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Restudy of Regalerpeton weichangensis (Amphibia: Urodela) from the Lower Cretaceous of Hebei, China


Abstract and Figures

Regalerpeton weichangensis was established in 2009 on an incomplete skeleton preserved mainly as an impression from the Lower Cretaceous of Hebei, China. However, several anatomical characters were misinterpreted due to distortion of the holotype, and its taxonomic position has been in debate. In this paper, R. weichangensis is redescribed based on eight new specimens and its diagnosis and phylogenetic position are reexamined. This work shows that R. weichangensis was a neotenic form with ossified carpals and tarsals. It has a series of unique combination of characteristics including the vomer with a transverse vomerine tooth row, anterior end of the cultriform process of the parasphenoid indented, basibranchial II triradiate, a long tail exceeded the snout-pelvis length and scapulocoracoid with a rectangular coracoid end. Phylogenetic analysis suggests Regalerpeton, Jeholotriton and Pangerpeton should be placed in the suborder Salamandroidea with three synapomorphies. Moreover, they also share unicapitate ribs with Cryptobranchoidea, which indicates that they represent an important stage of evolution in the Cryptobranchoidea-Salamandroidea split.
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古 脊 椎 动 物 学 报
DOI: 10.19615/j.cnki.1000-3118.170627
Restudy of Regalerpeton weichangensis (Amphibia:
Urodela) from the Lower Cretaceous of Hebei, China
RONG Yu-Fen1,2,3
(1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate
Paleontology and Paleoanthropology, Chinese Academy of Sciences Beijing 100044 rongyufen@ivpp.
(2 CAS Center for Excellence in Life and Paleoenvironment Beijing 100044)
(3 University of Chinese Academy of Sciences Beijing 100049)
Abstract Regalerpeton weichangensis was established in 2009 on an incomplete skeleton
preserved mainly as an impression from the Lower Cretaceous of Hebei, China. However, several
anatomical characters were misinterpreted due to distortion of the holotype, and its taxonomic
position has been in debate. In this paper, R. weichangensis is redescribed based on eight new
specimens and its diagnosis and phylogenetic position are re-examined. This work shows that
R. weichangensis was a neotenic form with ossied carpals and tarsals. It has a series of unique
combination of characteristics including the vomer with a transverse vomerine tooth row,
anterior end of the cultriform process of the parasphenoid indented, basibranchial II triradiate, a
long tail exceeded the snout-pelvis length and scapulocoracoid with a rectangular coracoid end.
Phylogenetic analysis suggests Regalerpeton, Jeholotriton and Pangerpeton should be placed in
the suborder Salamandroidea with three synapomorphies. Moreover, they also share unicapitate
ribs with Cryptobranchoidea, which indicates that they represent an important stage of evolution
in the Cryptobranchoidea-Salamandroidea split.
Key words Weichang, Hebei; Lower Cretaceous; Salamandroidea, Regalerpeton weichangensis;
morphology; phylogeny
Citation Rong Y F, 2018. Restudy of Regalerpeton weichangensis (Amphibia: Urodela) from the
Lower Cretaceous of Hebei, China. Vertebrata PalAsiatica, 56(2): 121–136
1 Introduction
Since the late 1990s, a large number of salamander fossils have successively been
unearthed from Jurassic and Cretaceous strata in northeastern China, and eleven taxa have
been established: Laccotriton subsolanus Gao et al., 1998, Liaoxitriton zhongjiani Dong
&Wang, 1998, Jeholotriton paradoxus Wang, 2000, Sinerpeton fengshanensis Gao & Shubin,
2001, Chunerpeton tianyiensis Gao & Shubin, 2003, Liaoxitriton daohugouensis Wang, 2004,
Pangerpeton sinensis Wang & Evans, 2006, Regalerpeton weichangensis Zhang et al., 2009,
Beiyanerpeton jianpingensis Gao & Shubin, 2012, and Qinglongtriton gangouensis Jia &
56卷 第2
pp. 121–136
gs. 1–8
122 Vertebrata PalAsiatica, Vol. 56, No. 2
Gao, 2016a, Nuominerpeton aquilonaris Jia & Gao, 2016b. These taxa provide a rich source
of information on primitive salamanders, and are significant in understanding the origin of
modern salamander clades. Among these taxa, R. weichangensis was erected on an incomplete
skeleton preserved mainly as impression. This preservation limited our understanding of its
morphology and phylogeny. In the original paper, Regalerpeton was placed as the sister taxon
of Chunerpeton plus living cryptobranchids (Zhang et al., 2009), whereas Skutschas and Gubin
(2012) suggested it as a sister taxon of Hynobiidae.
In this paper, eight new specimens which can be referred to R. weichangensis are
described from northern Hebei Province of China. These better preserved materials permit
a revised diagnosis of the taxon and allow a new phylogenetic analysis to be conducted that
includes most of the fossil taxa from China.
Abbreviations ac, acetabulum; ad. f, anterodorsal fenestra; am. f, anteromedial
fenestra; at, atlas; bb I–II, basibranchial I–II; c, centrale; cb I–II, ceratobranchial I–II; cr vent,
crista ventralis; d, dentary; dc, distal carpal; dt 1–3, distal tarsal 1–3; d 1–5, digit 1–5; e+o,
exoccipital+opisthotic; fe, femur; , bula; b, bulare; fo. i. den, inferior dental foramen; fr,
frontal; gf, gill lament; gr, gill raker; hb I–II, hypobranchial I–II; hu, humerus; i, intermedium;
il, ilium; in. c. a, internal carotid artery; isc, ischium; lac, lacrimal; m, maxilla; n, nasal; os,
orbitosphenoid; p+c, prearticular+coronoid; pa, parietal; ph 2–5, phalanx 2–5; pm, premaxilla;
pm. apr, ascending process of the premaxilla; prf, prefrontal; pro, prootic; ps, parasphenoid; pt,
pterygoid; qua, quadrate; ra, radius; rad, radiale; sa, sacral; sca, scapulocoracoid; sq, squamosal;
st, stapes; ti, tibia; tib, tibiale; ul, ulna, uln; ulnare; vo, vomer; vot, vomerine tooth row.
IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of
Sciences, Beijing.
2 Systematic paleontology
Class Amphibia Linnaeus, 1758
Subclass Lissamphibia Haeckel, 1866
Superorder Caudate Scopoli, 1777
Order Urodela Duméril, 1806
Family Salamandroidea Dunn, 1922
Genus Regalerpeton Zhang et al., 2009
Type species Regalerpeton weichangensis Zhang et al., 2009.
Holotype IVPP V 14391A, B, an incomplete skeleton impression preserved as part and
counterpart on slabs of siltstone.
