete or continuous slits in between. The nasals are separated, without a median contact, by the anterodorsal fenestra and the aforementioned narrow fissure (Figure 2; Figures S1 and S2). This condition differs from most of other cryptobranchoids, in which the nasals are tightly articulated with each other (Reese, 1906; Wu, 1982; Fei & Ye, 2016). Separation of the paired nasals was recently documented in the Late Jurassic stem hynobiid Linglongtriton, indicating that this derived feature indeed occurred in the early evolution of hynobiids (Jia & Gao, 2019). The anterior border of the nasal is bifurcated with a medial and a lateral process, with a subtriangular notch in between hosting the pars dorsalis of the premaxilla (Figures 2 and 5c). The lateral process is narrow, bordering the dorsomedial aspect of the external naris, whereas the medial process is wide, serving as the dorsolateral border of the anterodorsal fenestra. In contrast to other congeneric species, the median process in Ps. kuankuoshuiensis is shorter and narrower than the lateral process (Figures 2 and 5c). The medial border of the nasal is straight in LTHC 9460084 of Ps. shuichengensis, but is notched in Ps. kuankuoshuiensis and Ps. guizhouensis, or is notched posteromedially to the medial process in Ps. flavomaculatus, the paratype (CIB 85291) of Ps. jinfo and the other paratype (LTHC 9460085) of Ps. shuichengensis. In several specimens (CIB 17344, 17593, 17594, 17595, 17598) of Ps. flavomaculatus, the medial process of the nasal is further bifurcated anteriorly, with an extra stub‐like process extending into the anterodorsal fenestra (Figure S2). The anterior part of the nasal is penetrated by two to four foramina, which would have accommodated distal branches of the superficial ophthalmic nerve (CN V₁), to supply the skin of the anterolateral region of the snout (Figure 5; Jurgens, 1971).

The paired lacrimals are small and L‐shaped, with a lateral tubular part articulating with the pars facialis of the maxilla and a posteromedial extension overlapping the prefrontal (Figures 2 and 5). The tubular part of the lacrimal has a small foramen that opens on the dorsal surface close to the posterior border of lacrimal, and another small foramen that opens ventrally close to the anterior border. Through the canal connecting these two foramina passes the nasolacrimal duct as commonly seen in extant hynobiids (e.g. Fei et al., 2006; Jiang et al., 2018). The lacrimal participates in the posterior margin of the external naris in all species of Pseudohynobius; posteriorly, it forms part of the anterior border of the orbit as in Ps. flavomaculatus and Ps. jinfo, or is excluded from the border by a prefrontal‐maxilla contact as in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis.

The septomaxilla is a tiny bone positioned at the ventrolateral corner of the nasal cavity (Figure 2). It remains free from bony contacts with surrounding bones in most species, but fuses with the lacrimal as a variation in Ps. flavomaculatus (Figure S2; CIB 17343, 17593–17595). The septomaxilla in Ps. flavomaculatus, Ps. guizhouensis and Ps. shuichengensis is proportionally larger than in Ps. jinfo and Ps. kuankuoshuiensis. In the former three species, the septomaxilla is anteriorly constricted and posteriorly scrolled to form a canal around the nasolacrimal duct. In the latter two species, however, the septomaxilla is gutter‐like, with the ventral aspect remaining unossified; the nasolacrimal duct is exposed in ventral view.

The prefrontal is a strip‐like bone, setting between the maxilla and frontal at the anterodorsal border of the orbit (Figures 2, 4 and 6; Figure S1). The prefrontal is anteriorly expanded but narrows posteriorly to form a tongue‐like process that overlaps the frontal. Articulation of this element with both the lacrimal and the maxilla characterize all species of Pseudohynobius except Ps. flavomaculatus and Ps. jinfo, in which a maxilla contact is absent (Figure 4).

FIGURE 6.

Micro‐CT rendered reconstruction of prefrontal, frontal and parietal of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Right prefrontal in dorsal (left) and ventral (right) views, (f–j) left frontal in dorsal (left) and ventral (right) views, (k–o) left parietal in dorsal (left) and ventral (right) views. plpp, posterolateral process of parietal; qdp, quadrate process of parietal; sgcf, sagittal crest of frontal; sgcp; sagittal crest of parietal; tlf, trochlear foramen

The frontals are elongate plates that span the interorbital area in the skull roof (Figure 2). The frontal is proportionately longer in Ps. flavomaculatus and Ps. shuichengensis than in Ps. guizhouensis, Ps. jinfo and Ps. kuankuoshuiensis. It contacts the nasal anteriorly and underlies a part of the prefrontal anterolaterally. The frontal bifurcates under the nasal into a lateral process and a medial process, with the former wider and longer than the latter (Figure 6). The lateral borders of the frontals run parallel to form roughly one third the length of the orbital rim. The remaining two thirds of the border is formed by the prefrontal anteriorly and the parietal posteriorly (Figures 2 and 4). The lateral border of the frontal slightly curves ventrally to articulate with the orbitosphenoid. A spindle‐shaped frontoparietal fontanelle is present in Ps. flavomaculatus, Ps. guizhouensis and Ps. jinfo, but is absent in Ps. kuankuoshuiensis and Ps. shuichengensis. In ventral view, the frontal bears a low crest close to the lateral border and bears a concave area posterior to the level of the anterior border of the orbitosphenoid to hold the forebrain (Figure 6f–j).

The parietal is more or less L‐shaped, with an elongate anterior plate and a transverse posterolateral process (Figures 2 and 6). The anterolateral process wedges between the frontal and orbitosphenoid, terminating at the midlevel of the orbit (Figure 4). Posterior to the anterolateral process, the parietal bears a blunt process (quadrate process of Wilder, 1903) ventrally to articulate with both the prootic and the ascending process of the palatoquadrate (Figures 2 and 4; see below). CT images show that the base of the blunt process is obliquely penetrated by a tiny foramen that transmits the trochlear nerve (CN IV) to the extrinsic eye muscles (e.g. M. obliquus superior and M. levator bulbi; Francis, 1934). A trochlear foramen was recognized as piercing the parietal (Theron, 1952; Joubert, 1961; Jiang et al., 2018), orbitosphenoid (Aoyama, 1930; Cloete, 1961), or passing through the fissure between these two bones in several taxa of salamanders (Gaupp, 1911; Francis, 1934), but more attention should be paid in future studies to unveil its phylogenetic significance. The posterolateral process of the parietal is short compared to the anterior plate, with a blunt distal end having a limited contact with the squamosal in Ps. guizhouensis, Ps. jinfo and few specimens (CIB 17342, 17597) of Ps. flavomaculatus, or lacking bony contact with the squamosal in Ps. kuankuoshuiensis, Ps. shuichengensis and most specimens of Ps. flavomaculatus. Absence of a parietal‐squamosal contact is considered a derived feature within Cryptobranchoidea; currently known in Liua tsinpaensis (CIB 18349), Salamandrella keyserlingii (FMNH 83525, 83526) and Onychodactylus japonicus (FMNH 285321; Smirnov & Vassilieva, 2002), but present in all other extant hynobiids (Fei & Ye, 2016), cryptobranchids (Erdman & Cundall, 1984) and several fossil stem hynobiids (Jia & Gao, 2016a, 2019).

In dorsal view, the parietal table has a low ridge extending diagonally across the posterolateral process. Posteriorly, the parietal bends downward to form a concave area, onto which the anterior epaxial muscles attach (Carroll & Holmes, 1980). The posterior margin of the parietal overlaps the opisthotic‐exoccipital. In ventral view, the parietal bears a low crest lying slightly medial to the trochlear foramen along the lateral border. The crest is straight, extending along the anteroposterior axis between the posterior border of the orbitosphenoid and anterior edge of the posterolateral process of the parietal, and curves medially and terminates close to the base of the posterior process of the parietal. Similar to the frontal, the parietal has a concave area medial to the bony ridge to host the forebrain, optic lobe and cerebellum (Figure 6k–o).

3.3. Suspensorium

The squamosal is roughly T‐shaped, with an expanded proximal end and a slender distal process that slopes towards the craniomandibular joint (Figures 2 and 4). The expanded proximal end bears a short and blunt anterior process (otic process) overlapping the prootic and a much longer and pointed posterior process that contacts the opisthotic‐exoccipital complex. Between the anterior and posterior processes, the medial border of the squamosal is convex medially in Ps. jinfo and Ps. shuichengensis, concave in Ps. flavomaculatus and Ps. kuankuoshuiensis, and straight in Ps. guizhouensis (Figure 7a–e). The distal process of the squamosal has a slightly expanded terminus with a thin bony plate slanting posteroventrally to cover a large part of the quadrate (Figures 2 and 4). The anterior border of the squamosal is straight or slightly convex, flattened, whereas the posterior border of the squamosal is curved and descends posteroventrally, therefore the squamosal is convex dorsally and concave ventrally.