Referred specimens IVPP V 15677, an incomplete skeleton impression with a nearly
complete skull in dorsal view. V 16776A, B, an incomplete skeleton with a nearly complete
skull and partial postcranial skeleton that shows well-developed anterodorsal and anteromedial
fenestrae. V 16790A, B, an incomplete skeleton with disarticulated bones of the skull that
shows the morphology of the premaxilla, vomer, pterygoid, squamosal and dentary. V 16798A,
Rong - Restudy of Regalerpeton weichangensis
B, an incomplete skeleton impression with impression of the external gills and ossified
carpals. V 17989, a nearly complete skeletal impression with a well-preserved skull in ventral
view. V 16802A, B, an incomplete skeletal impression with ossified carpals. V 23342A, B,
an incomplete skeletal impression lacking part of the tail, with some bones displaced from
their original positions due to lateral compression. V 23343A, B, an incomplete skeleton
with disarticulated bones of the skull that shows the morphology of the premaxilla, maxilla,
prearticular+coronoid, parasphenoid and ilium.
Revised diagnosis Regalerpeton weichangensis, a neotenic salamander, is diagnosed
by the following unique combination of characters: premaxilla with a distinct ascending
process and bearing approximately 25 teeth; slender maxilla without the facial process and
bearing approximately 28 teeth; dentary bearing approximately 40 teeth; nasals without
midline contact; anterodorsal and anteromedial fenestrae present; lacrimal and prefrontal
present; parietal-prefrontal contact absent; squamosal with two proximal expansions; quadrate
ossication present; roughly pentagonal vomer with transversely oriented vomerine tooth row;
parasphenoid inverted arrow-shaped with an indented anterior end of the cultriform process;
internal carotid foramina penetrating parasphenoid; pterygoid triradiate with a vimineous
dentate anterior ramus; paired hypobranchial I and II ossified; basibranchial II triradiate;
prearticular and coronoid fused with two processes; opisthotic and exoccipital fused; stapes
present; three pairs of external gills present with ossied or calcied gill rakers; 16 presacral
vertebrae; atlas with bifid interglenoid tuberosity; trunk vertebrae amphicoelous and ribs
unicapitate; three pairs of free postsacral ribs; a long tail exceeded the snout-pelvis length;
coracoid portion of scapulocoracoid rectangular; ilium spoon-shaped; humerus with crista
ventralis; carpals and tarsals ossified; digit 2 in manus and digit 1 in pes short; phalangeal
formulae 2-2-3-2 in manus and 2-2-3-3-2 in pes.
Locality and horizon The specimens of Regalerpeton weichangensis are from three
different localities: Daobaziliang, Weichang County, Hebei Province, China (the holotype);
Xishunjing village, Weichang County (V 15677, V 16776A, B, V 16790A, B, V 16798A, B,
V 16802A, B, V 17989, V 23342A, B), and Yulinzi village, Weichang County (V 23343A, B).
Dabeigou Formation, Lower Cretaceous (Gao et al., 2013).
Remarks Because of incomplete preservation and distortion of the holotype, several
morphological characteristics are absent and misinterpreted. Further, series of reliable
characteristics can be concluded as below: the pterygoid is triradiate, with a vimineous dentate
anterior ramus. Although the original article is described as two pairs of ceratobranchials due
to poor preservation, it is reinterpreted as two pairs of hypobranchials. The coracoid portion
of scapulocoracoid is rectangular. In the original article, a detailed discussion about the shape
of coracoid portion shows that scapulocoracoid is the identification of the characteristics
of Regalerpeton. The study of new specimens also agrees that conclusion is reliable.
The vomerine tooth row is transversely oriented and vomer is pentagonal. The primitive
interpretation of the orientation is parallel to the maxillary arcade because of distortion of the
124 Vertebrata PalAsiatica, Vol. 56, No. 2
parasphenoid. New study on the holotype indicates that vomer should be roughly pentagonal.
The parasphenoid has prominent cultriform process. The angular is absent in the mandible.
The tarsals are ossied. Based on the comparison of the above characteristics combination, the
new specimen are referred to R. weichangensis.
3 Description
Among the eight new specimens, V 23342 (Figs. 1, 2) is the best preserved. The
following description is based on it unless otherwise noted.
Skull roof The skull roof is composed of the paired nasals, lacrimals, prefrontals,
frontals and parietals, showing no dermal sculpture.
The nasal (Fig. 3A) is a large triangular bone. It contacts the ascending process of
the maxilla anteriorly, frontal posteriorly and prefrontal laterally. It is not connected to the
lacrimal. The nasals are separated by a large anterodorsal fenestra (Fig. 3A, C).
The lacrimal is a small quadrilateral bone (V 16798) that forms the posterior edge of the
narial fenestra and the anterior edge of the orbit. The nasolacrimal ducts are not observed due
to poor preservation.
Fig. 1 Photograph (A) and line drawing (B) of Regalerpeton weichangensis (IVPP V 23342A) in dorsal view
Rong - Restudy of Regalerpeton weichangensis
The prefrontal (Fig. 3) is a cuneate bone. Its anterior border has inverted V-shape, with
the anteromedial side suturing with the nasal and the posterolateral side with the lacrimal.
Medially, it contacts the frontal, but not the parietal.
The frontal (Figs. 1, 3A) is a longitudinal bone with a small anterolateral extension.
Its anterior border completely contacts nasal. Posteriorly, the frontal overlies the parietal
The parietal (Figs. 1, 3A) is also a longitudinal bone. It is slightly shorter than frontal and
lies posterior to the frontal. The parietal is sutured along the posterior midline, but separated
anteriorly by an anteromedial fenestra (Fig. 3A, C). The parietal has a well-developed
anterolateral extension along the posterolateral margin of the frontal, but it fails to extent to the
prefrontal. Posteriorly, parietal has an obvious lateral extension that articulates with squamosal
(Fig. 3A). In lateral view, the parietal is seen to articulate with the orbitosphenoid ventrally.
Palate The oor of the skull consists of two completely separated vomers anteriorly and
one parasphenoid posteriorly.
The vomer (Figs. 3B, 4B) is roughly pen-tagonal with a prominent extension laterally.
Poster-olateral border of vomer has a slightly notch for the choana. There are about 20 teeth
Fig. 2 Photograph (A) and line drawing (B) of Regalerpeton weichangensis (IVPP V 23342B) in ventral view
126 Vertebrata PalAsiatica, Vol. 56, No. 2
(V 17989) in the vomer, they are monostichous
and nonpedicellate. The vomerine tooth row (Figs.
3B, 4B) is transversely oriented and runs from the
midline towards extension laterally.
The parasphenoid (Fig. 4D) is a large arrow-
shape bone. The anterior end of the parasphenoid
is serrated including four processes, the medial two
of which are cuspidal and the lateral two long and
obtuse. The lateral margins of the serrated part have
facets for the vomers. Posteriorly, the parasphenoid
broadens obviously and has a recognized conule of
the both side, for articulating with orbitosphenoids.
There is a groove on each side of the parasphenoid
wing, along which the internal carotid artery runs.
The orbitosphenoid (sphenethmoid) (Fig. 3B) is
approximately cuboid and they ossied to form the
lateral side of the neurocranium.
Suspensorium The suspensorium consists of
the paired pterygoids, squamosals and quadrates.
The pterygoid (Figs. 2, 3) is triradiate, with a
vimineous anteromedial process which is free and
curving and has ten teeth. Its posterolateral process
articulates with squamosal and quadrate. It has a
shorter medial process that points to parasphenoid.
There is no contact between the pterygoid and the
The squamosal (Fig. 3) is a slightly curving
lamellate of bone and presents as a transverse bar
in the posterior skull. It has two proximal ex-
pansions, anterior one smaller than the posterior
one. Ventrally, the squamosal articulates with the
pterygoid and the quadrate (Fig. 3B).
The quadrate (Fig. 3A, B) is a roughly
triangular bone that lies in the ventral surface of the
Fig. 3 Photograph of the impression of the
skull roof of Regalerpeton weichangensis
A. IVPP V 15677; B. V 16776 A;
C. V 16776B. Scale bars equal 5 mm
Otico-occipital region The otico-occipital region of Regalerpeton consists of three
endochondral bones: the prootic, a compound containing opisthotic and exoccipital, and
stapes (columella). Both prootic and opisthotic form the anterior and posterior walls of the
otic capsule respectively. The stapes (Fig. 2) is nail-shaped, with the head forming the round
Rong - Restudy of Regalerpeton weichangensis
footplate and its shaft forming a short stylus. Its footplate covers the lateral wall of the
otic capsule. In Regalerpeton, opisthotic and exoccipital form a compound which expose
completely in the parietal end (Fig. 3A). The opisthotic portion seems to have an obvious
expansion on account of deep impression preserved in V 15677 (Fig. 3A). However, more
information from otico-occipital region is not observed because most specimens are preserved
as impression.
Upper and lower jaws The upper jaw is formed by two dermal bones on each side: the
paired dentate premaxillae medially and the paired dentate maxillae posteriorly.
The premaxilla (Figs. 1, 2, 4A) has a distinct ascending process extending along about
one-third of the premaxilla near the midline, which contacts the anterior part of the nasal (Fig.
3A). The premaxilla bears approximately 25 teeth that are gracile, pointed and nonpedicellate.
The maxilla (Fig. 3B) is a slender bone that articulates with the premaxilla anteriorly. A
Fig. 4 Skeletons and impressions of Regalerpeton weichangensis
Impressions of right premaxilla (A), left ilium (C), parasphenoid (D), left dentary showing articular surface for
prearticular+coronoid (F) and left prearticular+coronoid (G); skeletons of right vomer (B), right dentary (E)
A, C, D from IVPP V 23343A; B, E, F from V 16790B; G from V 23343B
128 Vertebrata PalAsiatica, Vol. 56, No. 2
distinct facial process is easily observable. The maxilla is slightly longer than the premaxilla
and bears approximately 28 teeth which are similar to the premaxillary teeth.
The mandible consists of two distinct dermal bones: the dentary and a compound bone
comprising the prearticular and the coronoid. They surround Meckel’s cartilage.
The dentary (Fig. 4E) bears approximately 40 teeth on the lingual surface of the lower
jaw. The teeth are small, closely packed and nonpedicellate.
The compound of prearticular+coronoid possesses two processes (Figs. 1, 2, 4G). It has
a slender extension that runs along the lingual surface of the lower jaw (Fig. 4G). The inferior
dental foramen (Fig. 4G) is a conspicuous feature near the posterior of the compound bone
and it carried the ramus alveolaris of facial nerve and the alveolar artery (Francis, 1934). The
articular and the angular are not visible.
Hyobranchium The hyobranchium consists of three ossied elements: hypobranchial
I, hypobranchial II and basibranchial II (os thyroideum) (Figs. 2, 3B). Hypobranchials I and
II are paired, parallel to each other, and each of them is a slightly curved strip. The azygous
midline basibranchial II is positioned posterior to other hyobranchial elements and it is
Axial skeleton The vertebral column consists of 16 presacrals including the atlas and
15 trunk vertebrae, one sacral, and about 40 caudal vertebrae.
The atlas lacks free ribs and is shorter than the trunk vertebrae. It has two relatively
elongated transverse processes (Fig. 3A) and a bid interglenoid tuberosity (Figs. 3B, C) that
articulate with the exoccipitals. All the trunk vertebrae are amphicoelous and all the ribs are
unicapitate. The sacral vertebra is larger than the trunk vertebrae, and its ribs are long, thick
and expanded proximally. Regalerpeton has a very long tail that exceeds the snout-pelvis
length. The rst three caudosacrals bear free ribs. The remaining caudal vertebrae lack free
ribs, but bear elongate transverse processes, and distinct neural and haemal arches.
Appendicular skeleton The pectoral girdle lies roughly at the level of the 3rd to 4th
vertebrae. It consists of only one ossied bone: the scapulocoracoid. The coracoid portion is
almost rectangular, and the scapular portion is cuneate.
The humerus is claviform, with expanded proximal and distal portions. In V 23342, the
epiphysis of the humerus is wholly ossied and a crista ventralis is present (Figs. 1, 2, 5). The
radius is a slender and claviform bone, although the distal portion is inated. The ulna, which
is parallel to the radius, is a slightly curved bone that is longer than the radius (Figs. 1, 2, 5).
The carpals are ossied and consist of a radiale, ulnare, intermedium, one centrale, three distal
carpals (Fig. 6). There are four digits in the manus and phalanx 2 is very short (V 16776A, B;
V 17989). The phalangeal formula is 2-2-3-2.
The pelvic girdle has two ossied elements: a pair of ischia ventrally and a pair of ilia
laterally. Each ischium is approximately blade-shaped and the anterior portion is wider than
the posterior one. The ischium joins the cartilagious pubis anteriorly and the ilium laterally to
form the acetabulum for the articulation of the femur. The ilium is a spoon-shaped bone (Fig.
Rong - Restudy of Regalerpeton weichangensis
Fig. 5 Reconstruction of the skeleton of Regalerpeton weichangensis
A. skeleton in dorsal view; B. skull in ventral view; C. mandible in lingual view. Not to scale
4C). Its proximal portion is expanded and forms part of the acetabulum, while the posterior
dorsal portion is slightly curved, and articulates with the sacral rib to attach the pelvic girdle.