FIGURE 7.

Micro‐CT rendered reconstruction of squamosal and quadrate of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Left squamosal in dorsal (upper) and ventral (lower) views, (f–j) left quadrate in the following views from top to bottom: dorsal, ventral, anterior and posterior. acq, ascending process of quadrate; alq, anterolateral process of quadrate; aps, anterior process of squamosal; atq, articular process of quadrate; ppq, posterior process of quadrate; pps, posterior process of squamosal; ptq, pterygoid process of quadrate

The quadrate is irregularly shaped, constricted proximally and expanded distally (Figure 7f–j). Its ascending process is narrow, flat and ossified in all species but remains cartilaginous in Ps. shuichengensis (Figure 7j). The process spans roughly half the length of the quadrate along the transverse plane of the skull. The lateral end of the quadrate bears a subtriangular pterygoid process slanting ventromedially to articulate with the quadrate process of the pterygoid. The anterolateral process of the quadrate is short and exposes beyond the squamosal in dorsal view, with a blunt distal end. At the base of the anterolateral process, the quadrate is dorsally penetrated by a small foramen (quadrate foramen), which transmits a branch of the posterior condylar artery and vein as observed in Ps. flavomaculatus and Ps. shuichengensis, but not in Ps. guizhouensis, Ps. jinfo or Ps. kuankuoshuiensis. Such a foramen is previously found in Batrachuperus londongensis, Batrachuperus yenyuanensis and the stem hynobiid Nuominerpeton (Jia & Gao, 2016a; Jiang et al., 2018; Jia et al., 2019), but its distribution in other hynobiids is worth a thorough investigation. The posterior process of the quadrate is elongate, gracile and rod‐like in Ps. flavomaculatus and Ps. jinfo, whereas it is short and swollen in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis. The posterior process is not obvious in other hynobiid and cryptobranchids. In all species of Pseudohynobius, the posterior process of the quadrate has a small facet at the distal end (Figure 7) as the attachment site for the ligamentum hyo‐suspensoriale in connection with the ceratohyal (Kingsbury & Reed, 1909; hyoquadrate ligament of Schmalhausen, 1968). Between the anterolateral and posterior processes, the quadrate has a stout articular process projecting ventrolaterally. In all species of Pseudohynobius, the articular process ventrally has a bony crest that runs onto the pterygoid process of the quadrate; and forms a condyle with the anterolateral process to articulate with the bulging dorsal surface of the articular (see below). The articular process is proportionally more robust in Ps. shuichengensis than in the other four species.

The pterygoid is triradiate with a palatal (anterolateral), an otic (medial), and a quadrate (posterolateral) process (Figures 3 and 8). The palatal process is short, with its distal tip terminating posteriorly to the midlevel of the orbit as in most other extant hynobiids (Liua, Paradactylodon, Ranodon, Salamandrella and some species of Hynobius). However, it is slightly shorter than the quadrate process of the pterygoid in Ps. jinfo, but roughly equals to or slightly longer than the length of the latter process in other species of Pseudohynobius. In modern hynobiids, the proportionally shortest palatal process is found in Onychodactylus (FMNH 16095, 285321; Smirnov & Vassilieva, 2002), with its distal tip closely approaching the posterior border of the orbit; whereas the longest palatal process is found in Batrachuperus (Jiang et al., 2018), Pachyhynobius (Clemen & Greven, 2009) and some species of Hynobius (Sato, 1943), in which the distal tip of the palatal process surpasses the midlevel of the orbit. The palatal process in all species of Pseudohynobius has a shallow groove dorsally to hold the cartilaginous pterygoid process of the palatoquadrate as in other hynobiids (Jiang et al., 2018; Jia et al., 2019). The otic process of the pterygoid is merely a stub, with its anterior border curled dorsally to form a facet articulating with the ascending process of the palatoquadrate. CT images show that the otic process neither articulates with the parasphenoid nor the prootic. The quadrate process is a flat bony plate with its medial border curled dorsally to articulate with the quadrate (Figure 8).

FIGURE 8.

Micro‐CT rendered reconstruction of pterygoid (blue) and ossified portion of the ascending process of palatoquadrate (chartreuse), if any, of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Right pterygoid and ossified portion of the ascending process of palatoquadrate, if any, in dorsal (upper) and ventral (lower) views. app, ascending process of palatoquadrate; pt, pterygoid

The ascending process of the palatoquadrate (metapterygoid of Trueb, 1993 or epipterygoid of Rose, 2003) is retained as a cartilaginous strut between the medial process of the pterygoid and the quadrate process of the parietal in most specimens, but is partially ossified in several large individuals (TL ≥ 170 mm) of Ps. flavomaculatus (CIB 17342–17344, 17594), Ps. jinfo (CIB 85290) and Ps. shuichengensis (LTHC 9460084). It ossifies only at one side of the skull in all above specimens except CIB 17344 (Ps. flavomaculatus), CIB 85290 (Ps. jinfo) and LTHC 9460084 (Ps. shuichengensis), in which the ascending process of the palatoquadrate ossifies bilaterally as a short columnar bone that lies almost upright at the posteromedial corner of the orbit (Figures 2 and 8; Figure S1). In Cryptobranchoidea, such a bony pillar was found in several species of Batrachuperus (Regel, 1970; Jiang et al., 2018; Jia et al., 2019), Onychodactylus fischeri (Regel, 1970), Paradactylodon mustersi (FMNH 211936), some species of Hynobius (Carroll & Holmes, 1980) and Ranodon (Regel, 1970), but it was largely replaced as a ligament in Salamandrella keyserlingii and several salamandrids, ambystomatids or proteids (Regel, 1970), and absent in cryptobranchids (e.g. Reese, 1906; Wu, 1982), with the condition of other hynobiids awaiting to be investigated.

3.4. Palate and braincase

The palate consists of the vomer, anterior part of the parasphenoid and the pars palatina of both the premaxilla and maxilla (Figure 3). The palatine fails to ossify as a separate element, as commonly seen in other hynobiids. Exceptions in Hynobiidae have been known for several neotenic specimens of Batrachuperus londongensis, in which the palatine is a slender, toothless, bony rod set between the vomer and pterygoid (Jiang et al., 2018). The anteromedial fenestra is a large oval‐shaped opening bordered by the partes palatina of the premaxillae and the vomers. The anteromedial fenestra is present in all hynobiids but is absent in cryptobranchids as a derived feature (Reese, 1906; Wu, 1982; Fei & Ye, 2016; Jia & Gao, 2016a).

The paired vomers are irregular in shape, slightly longer than wide (Figures 3 and 9a–e). The vomers are widely separated by the anteromedial fenestra, and posteriorly approach to each other with a narrow fissure in between. The vomer has a broad plate anteriorly and a constricted posterior part that bears a row of vomerine teeth. The bony plate is perforated by two or three tiny foramina along its anterolateral rim to transmit the ramus ventralis of the trigeminal nerve (CN V₁) and the ramus palatinus of the facial nerve (CN VII). Posterolaterally, the vomer is deeply notched for the choana as in other hynobiids. Interestingly, the vomer has a tiny subtriangular process at the anterior edge of the choanal notch in Ps. flavomaculatus and Ps. guizhouensis, whereas such a process is absent in other species (Figure 9b,d,e; Ps. jinfo, Ps. kuankuoshuiensis and Ps. shuichengensis). The retrochoanal process of the vomer is poorly developed in all species of Pseudohynobius, with a short bony process carrying few teeth posterior to the choana. The vomerine tooth row bows anteriorly, with the inner and outer branch forming an almost right angle in all species of Pseudohynobius except Ps. shuichengensis, in which the angle is about 120°. The tooth rows on two vomers closely approach to but not contact each other medially (see below). The posterior process of the vomer is short and triangular in shape. The posterior process is proportionately shortest in Ps. kuankuoshuiensis, and longest in Ps. jinfo, with that of Ps. shuichengensis slightly longer than that of Ps. flavomaculatus and Ps. guizhouensis.

FIGURE 9.

Micro‐CT rendered reconstruction of vomer and parasphenoid of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Left vomer in ventral (upper) and dorsal (lower) views, (f–j) parasphenoid in ventral (upper), dorsal (middle) and left lateral (lower) views. cfpp, cultriform process of parasphenoid; icf, internal carotid foramen; lap, lateral ala of parasphenoid

The parasphenoid is a median element that dominates the ventral aspect of the skull (Figures 3 and 9f–j). The anteriorly projecting cultriform process is roughly rectangular with a slightly constricted distal end contacting the vomers. The lateral borders of the cultriform process parallel each other, curving dorsally to articulate with the orbitosphenoid; therefore, the parasphenoid is convex ventrally and concave dorsally (Figure 9). The base of the cultriform process is penetrated by a pair of internal carotid foramina in Ps. flavomaculatus, Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis, for the passage of the internal carotid arteries as in other hynobiids. However, the internal carotid foramen is present only on the left side of the parasphenoid in the holotype of Ps. jinfo (Figures 3b and 9g; CIB 85290), and is absent in the paratype (CIB 85291) of Ps. jinfo, in which the internal carotid artery may passes through the fissure between parasphenoid and the otic capsule as in Salamandra (Francis, 1934). The lateral wings (alae) of the parasphenoid are short, subtriangular and articulate dorsally with both the prootic and opisthotic‐exoccipital complex to floor the braincase and otic capsule. Like the lateral ala, the posterior process is short and subtriangular; it articulates with the opisthotic‐exoccipital complex to form the floor of the braincase and the ventral border of the foramen magnum.

The orbitosphenoids (orbitotemporal of Duellman & Trueb, 1986 or sphenethmoid of Trueb, 1993) are paired elements flanking the anterolateral wall of the braincase (Figure 4). The orbitosphenoid is convex laterally and concave medially (Figure 10a–e). The dorsal border of the orbitosphenoid is convex in lateral view, and dorsally articulates with the frontal and parietal, but the ventral border is straight. CT images show that each of the orbitosphenoid has a small anterovental process projecting medially to overlap the parasphenoid, but failed to meet with the contralateral element (Figure 10f–j). Such an anteroventral process is found in all hynobiid taxa investigated in this study (Table 3; e.g. Batrachuperus, Liua, Paradactylodon, Pseudohynobius), but is absent in cryptobranchids (Reese, 1906) and any of the known fossil stem hynobiids (Jia & Gao, 2016a, 2019). The anterior border of the orbitosphenoid is slightly notched for the orbitonasal fenestra for the passage of the deep ophthalmic branch of the trigeminal nerve (CN V₁), whereas the posterior border has a deep ‘U’‐shaped notch, within which opens the optic foramen (optic fenestra of Francis, 1934) to conduct the optic nerve (CN II) and the central retinal artery (Schmalhausen, 1968). In the holotype of Ps. guizhouensis, however, the foramen opticum opens at the posterior border of the left orbitosphenoid, but opens within the right orbitosphenoid as a variant condition (Figure 4c). The posteriorly notched orbitosphenoid was recognized as a diagnostic feature of hynobiids (Jia & Gao, 2016a, 2019; Jiang et al., 2018). As in other hynobiids, the oculomotor nerve, orbital artery and pituitary vein pass through the cartilaginous side‐wall of the braincase between the pila metoptica and pila antotica as seen in the hynobiid Onychodactylus (Ryke, 1950; Schmalhausen, 1968).

FIGURE 10.

Micro‐CT rendered reconstruction of orbitosphenoid of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Left orbitosphenoid in lateral (upper) and medial (middle) views, (f–j) orbitosphenoid in dorsal (left) and ventral (right) views. apo, anteroventral process of orbitosphenoid; fop, foramen opticum; obnf, orbitonasal foramen

The prootic is a paired endochondral bone, housing the anterior part of the membranous labyrinth (Figures 2, 3, 4). The prootic remains as an independent bone in all species of Pseudohynobius except Ps. jinfo, in which the prootic fuses with the opisthotic and exoccipital to form a prootic‐opisthotic‐exoccipital complex (Figure 11; POE complex). In dorsal view, the prootic or the prootic portion of the POE complex is bulbous with a strong swelling (the prominentia semicircularis anterioris of Villa et al., 2014) of the dorsal surface that houses the anterior semicircular canal. The prominence runs diagonally from the anterolateral corner to the position where the posterolateral process of the parietal contacting the prootic (Figures 2 and 11). Lateral to the anterior end of the prominentia semicircularis anterioris, the prootic bears a short, knob‐like process which has a blunt and round end projecting laterally. With the prominentia semicircularis anterioris, the lateral process defines a small concave area that accepts the otic process of the squamosal (Figures 2 and 11). In posterior view, the lateral semicircular canal in Ps. shuichengensis is higher than that in other congeneric species of Pseudohynobius, therefore the prootic is much more bulging posterior to the lateral process in the former than others (Figure 11). Medial to the prominentia semicircularis anterioris, the prootic has a flat, subtriangular anterior process that extends anteromedially along the anterodorsal edge of the prootic. The anterior process and the prominentia semicircularis anterioris together define a broad notch, which accepts the posterior part of the descending lateral border of the parietal. Ventral to the anterior process, the prootic bears a process along its anteroventral edge (Figure 11). This ventral projection is distinctively elongate in Ps. kuankuoshuiensis, whereas it is wide and subtriangular in the remaining species of Pseudohynobius. In ventral view, posterolateral to the ventral process, the prootic has a short, robust basal process, which is penetrated medially by the foramen palatinum and laterally by the foramen faciale. The foramen palatinum opens at the junction between the ventral and basal processes to conduct the palatine branch of the facial nerve (CN VII). This foramen is clearly visible in ventral view of the skull, unlike in all other hynobiids and the cryptobranchid Andrias japonicus (FMNH 60644), in which the foramen is completely covered by the parasphenoid. The foramen faciale opens laterally at the base of the basal process in Ps. flavomaculatus, Ps. jinfo, Ps. guizhouensis and Ps. kuankuoshuiensis, serving as the passageway of the hyomandibular branch of the facial nerve (CN VII). CT images reveal that these two foramina are connected via a short prootic canal in all species except Ps. shuichengensis, in which the connection is made via a transverse sulcus on the ventral surface of the prootic rather than a true canal (Figure 11). In lateral view, the anterior end of the prootic (or prootic portion of the POE complex) has a wide sulcus (sulcus petrosum lateralis of Villa et al., 2014) between the basal process and the anterior end of the prominentia semicircularis anterioris, for passage of the trigeminal and facial nerves (CNs V and VII), A. petrosa lateralis (Francis, 1934; see below) and a distinct notch at the posterior margin, which defines the prootic contribution to the border of the fenestra ovalis (Figure 11). In anterior view, the sulcus petrosum lateralis in the left prootic of the paratype (LTHC 9460084) of Ps. shuichengensis is so wide that it captures the bony ascending process of the palatoquadrate (Figure 4e; Figure S1E). Medial to this sulcus, the anterior and ventral processes define the large foramen prooticum (Figure 11). This opening house the Gasserian ganglion of the trigeminal nerve (CN V) as in Salamandrella (Francis, 1934). The medial wall of the prootic is poorly ossified, revealing the large vestibule of the inner ear; the wall is penetrated by one large and two small well‐defined foramina (foramina acustica of Ryke, 1950). The former conveys the facial nerve (CN VII), whereas the latter two reflect the path of the ramus anterior, medial and posterior of acustic nerve as in Onychodactylus (Ryke, 1950). CT images show that this large foramen leads a short canal, which connects both with the foramen palatinum and foramen faciale.

FIGURE 11.

Micro‐CT rendered reconstruction of prootic of four species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype); and the prootic‐opisthotic‐exoccipital complex of Ps. jinfo (CIB 85290, the holotype). (a–d) Left prootic and (e) left prootic‐opisthotic‐exoccipital complex in the following views from top to bottom: dorsal, ventral, lateral, medial, anterior and posterior. atpp, anterior process of prootic; auf, foramen acustica; bpp, basal process of prootic; enlf, foramen endolymphaticum; ff, foramen faciale; fnov, fenestra ovalis; fpro, foramen prooticum; lpp, lateral process of prootic; pf, foramen palatinum; prlf, foramen perilymphaticum; psa, prominentia semicircularis anterioris; psof, foramen post‐oticum; vpp, ventral process of prootic

The opisthotic‐exoccipital complex is a compound element representing the fusion of two endochondral bones—the opisthotic, which ossifies in the posterior half of the otic capsule, and the exoccipital, which ossifies in the occipital arch. This complex articulates with the prootic in all species of Pseudohynobius, with the exception of Ps. jinfo, in which the complex is further fused with the prootic (Figures 2, 3, 4, 11 and 12). In other hynobiids, a separate prootic with a fused opisthotic‐exoccipital complex prevails as a common pattern, whereas a complete fusion of the prootic, opisthotic and exoccipital was previously only reported in Onychodactylus (Smirnov & Vassilieva, 2002). In dorsal view, the prominentia semicircularis posterioris is clearly visible as a raised swelling in Ps. flavomaculatus, Ps. guizhouensis, Ps. jinfo and Ps. kuankuoshuiensis, extending perpendicular to the prominentia semicircularis anterioris. However, the same prominence is less distinct in Ps. shuichengensis, because the lateral semicircular canal is higher than that of other Pseudohynobius species when observed posteriorly (Figures 11 and 12).