The femur (Figs. 1, 2) is a long, straight claviform bone, with the proximal end narrower
than the distal one. The tibia and bula are relative short and straight (Figs. 1, 2). The proximal
portion of the tibia is more expanded than the distal end, whereas the bula is slight curved
and its distal portion is relatively inflated. There are seven ossified elements in the ankle
(Fig. 2). These consist of the tibiale and bulare proximally, intermedium, two centralia, and
130 Vertebrata PalAsiatica, Vol. 56, No. 2
three distal tarsals. There are ve digits in the pes. Digit 1 is relatively short (Fig. 1) and the
phalangeal formula is 2-2-3-3-2. (Fig. 1, 2)
Fig. 6 Regalerpeton weichangensis (IVPP V 16802B)
A. photograph of an incomplete skeletal impression; B. the details of gill rakers;
C. the details of ossied carpals
4 Comparison and discussion
4.1 Vomer and vomerine tooth row
The vomer is one of the important dentate elements on the palate for salamanders. The
vomerine teeth has played important role in the systematics of salamanders (Regal, 1966;
Rose, 2003; Fei and Ye, 2017). In the Cryptobranchidae the vomerine teeth is parallel to
the maxillary arcade (Qiu and Yang, 1986; Rose, 2003). In the Hynobiidae the vomerine
teeth varies in length, shape, location and orientation in different genus-species (Fei and
Ye, 2017). In the Salamandroidea the vomerine teeth have more complex types such as
transverse row along posterior border of the vomer in the Ambystomatidae, teeth covered the
posterior extension of the vomer in the Salamandridae and Plethodontidae, vomerine teeth
row paralleled to the maxillary arcade in the Proteidae and Amphiumidae (Regal, 1966; Rose,
2003; Darda and Wake, 2015; Gregory et al., 2016).
The vomer of the Regalerpeton is roughly pentagonal and the vomerine tooth row is
transversely oriented. Compared with other fossil salamanders (Fig. 7), the vomers do not meet
each other in the midline, as in Pangerpeton, Seminobatrachus, Chunerpeton, Jeholotriton
and Qinglongtriton. In contrast, a midline contact of the vomers is present in Liaoxitriton,
Valdotriton and Beiyanerpeton. Moreover, the pentagonal shape of the vomers is unique in
In Chunerpeton, Beiyanerpeton, Qinglongtriton, Seminobatrachus, and Valdotriton, the
vomerine tooth row is parallel to the maxillary arcade. In Jeholotriton paradoxus, the vomer
overgrows the vomerine teeth and has a gracile posterior palatal extension. In Liaoxitriton
Rong - Restudy of Regalerpeton weichangensis
daohugouensis, like Regalerpeton, the vomerine teeth row is transverse and medially placed
in the palate, but the vomers are larger and of a different shape (Fig. 7). The Nuominerpeton
aquilonaris (Jia and Gao, 2016b) from the Lower Cretaceous Guanghua Formation of Nei
Mongol, China, differs from Regalerpeton in having a Liaoxitriton-like vomer with short
multiple rows of vomerine teeth.
4.2 Parasphenoid
In most salamanders, the shape of anterior border of the parasphenoid cannot be well
displayed because it is covered with vomers (Regal, 1966; Rose, 2003; Darda and Wake, 2015;
Gregory et al., 2016; Fei and Ye, 2017). In Regalerpeton, parasphenoid is fully exposed in the
palate of the skull on account of separated vomer. It is clearly shown that Regalerpeton has
an arrow-shaped parasphenoid with an indented anterior end. Other than this taxon, this type
is only known in Liaoxitriton daohugouensis (Fig. 7), which has a relatively shorter anterior
process. Besides, it is unique that parasphenoid has an indented anterior end of the cultriform
process. The special morphology of the parasphenoid can be used as the identication feature
of Regalerpeton.
4.3 Hyobranchium
The hyobranchium lies in the oor of the mouth and supports the tongue. It varies in its
conguration in different species (Table 1). In Chunerpeton, Beiyanerpeton, Qinglongtriton,
and Regalerpeton, the hyobranchium consists of ossified hypobranchial I, hypobranchial II
Fig. 7 Comparison of vomer and vomerine tooth row of the fossil salamanders
A. Liaoxitriton zhongjiani (Dong and Wang, 1998); B. Seminobatrachus boltyschkensis (Skutschas and Gubin,
2012); C. Chunerpeton tianyiensis (Gao and Shubin, 2003); D. Pangerpeton sinensis (Wang and Evans, 2006);
E. Liaoxitriton daohugouensis (Wang, 2004); F. Regalerpeton weichangensis (IVPP V 17989);
G. Valdotriton (reconstruction) (Evans and Milner, 1996); H. Jeholotriton paradoxus (Wang, 2000);
I. Qinglongtriton gangouensis (Jia and Gao, 2016a); J. Beiyanerpeton jianpingensis (Gao and Shubin, 2012)
Not to scale
132 Vertebrata PalAsiatica, Vol. 56, No. 2
and basibranchial II, but the shape of basibranchial II differs among these taxa (Fig. 7). It is
triradiate in Regalerpeton. However, in Beiyanerpeton, basibranchial I and II are co-ossied
to form a trident-shaped element with slender arms extending anteriorly and anterolaterally
(Gao and Shubin, 2012). In Chunerpeton, basibranchial II is trident-shaped (Gao and Shubin,
2003), whereas in Qinglongtriton, it is more complex in shape, with paired anterolateral and
posterolateral processes fused to a median rod (Jia and Gao, 2016a). In modern salamanders,
the paired ossified hypobranchial I and hypobranchial II and paralleled to each other,
this occurs in the larval and neotenic salamanders like Siren intermedia, Pachyhynobius
shangchengensis, Batrachuperus mustersi, Pseudobranchus striatus, Proteus anguinus,
Amphiuma means, Desmognathus aeneus (Deban and Wake, 2000; Rose, 2003; Xiong et al.,
2013). Accordingly, pairs of hypobranchial I and hypobranchial II present in fossil taxon are
considered larval hyobranchium.
Table 1 The ossied elements of the hyobranchium in different fossil taxa
Taxa Ceratobranchial Hypobranchial Basibranchial
Kokartus honorarius1) 0 hb I, hb II 0
Laccotriton subsolanus2) ? ? bb II
Liaoxitriton zhongjiani3) cb I ? ?
Jeholotriton paradoxus4) 0 ? 0? 0?
Sinerpeton fengshanensis5) cb II ? ?
Chunerpeton tianyiensis6) 0 hb I, hb II bb II
Liaoxitriton daohugouensis7) cb I hb I bb II
Pangerpeton sinensis8) cb I, cb II 0 0
Regalerpeton weichangensis 0 hb I, hb II bb II
Beiyanerpeton jianpingensis9) 0 hb I, hb II bb I + bb II
Qinglongtriton gangouensis10) 0 hb I, hb II bb II
Nuominerpeton aquilonaris11) cb II hb II bb II
Based on: 1) Skutschas and Martin, 2011; 2) Gao et al., 1998; 3) Dong and Wang, 1998; 4) Wang, 2000; 5) Gao and
Shubin, 2001; 6) Gao and Shubin, 2003; 7) Wang, 2004; 8) Wang and Evans, 2006; 9) Gao and Shubin, 2012; 10) Jia and
Gao, 2016a; 11) Jia and Gao, 2016b.