FIGURE 12.

Micro‐CT rendered reconstruction of opisthotic‐exoccipital complex of four species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype); and the stapes of five species of Pseudohynobius in the following order from top to bottom: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, the paratype). (a–d) Left opisthotic‐exoccipital complex in the following views from top to bottom: dorsal, ventral, medial, anterior and posterolateral, (e–i) left stapes in lateral (left) and dorsal (right) views. fnov, fenestra ovalis; ocp, occipital condyle; prlf, foramen perilymphaticum; psp, prominentia semicircularis posterioris; psof, foramen post‐oticum; ts, tectum synoticum

Medial to the prominentia semicircularis posterioris, the tectum synoticum is bilaterally ossified and fused with the opisthotic‐exoccipital complex. In dorsal and anterior views, the ossified part of the tectum is flat and subtriangular, articulating with the parietal dorsally. Ventrally, the contralateral exoccipitals are in articulation in Ps. jinfo, but are separated in the other species (Figure S1). The opisthotic‐exoccipital complex is notched anteriorly as the posterior border of the fenestra ovalis. It is also penetrated at the base of the occipital condyle by the foramen post‐oticum (foramen jugulare of Gaupp, 1911; or foramen metoticum of Stadtmüller, 1936), which transmits the glossopharyngeus‐vagus nerves (CNs IX + X) and the vena cranialis post‐otica (Francis, 1934) out of the metotic fissure. The occipital condyle is short but stout, with an oval‐shaped facet facing posteromedially. In medial view, the ventral margin of the opisthotic‐exoccipital complex in Ps. flavomaculatus, Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis is penetrated by the foramen perilymphaticum, which conducts the perilymphatic duct into the vestibule of the inner ear as in the hynobiid Onychodactylus (Ryke, 1950). Similar location of this foramen is found in the medial aspect of the POE complex in Ps. jinfo, except that the latter species is further penetrated by the foramen endolymphaticum lying dorsal to the foramen auditoria, to conduct the endolymphatic duct of the inner ear as in Onychodactylus (Ryke, 1950). In Ps. flavomaculatus, Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis, however, the foramen endolymphaticum perforates the cartilaginous plate between the prootic and the opisthotic‐exoccipital complex. In addition, a large cavity opens posterior to the foramen endolymphaticum, which, as revealed by CT scanning, connects with the foramen post‐oticum by a short canal inside the opisthotic‐exoccipital complex as seen in all species of Pseudohynobius.

The stapes consists of an oval‐shaped footplate with a short stylus (Figures 4 and 12e–i; Figure S1). The footplate of the stapes is anteroposteriorly elongated, covering the fenestra ovalis, with all except part of its ventral margin articulating with the braincase. The stylus of the stapes is a short stub, with a blunt distal end serving for the attachment of the ligamentum squamoso‐columellare as seen in other salamanders (Kingsbury & Reed, 1909). In lateral view, the stylus stems from the middle part of the footplate and is perpendicular to the long axis of the latter (Figure 12e–i). The stapedial foramen is absent in all species of Pseudohynobius, but is found as perforating the base of the stylus in some but not all specimens of several other hynobiids, including Ranodon, Salamandrella and Batrachuperus (Schmalhausen, 1968; Jiang et al., 2018). In the holotype of Ps. jinfo, the footplate of the left stapes has a rounded notch at the middle part of the dorsal border, whereas the footplate on the right side remains as a normal oval‐shaped plate as seen in other species of Pseudohynobius (Figure 12f).

3.5. Mandible

In Pseudohynobius, bony components of the mandible include the dentary, prearticular and the angular (Figure 13). The mentomeckelian is an endochondral bone and fuses with the dentary as a mental process of the latter at the jaw symphysis.

FIGURE 13.

Micro‐CT rendered reconstruction of lower jaw of five species of Pseudohynobius in the following order from top to bottom: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). Left lower jaw in (a) dorsal, (b) ventral, (c) medial, (d) lateral views. an, angular; anf, angular foramen; den, dentary; idf, inferior dental foramen; mf, mental foramen; pra, prearticular

The dentary is elongated, covering almost the whole lateral and ventral aspect of the lower jaw (Figure 13). The dentary bears a tooth row extending from the jaw symphysis posteriorly to a level at the anterior edge of the coronoid process of the prearticular (see below). Below the tooth row, the dentary has a short, ledge‐like bony lamina extending medially. The bony lamina starts from the jaw symphysis, gradually raises its height posteriorly and merges with the dorsal margin of the dentary immediately posterior to the tooth row (Figure 14a–e). In medial view, another bony ridge extends as a short, horizontal crest close to the posterior end of the dentary, with the ventral surface of this crest articulating with the angular (Figures 13 and 14). The lateral surface of the dentary is smooth, but is penetrated by two to four mental foramina that conduct external branches of the mandibular nerve (CN V₃). The mental foramina distribute mainly within the anterior half of the dentary in Ps. flavomaculatus, Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis, whereas the posterior most mental foramen in the holotype (CIB 85290) of Ps. jinfo is located close to the posterior end of the tooth row (Figures 13 and 14).

FIGURE 14.

Micro‐CT rendered reconstruction of dentary, prearticular and angular of five species of Pseudohynobius in the following order from left to right: Ps. flavomaculatus (CIB 17342, the holotype), Ps. jinfo (CIB 85290, the holotype), Ps. guizhouensis (LTHC 0711005, the holotype), Ps. kuankuoshuiensis (ZMC 7504023, the holotype) and Ps. shuichengensis (LTHC 9460084, a paratype). (a–e) Left dentary in the following views from top to bottom: lateral, medial, dorsal and ventral, (f–j) left prearticular in the following views from top to bottom: lateral, medial, dorsal and ventral, (k–o) left angular in lateral (upper) and medial (lower) views. anf, angular foramen; idf, inferior dental foramen; mf, mental foramen

The prearticular is an elongate structure that covers about two thirds the medial aspect of the mandible, extending from the craniomandibular joint roughly to the midlevel of the dentary teeth (Figure 13a,c). The anterior process of the prearticular is slender distally, but it expands proximally into a large coronoid process. The coronoid process ascends from the midlevel of the prearticular, with the dorsal border curled medially, and thus providing a concave facet facing dorsolaterally, onto which the M. adductor mandibulae internus inserts (Carroll & Holmes, 1980). At the base of the coronoid process, the prearticular is obliquely penetrated by the slit‐like inferior dental foramen on the medial side (Figure 13b,c). This foramen connects with a short canal inside the prearticular, which in turn merges with a shallow groove on the lateral surface of the prearticular. These structures collectively transmit the inferior alveolar ramus of the facial nerve (CN VII) and the alveolar artery as seen in other hynobiids (Jiang et al., 2018). Anteroventral to the inferior dental foramen, the prearticular is notched at the ventral edge by the angular foramen (mandibular foramen of Wilder, 1903) for the passage of the mylohyoid branch of the profundus nerve (CN V₃).

The angular is a straight, dagger‐like bone posteroventrally set between the dentary and the prearticular (Figure 13). The angular is about half the length of the prearticular, its greatest exposure is in ventral view and its anterior‐most length is concealed by the surrounding elements. In lateral view, the angular develops a shallow groove below the dorsal border to articulate with the ventral edge of the dentary (Figure 14k–o). The anterior end of the angular pinches out, whereas the posterior end is expanded—in several specimens of Ps. flavomaculatus (CIB 17342–17344, 17592, 17598), Ps. jinfo (CIB 82590) and Ps. kuankuoshuiensis (ZMC 7604025) the latter even has a hook‐like, anterodorsally directed process (Figure 14k,l). This hook‐like process is an unusual structure within hynobiids, and indicates that the articular fuses with the angular as documented in few hynobiids (Hynobius nebulosus and Onychodactylus japonicus) and cryptobranchids (Stadtmüller, 1936; Rose, 2003). Whereas the articular in Ps. guizhouensis, Ps. shuichengensis and other specimens of the remaining species of Pseudohynobius remains cartilaginous, because the hook‐like process is absent in the angular. The articular is present as a bony element in most extant and fossil hynobiids, but is present as cartilaginous in Liua tsinpaensis (CIB 18349; Xiong et al., 2016; see below).

3.6. Dentition

As in other hynobiids, tooth‐bearing elements include the premaxilla, maxilla, dentary and vomer, with each of them bearing a single row of teeth. Both the marginal teeth in the upper and lower jaws and the palatal teeth in the vomer are pedicellate with a weak ossification area separating the pedicel and the tooth crown; all of the tooth crowns are bicuspid.