4.4 Neoteny
In salamanders, neoteny is a phenomenon in which an animal retains the larval
conguration while attaining reproductive maturity (Pierce and Smith, 1979; Shaffer, 2013).
There are gill filament impressions (three pairs) (Fig. 3B, C) and ossified or calcified gill
rakers (Figs. 1, 2, 6) present in our newly discovered specimens. As in modern relatives, the
gill laments or rakers are indicative of external gills. Regalerpeton must therefore have been
neotenic, as the adult individuals have 1) external gills; 2) larval-shaped pterygoids; 3) a larval
hyobranchium. Regalerpeton therefore resembles Chunerpeton (Gao and Shubin, 2003),
Beiyanerpeton (Gao and Shubin, 2012) and Qinglongtriton (Jia and Gao, 2016a). However,
Regalerpeton differs from the other three taxa in having the carpals and tarsals fully ossied in
the adult stage. It is implied that Regalerpeton has the ability to support the body.
Rong - Restudy of Regalerpeton weichangensis
Fig. 8 50% majority rule consensus tree of the most
parsimonious trees obtained in TNT (Goloboff and
Catalano, 2016) for caudate phylogeny
Clade A. Cryptobranchoidea; B. Salamandroidea;
C. Cryptobranchidae; D. Hynobiidae
5 Phylogenetic analysis
In order to determine the phy-
logenetic position of Regalerpeton,
it is added into the latest data matrix
of caudates as show in Jia and Gao
(2016a). The character states of Jehol-
otriton were also added based on
descriptions in Wang (2000), Wang and
Rose (2005) and Carroll et al. (2012).
Some character states of Pangerpeton
were modied by reexamination of the
holotype (see Appendix 1). Karaurus
was the designated outgroup taxon, and
all the characters were unordered and
unweighted as in Jia and Gao (2016a).
The implicit enumeration search
algorithm using TNT (Goloboff and
Catalano, 2016) resulted in fifteen
most parsimonious trees (tree length =
263 steps, consistency index = 0.464,
retention index = 0.702). 50% majority
rule consensus of
15 most parsimonious
trees (Fig. 8) suggests that Jeholotriton
and Pangerpeton
are sister taxa with
three synapomorphies: vomer with
greatly elongated process extending
along lateral border of parasphenoid
[character state 13(2)]; ossified hypo-
branchial I absent [character state
33(1)]; pterygoid without teeth [cha-
racter state 39(0)]. Regalerpeton
forms a sister taxon to the “Jeholotriton + Pangerpeton” clade. The analysis also reveals that
Regalerpeton, Jeholotriton, Pangerpeton, Qinglongtriton and Beiyanerpeton together form a
clade that is sister-taxon to the rest of Salamandroidea. This sister clade is supported by three
synapomorphies: ossied nasal without midline contact [character state 8(1)]; angular fused
to prearticular [character state 26(1)]; articular absent or by fusion with prearticular [character
state 29(1)]. Besides, Regalerpeton, Jeholotriton and Pangerpeton (from the Early Cretaceous
of Hebei, Middle/Late Jurassic of Nei Mongol and Liaoning, respectively) share unicapitate
ribs with the suborder Cryptobranchoidea. This is in accordance with their transitional position
134 Vertebrata PalAsiatica, Vol. 56, No. 2
between Cryptobranchoidea and Salamandroidea, which indicates that the two groups may
have started to split in the Middle to Late Jurassic. There is also mitochondrial genomes
evidence showing the Cryptobranchoidea-Salamandroidea split in the Mid-Jurassic (~171 Ma)
(Zhang and Wake, 2009). Therefore, Regalerpeton, Jeholotriton and Pangerpeton represent an
important stage of evolution in the history of salamanders.
6 Conclusions
The following conclusions can be drawn from this study:
(1) R. weichangensis differs from other salamanders mainly in the following charac-
teristics: it is a neotenic salamander with ossified carpals and tarsals; roughly pentagonal
vomer with transversely oriented vomerine tooth row; presence of triradiate basibranchial II;
parasphenoid inverted arrow-shaped with an indented anterior end of the cultriform process;
a long tail exceeded the snout-pelvis length coracoid portion of scapulocoracoid rectangular.
These features support the generic distinction of Regalerpeton.
(2) New phylogenetic analysis places Regalerpeton, Jeholotriton and Pangerpeton into
the suborder Salamandroidea. They also share unicapitate ribs with Cryptobranchoidea, so
the three taxa Regalerpeton, Jeholotriton and Pangerpeton represent an important transitional
stage in the evolution of salamanders.
Acknowledgments Thanks to Dr. Dong Liping for discussions on the phylogenetic analysis
and Mr. Zhang Shaoguang for taking photographs. I am grateful for my MSc advisor Prof.
Wang Yuan for his support and revisions of early versions of this manuscript. Thanks also
to reviewers Profs. Susan Evans (UCL), Liu Jun (IVPP) and Dr. Chen Jianye (AMNH)
for their constructive revisions on the paper. This work was supported by grants from the
NNSF of China (Grant NO. 41472018) and the Chinese Academy of Sciences (Grant NO.
河北围场下白垩统围场皇家螈(Regalerpeton weichangensis)
(1 中国科学院古脊椎动物与古人类研究所,中国科学院脊椎动物演化与人类起源重点实验室 北京 100044)
(2 中国科学院生物演化与环境卓越创新中心 北京 100044)
(3 中国科学院大学 北京 100049)
摘要:围场皇家螈(Regalerpeton weichangensis)2009年基于一件产自河北围场下白垩统不
Rong - Restudy of Regalerpeton weichangensis
中图法分类号Q915.863 文献标识码A 文章编号1000–3118(2018)02–0121–16
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Appendix 1 Additions and changes to the date matrix (Jia and Gao, 2016a) used in the phylogenetic analysis
Regalerpeton weichangensis 0000100100 1201000010 0001?11?10 000?001?10 0002?0110? ?01??0?101
000100??10 0??100???? ?????????? 1111????0? 000??
Jeholotriton paradoxus 0000100000 1320000010 ?111??1??0 0?1??01?00 0012?0?10? ?01???0100
000100??10 0??100???? ?????????? 1111????0? 000??
Pangerpeton sinensis 0000??1??? 1020000010 0????11??? 0010001000 ?00?00110? ???????1??
????1???00 0????????? ?????????? ?111????0? ?00??
... 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;, and further obstructs our understanding of the paleobiology of these early salamanders (e.g. life history, ecological preferences). ...
... 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. ...
... Formation of northern Hebei, China (Rong, 2018) and Linglongtriton from the Upper Jurassic Tiaojishan Formation of Liaoning Province, ...
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.
... The Asian record is rich in the Early Cretaceous. Chinese taxa are represented by the proposed stemhynobiids Nuominerpeton aquilonaris and Liaoxitriton zhongjiani [42,61], the possible salamandroid Regalerpeton weichangensis [62,63], as well as Jeholotriton paradoxus [64] (figures 1 and 3). However, the phylogenetic positions of all of these Chinese species are uncertain [43], with cryptobranchoid [65], salamandroid [63] and stem caudate [30] affinities proposed. ...
... Chinese taxa are represented by the proposed stemhynobiids Nuominerpeton aquilonaris and Liaoxitriton zhongjiani [42,61], the possible salamandroid Regalerpeton weichangensis [62,63], as well as Jeholotriton paradoxus [64] (figures 1 and 3). However, the phylogenetic positions of all of these Chinese species are uncertain [43], with cryptobranchoid [65], salamandroid [63] and stem caudate [30] affinities proposed. Sinerpeton fengshanensis and Laccotriton subsolanus are non-salamandroid caudates of uncertain placement described by Gao & Shubin [66] and Gao et al. [67], respectively. ...
Full-text available
The disjunct geographical range of many lineages of caudates points to a complex evolutionary and biogeographic history that cannot be disentangled by only considering the present-day distribution of salamander biodiversity. Here, we provide a critical reappraisal of the published fossil record of caudates from the Palearctic and quantitatively evaluate the quality of the group's fossil record. Stem-Urodela and Karauridae were widespread in the Palearctic in the Middle Jurassic, suggesting an earlier, unsampled diversification for this group. Cryptobranchidae reached Europe no later than the Oligocene, but this clade was subsequently extirpated from this continent, as well as from western and central Asia. The relatively recent appearance of hynobiids in the fossil record (Early Miocene) is most likely an artefact of a taphonomic bias against the preservation of high-mountain, stream-type environments which early members likely inhabited. Salamandroids first appear in Europe, expanding into Asia by the Miocene. The apparently enigmatic and disjunct distribution of extant caudate lineages is therefore explained by a wider past geographical range, as testified by the fossil record, which was fragmented during the late Cenozoic by a combination of tectonic (i.e. the uplift of the Tibetan Plateau) and climatic drivers, resulting in regional extirpations.
... iScience 24, 102744, July 23, 2021 5 iScience Article external gills, and an anteromedial directed palatal process of the pterygoid (Rong, 2018). Based largely on the absence of these larval features and presence of an anterolaterally directing palatal process of the pterygoid in adult specimens, several other taxa (Laccotriton, Liaoxitriton, Linglongtriton, Nuominerpeton, and Neimengtriton) are considered metamorphic (e.g., Dong and Wang, 1998;Gao et al., 1998;Gao and Shubin, 2001;Wang 2004a;Gao, 2016a, 2019). ...
... Our taxonomic revision of the genus and species as a new combination is primarily based on the information from PKUP V0515, supplemented by published accounts of other conspecific specimens (Wang, 2004a;Wang et al., 2008;Sullivan et al., 2014). Our comparative study of fossil forms with extant hynobiids is largely based on published accounts (e.g., Gao et al., 1998;Gao and Shubin, 2001;Wang and Evans, 2006b;Jia and Gao, 2016a;Rong, 2018), along with direct examination of specimens in various collections noted in supplemental information. ...
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The Hynobiidae are an early-diverging clade of crown-group salamanders (urodeles) with an important bearing on the evolution of urodeles. Paleobiology and early branching patterns of the Hynobiidae remain unclear due to a poorly documented fossil record. We reported a newly referred specimen to the stem hynobiid, originally named as “Liaoxitriton daohugouensis”, but here as Neimengtriton daohugouensis comb. nov., and predates the previously estimated origination time of Hynobiidae for at least 8 Myr. We interpret Neimengtriton daohugouensis as semi-aquatic at adult stage, a previously unknown paleoecological preference among Mesozoic salamanders. Phenotypic variations of Neimengtriton daohugouensis enlighten an unrecognized association between caudosacral vertebrae and fertilization modes in the early evolution of urodeles. Our cladistic analyses based on morphological characters not only recognize several stem hynobiids and establish Panhynobia nomen cladinovum for the total-group hynobiids, but also shed light on the sequential evolution of morphological features in this primitive urodele clade.
... Subsequent phylogenetic analyses typically grouped Chunerpeton with the cryptobranchids and placed it as either a crown or stem member (Gao and Shubin, 2003;Wang and Evans, 2006;Gao, 2016a, 2019;Rong, 2018), but the cryptobranchid affinities of Chunerpeton were questioned by Vasilyan et al. (2013). As the presumed earliest cryptobranchid, Chunerpeton has been used to calibrate the molecular clock for the split between Cryptobranchidae and Hynobiidae (e.g., San Mauro et al., 2005;Bossuyt et al., 2006;Chen et al., 2015;Irisarri et al., 2017). ...
... Chunerpeton further differs from Valdotriton, as follows: (1) premaxilla bearing a short pars praenasalis versus long pars dorsalis in Valdotriton; (2) unfused prootic, exoccipital, and opisthotic versus fused prootic, exoccipital, and opisthotic in Valdotriton; (3) unicapitate ribs versus bicapitate ribs in Valdotriton; and (4) single row of vomerine teeth versus two or three rows in Valdotriton. Chunerpeton can be further distinguished from Regalerpeton weichangense (Early Cretaceous, China), as follows: (1) long and narrow vomer with posterolaterally oriented vomerine tooth row versus roughly pentagonal vomer with transversely oriented vomerine tooth row in Regalerpeton; (2) separate opisthotic and exoccipital versus fused opisthotic and exoccipital in Regalerpeton; and (3) arrow-shaped basibranchial II versus triradiate in Regalerpeton (Rong, 2018). Chunerpeton differs from Jeholotriton paradoxus in having monostichous vomerine teeth versus multiple vomerine tooth rows in Jeholotriton (Wang and Rose, 2005). ...