The premaxillary teeth are of uniform size, whereas the maxillary teeth gradually decrease in size posteriorly, with the last tooth merely half the height of the first one. These upper marginal teeth are closely aligned along the lingual aspect of the pars dentalis of the maxilla and premaxilla, forming a tooth row that follows the curvature of the upper jaw. Both the premaxillary and maxillary teeth are proportionately taller in Ps. flavomaculatus and Ps. jinfo than that in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis (Figure 5a,b). The premaxilla and maxilla on each side of the skull have 13–17 and 22–30 teeth, respectively, in Ps. flavomaculatus, 16–18 and 30–32 teeth in Ps. guizhouensis, 15–17 and 23–26 teeth in Ps. jinfo, 14–17 and 26–32 teeth in Ps. kuankuoshuiensis, and 17–20 and 33–39 teeth in Ps. shuichengensis. Similar to the maxillary teeth, the dentary teeth decrease in size posteriorly. However, the dentary tooth row exceeds that of the upper jaw by additional of four to five teeth. The dentary on each side of the mandible carries 42–49 teeth in Ps. flavomaculatus, 46–48 teeth in Ps. guizhouensis, 41–44 teeth in Ps. jinfo, 42–45 teeth in Ps. kuankuoshuiensis, and 51–56 teeth in Ps. shuichengensis.

The right and left vomerine tooth rows collectively form a flattened ‘M’ in palatal view (Figure 3). Each tooth row bows anteriorly, terminating medially near the vomer‐parasphenoid contact. The forward‐most point of the arch lies along a transverse plane that approximates the anterior margin of the choanae. The medial branch of the tooth row arch is slightly longer than the lateral branch, with three or four more teeth in the former. One exception within Pseudohynobius exists as an asymmetric variation in the right vomer of the holotype (CIB 85290) of Ps. jinfo, in which the lateral branch (eight teeth) is slightly longer than the medial branch (five teeth). Each vomer has a total of 11–15 teeth in Ps. flavomaculatus, 14–16 teeth in Ps. guizhouensis, 10–13 teeth in Ps. jinfo, 11–14 teeth in Ps. kuankuoshuiensis, and 14–15 teeth in Ps. shuichengensis. The vomerine tooth counts of the five species of Pseudohynobius (10–16) are larger than that of closely related hynobiids: L. shihi (8–9 of Zhang, 1985; 4–7 of Xiong et al., 2016), L. tsinpaensis (7–10 of Fei & Ye, 2016; 6–9 of Xiong et al., 2016), and species of Batrachuperus (4–6 in B. tibetanus and B. pinchonii of Fei et al., 2006; 4–8 in B. londongensis of Jiang et al., 2018; 4–6 in B. yenyuanensis of Jia et al., 2019).

3.7. Hyobranchial apparatus

The hyobranchium are largely cartilaginous with basibranchial II and paired hypobranchial II and ceratobranchial II ossified as a standard pattern in Pseudohynobius (Figure 15). Deviations from this pattern are only found in few specimens of few species where hypobranchial I and ceratohyal are partially ossified (see below).

FIGURE 15.

Micro‐CT rendered reconstruction of the ossified hyobranchium of the five species of Pseudohynobius in ventral view: (a) Ps. flavomaculatus (CIB 17342, the holotype), (b) Ps. jinfo (CIB 85290, the holotype), (c) Ps. guizhouensis (LTHC 0711005, the holotype), (d) Ps. kuankuoshuiensis (ZMC 7504023, the holotype), (e) Ps. shuichengensis (LTHC 9460084, a paratype). bb, basibranchial; cb, ceratobranchial; hb, hypobranchial

Basibranchial II is the most posteroventral component in the hyobranchium, and has no direct contact with any other hyobranchial elements. The basibranchial II is a transverse bar in Ps. flavomaculatus and Ps. kuankuoshuiensis. It keeps this form in Ps. jinfo and Ps. guizhouensis, but bears a stub‐like anterior process in the middle of the transverse bar. In Ps. shuichengensis, however, the basibranchial II is flattened, broad inverted ‘V’‐shaped, and is less than half the length of hypobranchial II. The basibranchial II is proportionally longer in Ps. flavomaculatus than others by having a similar length to or slightly shorter than the ossified portion of the hypobranchial II, whereas it is less than half the length of hypobranchial II in other congeneric species.

The hypobranchial I remains cartilaginous in all species except found as partially ossified in a specimen of Ps. kuankuoshuiensis (ZMC 8208010), where the hypobranchial I is a rectangular, thin and spatulate bone lying anterolateral to hypobranchial II. The ossified portion of the hypobranchial I is straight with both ends having a shallow groove to articulate with cartilaginous part of hypobranchial I. The hypobranchial I is wider but slightly shorter than hypobranchial II. Ossification of hypobranchial I has not been found in most stem and extant hynobiids, with only few exceptions documented in Pachyhynobius shangchengensis (Clemen & Greven, 2009), and some large neotenic individuals of Batrachuperus londongensis (Jiang et al., 2018).

Both hypobranchial II and ceratobranchial II are paired rod‐like bones, with the former articulating with the latter posterolaterally (Figure 15). The hypobranchial II arches posteromedially and has both ends expanded and a constricted shaft, whereas the ceratobranchial II bows anterolaterally and tapers posteriorly without constriction in the shaft. The posterior end of the ceratobranchial II is constricted with a diameter that is about half the size as the anterior end. The hypobranchial II is in similar length with the ceratobranchial II. Separated ossification of hypobranchial II and ceratobranchial II is seen in all extant and extinct hynobiids.

The ceratohyal remains cartilaginous in most specimens of Pseudohynobius, but with its distal end partially ossified as an intraspecific variation in a few specimens of Ps. flavomaculatus (CIB 17343, 17593, 17598) and Ps. jinfo (CIB 85290). However, ossification of ceratohyal in these two species should not be deemed solely as a late‐stage ontogenetic transformation, but rather a true individual variation. This is because the variable ossification occurs in both small (TL < 161 mm; CIB 17593, 17598) and large (TL > 178 mm; CIB 17343, 85290) specimens, and even the extent of ceratohyal ossification is not obviously correlated with body size.

4. DISCUSSION

The above morphological description and previous studies on Batrachuperus (Jiang et al., 2018; Jia et al., 2019) enable us to conduct comparisons on the cranial morphology among these five species of Pseudohynobius, and provide discussions on several characters in the cranium that are informative in terrestrial adaptation and evolution in Hynobiidae.

4.1. Osteological comparison of the cranium in five species of Pseudohynobius

Among the five species of Pseudohynobius, Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis are distributed within Guizhou Province, whereas both Ps. jinfo and Ps. flavomaculatus are distributed outside of Guizhou Province, with the former living in Chongqing Municipality and the latter living in Chongqing Municipality, and Hunan and Hubei provinces (Fei & Ye, 2016). Interestingly, species living inside and outside of Guizhou Province were previously recovered as two different subclades, respectively (Figure 1; Yang et al., 2013; Zhao et al., 2016). Our study reveal that members within each of these two subclades share several similar features: (1) the pars facialis of maxilla articulates both with the lacrimal and prefrontal in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis; but only articulates with the lacrimal in Ps. flavomaculatus and Ps. jinfo; (2) the lacrimal enters the external naris but not the orbit in Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis; but contributes to both margins in Ps. flavomaculatus and Ps. jinfo; (3) the premaxillary and maxillary teeth are longer in proportion to the upper jaws in Ps. flavomaculatus and Ps. jinfo than that of Ps. guizhouensis, Ps. kuankuoshuiensis and Ps. shuichengensis; (4) the posterior process of the quadrate is elongate in Ps. flavomaculatus and Ps. jinfo but swollen as a short bump in the other three taxa. The second feature had been recognized by Jia and Gao (2016b), who pointed out that lacrimal enters into the external naris only as a primitive feature in hynobiids, but phylogenetic signals of the remaining characters remain to be tested in future cladistic analyses.

Species across these two clades share nine features: (1) anterodorsal fenestra longest in Ps. flavomaculatus, but shortest in Ps. shuichengensis; with that in the remaining three species being moderate by terminating posteriorly at the level corresponding to the septomaxilla; (2) foramen lateral nasi absent in the maxilla of Ps. flavomaculatus and Ps. guizhouensis but present in the other species; (3) frontoparietal fontanelle absent in Ps. kuankuoshuiensis and Ps. shuichengensis, but present in Ps. flavomaculatus, Ps. guizhouensis and Ps. jinfo; (4) squamosal/parietal contact present in Ps. guizhouensis, Ps. jinfo and few specimens of Ps. flavomaculatus (CIB 17342, 17597) but absent in Ps. kuankuoshuiensis, Ps. shuichengensis and most specimens of Ps. flavomaculatus; (5) medial border of the proximal head of the squamosal is convex in Ps. jinfo and Ps. shuichengensis, concave in Ps. flavomaculatus and Ps. kuankuoshuiensis, and straight in Ps. guizhouensis; (6) quadrate foramen pierces the anterolateral process of the quadrate in Ps. flavomaculatus and Ps. shuichengensis but is absent in Ps. guizhouensis, Ps. jinfo or Ps. kuankuoshuiensis; (7) ascending process of the palatoquadrate is partially ossified in Ps. flavomaculatus, Ps. jinfo and Ps. shuichengensis, but remain cartilaginous in other two species; (8) vomer bears a tiny subtriangular process at the anterior edge of the choanal notch in Ps. flavomaculatus and Ps. guizhouensis, but such a process is absent in the other species; (9) fusion of the articular with the angular occurs in Ps. flavomaculatus, Ps. jinfo and Ps. kuankuoshuiensis, but not in Ps. guizhouensis and Ps. shuichengensis.