Full-text available
Lacustrine deposits of Juro-Cretaceous age in northeastern China have yielded some of the best-preserved fossils of early crown salamanders. One of those taxa, Chunerpeton tianyiense, has been considered as a crown or stem member of the family Cryptobranchidae, significant for implying a long evolutionary history for cryptobranchids and for calibrating the molecular clock of Caudata evolution. Building on the most recent large-scale phylogenetic analysis of relationships among fossil and recent salamanders and utilizing new specimens of Chunerpeton, we update the osteological description and diagnosis for Chunerpeton and reconsider its phylogenetic relationships. On the basis of recently collected Chunerpeton skeletons from the type locality at Daohugou, Inner Mongolia, China and available literature, we update the taxon-character matrix and run phylogenetic analyses with constraints on the relationships among families using a molecular backbone. We redescribe the osteology of Chunerpeton, revise and identify some new characters including large anterodorsal fenestra bordered by paired premaxillae, nasals, and frontals; nasals separate and wider than frontal; contact between nasal and prefrontal present; lacrimal present; and contact between pterygoid and parasphenoid absent. Osteological comparisons between Chunerpeton and living cryptobranchids reveal a suite of distinct differences in snout shape and in configurations, positions, and contacts of certain skull bones. Our phylogenetic analyses consistently place Chunerpeton as a stem Caudata outside of Cryptobranchidae and crown salamanders. Exclusion of Chunerpeton from Cryptobranchidae will require reconsideration of the origin time for Cryptobranchidae and recalibration of the molecular clock for the whole caudatan tree.
... Our finding of Chunerpeton as a stem-salamander strengthens doubts regarding its cryptobranchid affinity (28,70,71) and argues against its widespread use as a molecular constraint to calibrate the cryptobranchoid crown-group node (9, 14, 71-81) (SI Appendix, Part N). Our analysis also places Iridotrition, Linglongtriton, Neimengtriton, and Regalerpeton (previously referred to the stem of Hynobiidae, and therefore within the cryptobranchoid crown-group) (23,24,27,52,66) in an unresolved polytomy with Cryptobranchoidea and Salamandroidea. Nuominerpeton is placed outside Cryptobranchidae + Hynobiidae, as a stem cryptobranchoid. ...
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Salamanders are an important group of living amphibians and model organisms for understanding locomotion, development, regeneration, feeding, and toxicity in tetrapods. However, their origin and early radiation remain poorly understood, with early fossil stem-salamanders so far represented by larval or incompletely known taxa. This poor record also limits understanding of the origin of Lissamphibia (i.e., frogs, salamanders, and caecilians). We report fossils from the Middle Jurassic of Scotland representing almost the entire skeleton of the enigmatic stem-salamander Marmorerpeton . We use computed tomography to visualize high-resolution three-dimensional anatomy, describing morphologies that were poorly characterized in early salamanders, including the braincase, scapulocoracoid, and lower jaw. We use these data in the context of a phylogenetic analysis intended to resolve the relationships of early and stem-salamanders, including representation of important outgroups alongside data from high-resolution imaging of extant species. Marmorerpeton is united with Karaurus , Kokartus , and others from the Middle Jurassic–Lower Cretaceous of Asia, providing evidence for an early radiation of robustly built neotenous stem-salamanders. These taxa display morphological specializations similar to the extant cryptobranchid “giant” salamanders. Our analysis also demonstrates stem-group affinities for a larger sample of Jurassic species than previously recognized, highlighting an unappreciated diversity of stem-salamanders and cautioning against the use of single species (e.g., Karaurus) as exemplars for stem-salamander anatomy. These phylogenetic findings, combined with knowledge of the near-complete skeletal anatomy of Mamorerpeton, advance our understanding of evolutionary changes on the salamander stem-lineage and provide important data on early salamanders and the origins of Batrachia and Lissamphibia.
... The assemblages are characterized by relatively few taxa and the lack of some of the most typical elements of the middle-late phases of the Jehol Biota, such as the fish Lycoptera Müller, the spinicaudatans Eosestheria Chen and the ostracod Cypridea Bosquet (non-Cypridea fauna in Figs. 5, 6). However, some important vertebrate fossils already occur at this horizon, such as mammal (Luo et al., 2007;Yang et al., 2020), amphibian (Rong, 2018), and dinosaur fossils (Jin et al., 2008). ...
The Early Cretaceous Jehol Biota in northern China is a terrestrial lagerstätte that contains exceptionally-preserved fossils, including birds, dinosaurs, pterosaurs, mammals, insects, and flowering plants. The biota underwent three developmental phases, with relatively limited biodiversity in an early phase that rapidly diversified in a middle phase; however, the relationship between this biological radiation and climate remains uncertain. In this paper, we study fossils from the early-to-middle phases of the Jehol Biota preserved in the Lower Cretaceous (middle Valanginian-lower Barremian) Dabeigou and Dadianzi formations of Hebei Province to ascertain climatic impact on biotic evolution. The occurrence of a cool to warm climate turnover during the deposition of these strata is inferred based on a synthesis of geochemical and paleontological evidence. Palaeogeographic distribution of the middle phase of the biota is wider and positioned more southerly than that of the early phase, possibly indicating that the biota in the early and middle phases lived in boreal and temperate climate realms, respectively. Biotic diversity shows an increasing trend from the early phase to the middle phase of the Jehol Biota, closely coinciding with the cool to warm turnover of the climate. The body sizes of some taxa in the middle phase were significantly smaller than those in the early phase, which is also interpreted as a climatic effect. This study represents the first attempt to correlate the response of terrestrial evolution of the Jehol Biota to climate change, with a focus on Early Cretaceous paleotemperatures.
... external gills, gill rakers) than living cryptobranchids; however, we did not differentiate subgroups within neotenic taxa due to our sampling scope. Living habitats of cryptobranchoids in the adult stage outside the breeding season have been broadly classified into three types, namely aquatic, semiaquatic, and terrestrial (Fei et al., 2006;Rong, 2018;Fei and Ye, 2016;AmphibiaWeb, 2021). In order to understand if factors like taxonomic affiliations at the genus and family level, life history strategy and living habitat would contribute to morphological disparities of the palate, we conducted phylogenetic Procrustes ANOVAs for the non-allometric symmetric shape components of the 24-Procrustes-landmark dataset using the function ' procD. ...
Full-text available
Ecological preferences and life history strategies have enormous impacts on the evolution and phenotypic diversity of salamanders, but the yet established reliable ecological indicators from bony skeletons hinder investigations into the paleobiology of early salamanders. Here we statistically demonstrate, by using time-calibrated cladograms and geometric morphometric analysis on 71 specimens in 36 species, that both the shape of the palate and many non-shape covariates particularly associated with vomerine teeth are ecologically informative in early stem- and basal crown-group salamanders. Disparity patterns within the morphospace of the palate in ecological preferences, life history strategies and taxonomic affiliations were analyzed in detail, and evolutionary rates and ancestral states of the palate were reconstructed. Our results show that the palate is heavily impacted by convergence constrained by feeding mechanisms and also exhibits clear stepwise evolutionary patterns with alternative phenotypic configurations to cope with similar functional demand. Salamanders are diversified ecologically before the Middle Jurassic and achieved all their present ecological preferences in the Early Cretaceous. Our results reveal that the last common ancestor of all salamanders shares with other modern amphibians a unified biphasic ecological preference, and metamorphosis is significant in the expansion of ecomorphospace of the palate in early salamanders.