Several unique features are found for Ps. jinfo within the Pseudohynobius: (1) palatal process of the pterygoid is shorter than the quadrate process; (2) internal carotid foramina are absent in the paratype (CIB 85291) and are present only on the left side of the parasphenoid in the holotype (CIB 85290), whereas they consistently characterize the other species; (3) prootic fused with the opisthotic‐exoccipital complex, but remain independent in others; (4) the opisthotic‐exoccipital complex articulates with each other ventrally, but remain separated from each other in others; (5) the mental foramina locate more posteriorly with the most posterior one reaching close to the rear end of the tooth row in the dentary in Ps. jinfo than others. Considering that the available specimens in most species of Pseudohynobius are limited in number, two for Ps. jinfo, one for Ps. guizhouensis, three for Ps. kuankuoshuiensis and two for Ps. shuichengensis (Table 1), both intra‐ and interspecific differences need to be further investigated when larger samples become available. Morphological diagnosis of the genus Pseudohynobius will not be screened until the accomplishment of morphological investigations on the complex Liua‐Protohynobius‐Pseudohynobius.

4.2. Morphological adaptations for terrestrial life in Hynobiidae

Hynobiids were divided into two ecological types at adult stage: terrestrial and aquatic (e.g. Fei et al., 2006). As typical metamorphosed salamanders, species of Pseudohynobius have gilled larvae living in water and terrestrial adults that are often found under thick layers of leaves, roots of bushes or within soil tunnels except the breeding period (Fei et al., 2006; Fei & Ye, 2016). During non‐breeding seasons, several other species are found mainly living on land, including Liua tsinpaensis, and species of Hynobius, Onychodactylus, Protohynobius and Salamandrella (Fei et al., 2006; Fei & Ye, 2016; AmphibiaWeb, 2020). Alternatively, other hynobiids live in water even though they have been going through metamorphosis, including species of Batrachuperus and Paradactylodon, Ranodon sibiricus, Liua shihi and Pachyhynobius shangchengensis (Fei et al., 2006).

Both life history and living habitats have previously been recognized as factors in contributing to the diversity of cranial morphology in different groups of salamanders (e.g. plethodontids of Wake, 1966; salamandrids of Özeti & Wake, 1969; hynobiids of Zhao & Zhang, 1985; Deban & Wake, 2000). Since all extant hynobiids (except the facultative neotenic B. londongensis) are metamorphosed, living habitats (aquatic/terrestrial) play a more direct role in shaping observed morphological diversity. Species of Pseudohynobius feature as having a large rounded snout and weak articulation between neighboring bones in the skull (Figure 2; Figures S1 and S2): the nasals are widely separated from each other by the anterodorsal fenestra; other bones along the midline of the skull roof (frontal, parietal) and the palate (vomer) are separated from their contralateral element by a narrow fissure; the strip‐like prefrontal has a loose contact with the nasal. This configuration of the skull is also found in other terrestrial hynobiids (Onychodactylus of Smirnov & Vassilieva, 2002; Liua tsinpaensis of Xiong et al., 2016), with the extreme condition found in Salamandrella keyserlingii (Lebedkina, 2004), in which the frontal and parietal are separated by a wide frontoparietal fontanelle (see below). Alternatively, in aquatic hynobiids the skull is elongate with squarish snout region as seen in Batrachuperus (e.g. Jiang et al., 2018; Jia et al., 2019) and Liua shihi (e.g. CIB 17600). Moreover CT images show that the bones in the cranium of species of Pseudohynobius are much thinner than that of aquatic hynobiids, in which, as in Liua shihi (CIB 17600) and species of Batrachuperus, the cranial bones are more compact and rigid with thick bony elements tightly articulating with each other (Zhao & Zhang, 1985; Xiong et al., 2016; Jiang et al., 2018; Jia et al., 2019). The loose articulation between cranial bones seen in the species of Pseudohynobius may have no effect on the strength of the whole skull bone as in Salamandrella (Zhou et al., 2017), and provide more flexibility for the dermal skull roof (see below).

Number and arrangement pattern of vomerine teeth are important taxonomic features of Hynobiidae (e.g. Fei et al., 2006; Zhang et al., 2006). As mentioned above, the vomerine teeth of species in Pseudohynobius are larger in number (10–16 teeth on each vomer) than that of aquatic hynobiids: Liua shihi (4–9), species of Batrachuperus (4–8), Pachyhynobius shangchengensis (7–12; Fei et al., 2006; Clemen & Greven, 2009) and Ranodon sibiricus (6–8; Jömann et al., 2005; Fei et al., 2006). The vomerine tooth row in adult individuals of Pseudohynobius species is transversely arranged and lies posteriorly close to the vomer‐parasphenoid suture. A similar pattern of the vomerine tooth row is found in some terrestrial hynobiids: Onychodactylus (Smirnov & Vassilieva, 2002) and Liua tsinpaensis (CIB 18349; Xiong et al., 2016); whereas other terrestrial species, Salamandrella (FMNH 83525, 83526; Lebedkina, 2004) and Hynobius (Sato, 1943), are specialized in having the vomerine tooth row as elongated and curved in the palate. Despite having variable arrangements, the paired vomerine tooth rows in terrestrial hynobiids span most of the transverse dimension of the palate, and therefore would have coordinated with the protrusible tongue commonly seen in terrestrial salamanders for prey manipulation (Regal, 1966; Trueb, 1993). In contrast, the paired vomerine tooth rows in aquatic hynobiids only possess limited part of the transverse dimension of the palate, because they are separated from each other by a large gap in between, and therefore may facilitate the influx of a mix of water and prey when using buccal pumping in water. In these aquatic species, the vomerine tooth rows are short and obliquely arranged in the palate, as seen in Liua shihi (CIB 17600; Fei and Ye, 2016; Xiong et al., 2016), Batrachuperus (Jiang et al., 2018; Jia et al., 2019), Pachyhynobius (CIB 72887, 72891; Clemen & Greven, 2009) and Ranodon (FMNH 83050, 83051; Jömann et al., 2005). The long axis of the vomerine tooth row in these aquatic hynobiids extends either posterolaterally (Ranodon of Jömann et al., 2005; Liua shihi of Xiong et al., 2016; Batrachuperus londongensis of Jiang et al., 2018) or posteromedially (Pachyhynobius of Clemen & Greven, 2009), or resembles that of terrestrial hynobiids in being transversely arranged in different position of the vomer (e.g. Batrachuperus yenyuanensis of Jia et al., 2019).

In hyobranchial apparatus, the ceratohyal remains cartilaginous as a standard pattern in Pseudohynobius. Similar to Pseudohynobius, the ceratohyal is not ossified in most terrestrial hynobiids that have been documented, including Hynobius formosanus (Vassilieva et al., 2015), Hynobius guabangshanensis (Xiong et al., 2010), Hynobius leechii (Ma & Ma, 1987), Liua tsinpaensis (CIB 18349; Xiong et al., 2016) and Salamandrella keyserlingii (FMNH 83525, 83526). The ceratohyal was reported to be ossified distally in the terrestrial taxon Onychodactylus fischeri (Smirnov & Vassilieva, 2002), but it remains cartilaginous in Onychodactylus japonicus (FMNH 16095, 285321; Vassilieva et al., 2013). A cartilaginous ceratohyal was interpreted as reducing the structural constraints limiting tongue protraction (Xiong et al., 2016), and therefore may increase the feeding ability in terrestrial salamanders. However, the ceratohyal is distally ossified and was reported in all aquatic hynobiids, including Batrachuperus (e.g. Jiang et al., 2018), Pachyhynobius shangchengensis (CIB72887, 72891; Clemen & Greven, 2009, Liua shihi (CIB 17600, 19910401; Xiong et al., 2016), Paradactylodon mustersi (FMNH 211936), Ranodon sibiricus (FMNH 83050, 83051). All stem hynobiids known to date (e.g. Nuominerpeton aquilonaris, Linglongtriton daxishanensis) have cartilaginous ceratohyals, including the neotenic Regalerpeton (Rong, 2018); indicating that the ossification of the ceratohyal represents a derived character that may convergently evolved in aquatic hynobiid lineages, a hypothesis awaits to be tested in future studies.

4.3. Distribution and formation of the frontoparietal fontanelle

The frontoparietal fontanelle (median fontanelle) is an elongate midline opening in the skull roof defined by the paired frontals anteriorly and the parietals posteriorly (Trueb, 1993). Presence or absence of this fontanelle is an important character in taxonomy of hynobiids (Zhao & Hu, 1984). In Pseudohynobius, the fontanelle is absent in Ps. kuankuoshuiensis and Ps. shuichengensis, but is present in Ps. flavomaculatus, Ps. guizhouensis and Ps. jinfo, in which the fontanelle is a narrow fissure occupying <20% of the interorbital width. In Cryptobranchoidea, the frontoparietal fontanelle is closed in all cryptobranchids (Reese, 1906; Wu, 1982), some extant hynobiids: Batrachuperus (Fei & Ye, 2016; Jiang et al., 2018), Hynobius (Fei & Ye, 2016), Liua shihi (Fei & Ye, 2016), Pachyhynobius (Clemen & Greven, 2009) and Paradactylodon (FMNH 211936); and most basal cryptobranchoids. Previous studies show that the fontanelle is present in Liua tsinpaensis (Fei & Ye, 2016), Onychodactylus (Smirnov & Vassilieva, 2002), Protohynobius (Fei & Ye, 2016), Ranodon (Schmalhausen, 1968; Lebedkina, 2004) and Salamandrella (Fei & Ye, 2016). This fontanelle is as narrow as in Pseudohynobius in most of these taxa, but it was retained in Salamandrella as a wide opening with the maximum width of which exceeding two‐thirds of the interorbital width. In basal cryptobranchoids, the fontanelle was reported to be present in Jeholotriton from the Middle Jurassic Haifanggou Formation of Inner Mongolia (Wang & Rose, 2005), and two stem hynobiids: Regalerpeton from the Lower Cretaceous Dabeigou Formation of northern Hebei, China (Rong, 2018) and Linglongtriton from the Upper Jurassic Tiaojishan Formation of Liaoning Province, China (Jia & Gao, 2019). Despite the fontanelle appears in some of the Mesozoic stem hynobiids, a recent study shows that the presence of the frontoparietal fontanelle seems to be a derived feature in Hynobiidae (Jia & Gao, 2019).

In non‐cryptobranchoid salamanders, the frontoparietal fontanelle is absent in most groups, but is present as a derived feature in several genera of Plethodontidae, including Batrachoseps (Marlow et al., 1979), Chiropterotriton (Darda & Wake, 2015), Eurycea (Chippindale et al., 1993), Nototriton (McCranie et al., 1998), Nyctanolis (Elias & Wake, 1983), Oedipina (Sunyer et al., 2010), Pseudoeurycea (Hanken, 1984) and Thorius (Hanken & Wake, 1998). A small frontoparietal fontanelle is present in an adult specimen (PKUP V0605; SKL = 33.29 mm) of a basal salamandroid Beiyanerpeton jianpingensis from the Upper Jurassic Tiaojishan Formation of western Liaoning, China (Gao & Shubin, 2012); but such a fontanelle is absent in a larger specimen (PKUP V0608) of this taxon (SKL = 51.82 mm), indicating that this fontanelle in Beiyanerpeton is closed ontogenetically.

The presence/absence of frontoparietal fontanelle has no strict correlation with habitat preferences of the species, because the fontanelle is present in taxa that are either terrestrial (e.g. Pseudohynobius, Salamandrella, Nyctanolis) or aquatic (e.g. Eurycea, Jeholotriton, Ranodon). In salamanders, the frontoparietal fontanelle is covered by an elastic membrane (or connective tissue), and thus contribute little to the rigidity of skull, providing little protection to the brain dorsally (Hanken, 1984). Presence of the frontoparietal opening was previously explained by Lebedkina (2004) as its assistance in controlling the intracranial pressure under conditions of a rigid, hermetically encapsulated braincase. A recent biomechanical study investigating the feeding abilities of adult specimens of Salamandrella keyserlingii based on 3D Finite Element Analysis, shows that presence of the frontoparietal fontanelle has a trivial effect on the strength of skull bones; but instead provides more flexibility of frontals and parietals with the elastic nature of the connective tissue (Zhou et al., 2017).

Bones surrounding the fontanelle provide attachment sites for the origin of muscles that function in jaw closure: M. adductor mandibulae internus (superficial and deep branches; Francis, 1934). As documented in previous studies, hynobiid taxa that either bear (Ranodon sibiricus of Iordansky, 1990; Salamandrella keyserlingii of Lebedkina, 2004) or lack (Hynobius of Carroll & Holmes, 1980) the frontoparietal fontanelle have no significant differences regarding the arrangement of this muscle. Because the M. superficial adductor mandibulae internus originates from the lateral side on dorsal surface of the parietal and prootic, and the profound adductor mandibulae internus from the dorsal surfaces of frontals and parietals (Luther, 1914).

Ontogenetically, the frontoparietal fontanelles persists as a wide opening during early developmental stages of salamanders, because both frontal and parietal first appear as elongated bony strips along the medial rim of the orbit (Hanken, 1984; Rose, 2003). Typically, this opening closes after metamorphosis as frontals and parietals approaching medially with extensive ossification (Lebedkina, 2004; Darda & Wake, 2015). Retention of the fontanelle in adult individuals of salamanders like most species of Pseudohynobius may stem from the failure of the paired frontals and parietals to abut medially before skeletal growth ceases at sexual maturity (Hanken, 1984; Lebedkina, 2004; Zhou et al., 2017).

4.4. The endochondral orbitosphenoid and its phylogenetic significance

In lateral view, the orbitosphenoid in all five species of Pseudohynobius is a rectangular bone that stands vertically between the lateral borders of the parasphenoid ventrally and the frontals and parietals dorsally; it serves as the anterolateral wall of the braincase. Presence/absence, ossification extent and spatial relationship between orbitosphenoid with optic and oculomotor nerves are phylogenetic informative.

To the best of our knowledge, a bony orbitosphenoid is present in all cryptobranchoids and most extant salamandroids, but is absent in proteids (Wiedersheim, 1877; Wilder, 1903; Gilbert, 1973) and some neotenic species of the plethodontid Eurycea (Wake, 1966; Potter & Sweet, 1981). In the proteid Necturus, stout bony processes descend from the ventrolateral borders of frontals and parietals, and are reinforced externally by the cartilaginous trabecular arch in adults, as a substitute of orbitosphenoid, to form the sidewalls of the braincase (Wilder, 1903). However, in Eurycea rathbuni, the absence of a bony orbitosphenoid was explained to be related to the reduced need for lateral bony support in its flattened skull (Potter & Sweet, 1981). The plethodontid Eurycea tridentifera, a small neotenic salamander, was claimed to lack a bony orbitosphenoid based on three cleared and stained specimens (Wake, 1966). However, later studies (Mitchell & Smith, 1972; Potter & Sweet, 1981) demonstrate that this bone is indeed present in larger specimens of the same taxon, leading to the conclusion that ossification of orbitosphenoid is delayed. In extant salamanders, the orbitosphenoid starts to ossify relatively late during development (Rose, 2003; Lebedkina, 2004), and the timing of its ossification varies across clades of salamanders that metamorphose: before (e.g. Ambystoma texanum of Bonebrake & Brandon, 1971; Salamandrella keyserlingii and Triturus karelinii of Lebedkina, 2004), during (Ranodon sibiricus and Pleurodeles waltl of Lebedkina, 2004), or after (e.g. Onychodactylus japonicus of Smirnov & Vassilieva, 2002; Hynobius formosanus of Vassilieva et al., 2015) metamorphosis. Interestingly, a bony orbitosphenoid was recently found as absent in adult specimens of the basal‐most salamandroid Qinglongtriton gangouensis from the Upper Jurassic Tiaojishan Formation of Hebei Province, China (Jia & Gao, 2016b), indicating that the absence of a bony orbitosphenoid exists in the early evolution of salamandroids, and its disappearance in some proteids and plethodontids is likely derived from convergent evolution.

As an endochondral bone, the orbitosphenoid gradually replaces the cartilaginous lateral wall of the neurocranium during development. The lateral wall of the neurocranium consists of three cartilages, from the anterior to posterior, the preoptic root (or pila praeoptica of Lebedkina, 2004), pila metoptica and pila antotica, with the foramen opticum located between the preoptic root and pila metoptica, and the oculomotor foramen between the pila metoptica and pila antotica (Rose, 2003). The orbitosphenoid first arises as a small bony plate on the anterodorsal surface of the preoptic root of the neurocranium (e.g. Sweet, 1977; Rose, 1995, 2003), in contrast to the conditions of reptiles, in which the orbitosphenoid starts to ossify from the pila metoptica (Romer, 1956). Once the ossification begins, the orbitosphenoid expands anteriorly toward the nasal capsule, ventrally to contact the parasphenoid and posteriorly toward the pila metoptica (e.g. Marconi & Simonetta, 1988; Lebedkina, 2004). Along the posterior expansion, the orbitosphenoid may expand onto the pila metoptica region as seen in several groups of salamanders (e.g. cryptobranchids, some species of plethodontids), in which case the foramen opticum is fully enclosed by the bony orbitosphenoid. In other scenarios, the orbitosphenoid extends further onto or even surpasses the boundary between pila metoptica and pila antotica in other taxa, in the case of which the oculomotor foramen is notched at the posterior margin of the orbitosphenoid (e.g. some species of plethodontids, some species of salamandrids, dicamptodontids, and amphiumids) or is fully encircled by the bony orbitosphenoid (e.g. some species of plethodontids, some salamandrids, and sirenids). Distinctively, in hynobiids, the posterior expansion of orbitosphenoid are restricted within the preoptic root region and does not expand onto the pila metoptica. As a result, the posterior border of the orbitosphenoid is deeply notched for the foramen opticum in hynobiids, and this pattern was recently recognized as a diagnostic feature of crown and stem hynobiids (Jia & Gao, 2016a, 2019).

Based on CT scan, we found that the orbitosphenoid in all species of Pseudohynobius bears a short anteroventral process. This process extends medially and firmly articulates with the parasphenoid, providing a strong support to the nasal capsule. To our knowledge, such an anteroventral process is reported in Batrachuperus yenyuanensis (Jia et al., 2019), but has not been previously documented in any other hynobiid taxa, because the anteroventral process is completely hidden within the braincase, and is not readily visualized under microscope, a traditional way in conducting morphological research on hynobiids. During development, the orbitosphenoid incorporates several structures of the nasal capsule, including the lateral margin of the cartilage infranasalis (internasal planum of Rose, 2003; planum internasale of Lebedkina, 2004), the medial portion of the cartilage antorbitalis (process antorbitalis of Lebedkina, 2004), and the lamina orbitonasalis as present in some taxa (Marconi & Simonetta, 1988). Based on the similar position, the anteroventral process of the orbitosphenoid as observed in specimens of Pseudohynobius indicates that the ossification of the orbitosphenoid likely extends onto the lateral margin of the cartilage infranasalis.

In a previous study, the ossification extent of the anteroventral process of the orbitosphenoid in Batrachuperus yenyuanensis tend to increase during development (Jia et al., 2019). In this study, such an anteroventral process of the orbitosphenoid is found in several extant cryptobranchoids based on μCT scanning of adult specimens available to us, including Pseudohynobius (this study), Liua (CIB 17600, 18349), Ranodon (FMNH 83051), Batrachuperus yenyuanensis (FMNH 49371), Batrachuperus karlschmidti (FMNH 49380), Paradactylodon mustersi (FMNH 211936), Salamandrella keyserlingii (FMNH 83525, 83526), Onychodactylus japonicus (FMNH 16095) and Pachyhynobius (CIB 72887, 72891). However, the anteroventral process is absent in Batrachuperus londongensis (Jiang et al., 2018), Andrias (FMNH 60644), Cryptobranchus (FMNH 33838), and several salamandroids, including Dicamptodon ensatus (Wake, 2001), Dicamptodon tenebrosus (FMNH 59246), Amphiuma tridactylum (FMNH 7181702) and Rhyacotriton variegatus (AmphibiaTree, 2007). Presence of the bony anteroventral process of orbitosphenoid likely represents a potential synapomorphy of crown group hynobiids, because no fossil stem hynobiids have been found with this part ossified (e.g. Jia & Gao, 2019).

4.5. The endochondral articular

The articular is an endochondral bone, as a major component of the craniomandibular joint. In Hynobiidae, a bony articular is present in most extant taxa, and is absent in a few derived hynobiids, including Liua tsinpaensis (CIB 18349) and Protohynobius (Xiong et al., 2011, 2016) and Pseudohynobius (this study). Similarly, a bony articular is present in most stem hynobiids, and is only absent in the recently reported Linglongtriton (Jia & Gao, 2019). Absence of a bony articular in Hynobiidae was previously ascribed to its fusion with the angular (Stadtmüller, 1936; Rose, 2003), and such a statement was confirmed here in several specimens of Ps. flavomaculatus, Ps. jinfo and Ps. kuankuoshuiensis, where the articular is partially ossified and fused with the angular as a hook‐like bony process presents at the posterior end of the latter bone. An independent bony articular is absent in all extant and fossil cryptobranchids (Elwood & Cundall, 1994; Gao & Shubin, 2003) and the stem urodele Karaurus, indicating that an independent bony articular is probably a synapomorphy uniting both the stem and crown Hynobiidae. In Salamandroidea, a bony articular is absent in amphiumids, proteids and plethodontids, and in some members of ambystomatids and salamandrids (Trueb, 1993), and basalmost salamandroids: Beiyanerpeton and Qinglongtriton (Gao & Shubin, 2012; Jia & Gao, 2016a). However, different patterns of the articular fusing with its neighboring bones were recorded: the articular fused with the prearticular in Ambystoma maculatum, Ambystoma macrodactylum, Pseudotriton ruber (Theron, 1952; Papendieck, 1954; Joubert, 1961); with the angular in amphiumids (Erdman & Cundall, 1984); or with both the prearticular and dentary in ambystomatid Ambystoma opacum (Stadtmüller, 1936).

5. CONCLUSIONS

The following conclusions are drawn upon our study on the cranial osteology of the five species of Pseudohynobius: (1) A bone‐by‐bone investigation of five species of Pseudohynobius are provided based on Micro‐CT scan of 18 adult specimens, with the finding of several similar features shared by or across species living outside (Ps. flavomaculatus, Ps. jinfo) or inside (Ps. guizhouensis, Ps. kuankuoshuiensis, Ps. shuichengensis) of Guizhou Province, China. (2) Each of the five species of Pseudohynobius shows a combination of differences in morphology, proportions and articulation patterns in both dermal and endochondral bones; and morphological diagnosis of the genus Pseudohynobius will be provided upon the accomplishment of morphological investigations on the hynobiid complex Liua‐Protohynobius‐Pseudohynobius. (3) Several cranial morphological features indicative of the terrestrial adaptation in Hynobiidae are recognized, including: loose articulations between cranial bones in the dermal skull roof (nasals, frontals, parietals, prefrontal‐nasal contact) and in the palate (vomers); vomerine teeth large in number; vomerine tooth row arranged posteriorly close to the vomer‐parasphenoid suture, spanning the transverse dimension of the palate; and cartilaginous ceratohyal. (4) Presence of a frontoparietal fontanelle in some species of Pseudohynobius stems from the cessation of medial growth of frontal and parietal; but presence or absence of the frontoparietal fontanelle has no indication of terrestrial or aquatic adaptation in hynobiids. (5) The orbitosphenoid in species of Pseudohynobius has an anteroventral process formed from the progressive ossification of the lateral margin of the cartilage infranasalis. Presence of this anteroventral process in orbitosphenoid represents a potential synapomorphy of crown Hynobiidae. (6) The articular fused with the angular as a hook‐like process at the posterior edge of the angular in some specimens of Ps. flavomaculatus, Ps. jinfo and Ps. kuankuoshuiensis. Presence of an independent bony articular is a derived feature in urodeles, and may represent a synapomorphy of the stem and crown Hynobiidae.

CONFLICT OF INTEREST

The authors declare there are no competing interests.

AUTHOR CONTRIBUTIONS

Jia Jia conceived and designed the experiments, analyzed the data, performed the CT scanning and 3D reconstruction, prepared the figures and tables, and wrote and revised drafts of the paper. Ke‐Qin Gao conceived and designed the experiments, analyzed the data, and wrote and revised drafts of the paper. Jian‐ping Jiang conceived and designed the experiments, analyzed the data, and wrote and revised drafts of the paper. Gabriel S. Bever helped interpret the data, and wrote and revised drafts of the paper. Rongchuan Xiong contributed specimens and reviewed drafts of the paper. Gang Wei contributed specimens and reviewed drafts of the paper.

Supporting information

Figure S1‐S2

Jia J, Gao K-Q, Jiang J-P, Bever GS, Xiong R, Wei G. Comparative osteology of the hynobiid complex Liua‐Protohynobius‐Pseudohynobius (Amphibia, Urodela): Ⅰ. Cranial anatomy of Pseudohynobius . J. Anat. 2021;238:219–248. 10.1111/joa.13311

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.