... Chinese cryptobranchoids or even some of the more crownward stem-urodeles toward them, even if some 824 (Rong, 2018) or most (Jia and Gao, 2019) end up in the hynobiid total group rather than in 825 ...
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Molecular divergence dating has the potential to overcome the incompleteness of the fossil record in inferring when cladogenetic events (splits, divergences) happened, but needs to be calibrated by the fossil record. Ideally but unrealistically, this would require practitioners to be specialists in molecular evolution, in the phylogeny and the fossil record of all sampled taxa, and in the chronostratigraphy of the sites the fossils were found in. Paleontologists have therefore tried to help by publishing compendia of recommended calibrations, and molecular biologists unfamiliar with the fossil record have made heavy use of such works. Using a recent example of a large timetree inferred from molecular data, I demonstrate that calibration dates cannot be taken from published compendia without risking strong distortions to the results, because compendia become outdated faster than they are published. The present work cannot serve as such a compendium either; in the slightly longer term, it can only highlight known and overlooked problems. Future authors will need to solve each of these problems anew through a thorough search of the primary paleobiological and chronostratigraphic literature on each calibration date every time they infer a new timetree; over 40% of the sources I cite were published after mid-2016. Treating all calibrations as soft bounds results in younger nodes than treating all calibrations as hard bounds. The unexpected exception are nodes calibrated with both minimum and maximum ages, further demonstrating the widely underestimated importance of maximum ages in divergence dating.
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Early limb skeletogenesis in salamanders is characterized by preaxial elements, digits I and II forming earlier than their postaxial counterparts (digits III to V), a phenomenon known as preaxial dominance, whereas in amniotes and anurans, these developmental sequences are reversed. This pattern characterizes the late skeletogenesis of digits and zeugopodium of anamniote tetrapods but remains unknown in carpals/tarsals. To correct this gap in knowledge, we investigate the ossification patterns of the carpals/tarsals in six salamander families/clades based on micro-computed tomography scans. We found that preaxial dominance is seen in the distal carpals/tarsals of several salamander clades and diverse early tetrapods, such as temnospondyls and amniotes. This distribution suggests that preaxial dominance is a primitive developmental pattern in tetrapods. Our results demonstrate that the distal carpals/tarsals are developmentally and evolutionarily independent in the autopodium, and preaxial dominance facilitates stabilization of the number of distal carpals/tarsals during fin-to-limb transition and digit reduction in early tetrapods.
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Molecular divergence dating has the potential to overcome the incompleteness of the fossil record in inferring when cladogenetic events (splits, divergences) happened, but needs to be calibrated by the fossil record. Ideally but unrealistically, this would require practitioners to be specialists in molecular evolution, in the phylogeny and the fossil record of all sampled taxa, and in the chronostratigraphy of the sites the fossils were found in. Paleontologists have therefore tried to help by publishing compendia of recommended calibrations, and molecular biologists unfamiliar with the fossil record have made heavy use of such works (in addition to using scattered primary sources and copying from each other). Using a recent example of a large node-dated timetree inferred from molecular data, I reevaluate all 30 calibrations in detail, present the current state of knowledge on them with its various uncertainties, rerun the dating analysis, and conclude that calibration dates cannot be taken from published compendia or other secondary or tertiary sources without risking strong distortions to the results, because all such sources become outdated faster than they are published: 50 of the (primary) sources I cite to constrain calibrations were published in 2019, half of the total of 280 after mid-2016, and 90% after mid-2005. It follows that the present work cannot serve as such a compendium either; in the slightly longer term, it can only highlight known and overlooked problems. Future authors will need to solve each of these problems anew through a thorough search of the primary paleobiological and chronostratigraphic literature on each calibration date every time they infer a new timetree, and that literature is not optimized for that task, but largely has other objectives.
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A new fossil salamander, Nuominerpeton aquilonaris (gen. et sp. nov.), is named and described based on specimens from the Lower Cretaceous Guanghua Formation of Inner Mongolia, China. The new discovery documents a far northern occurrence of Early Cretaceous salamanders in China, extending the geographic distribution for the Mesozoic fossil record of the group from the Jehol area (40th–45th parallel north) to near the 49th parallel north. The new salamander is characterized by having the orbitosphenoid semicircular in shape; coracoid plate of the scapulocoracoid greatly expanded with a convex ventral and posterior border; ossification of two centralia in carpus and tarsus; and first digit being about half the length of the second digit in both manus and pes. The new salamander appears to be closely related to hynobiids, although this inferred relationship awaits confirmation by research in progress by us on a morphological and molecular combined analysis of cryptobranchoid relationships. Comparison of adult with larval and postmetamorphic juvenile specimens provides insights into developmental patterns of cranial and postcranial skeletons in this fossil species, especially resorption of the palatine and anterior portions of the palatopterygoid in the palate and the coronoid in the mandible during metamorphosis, and postmeta-morphic ossification of the mesopodium in both manus and pes. Thus, this study provides a rare case study of developmental features in a Mesozoic salamander.
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Despite being widely regarded as generalist predators, amphibians exhibit a diversity of tooth shapes and dentition patterns, which may indicate the influence of dietary specialization on the evolution of tooth morphology. Very few studies have analysed the relationship between tooth morphology and diet (i.e., prey items) in amphibians, and those existing studies are highly speculative. We investigated the evolution of salamander teeth and the relationship between tooth morphology and diet in a phylogenetically independent fashion. We used a phylogeny of 23 species of salamander representing three families (Ambystomatidae, Plethodontidae, and Salamandridae) to, first, analyse the divergence of tooth morphology and its relationship to phylogeny and, second, to analyse the relationship between tooth morphology and diet diversity. We used electron scanning microscopy and a statistical comparative approach using Spatial Evolutionary and Ecological Analysis (SEEVA) and phylogenetic generalized least-squares regression in R. Our results indicated significant divergence in tooth morphology at major phylogenetic splits. Moreover, there was a significant, phylogenetically independent relationship between tooth morphology and diet diversity. The relationship between diet and tooth morphology indicates not only a reflection of phylogenetic history, but also a degree of dietary specialization, indicating that evolution in tooth morphology has had an adaptive aspect in relation to salamander diet.
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A new salamandroid salamander, Qinglongtriton gangouensis (gen. et sp. nov.), is named and described based on 46 fossil specimens of juveniles and adults collected from the Upper Jurassic (Oxfordian) Tiaojishan Formation cropping out in Hebei Province, China. The new salamander displays several ontogenetically and taxonomically significant features, most prominently the presence of a toothed palatine, toothed coronoid, and a unique pattern of the hyobranchium in adults. Comparative study of the new salamander with previously known fossil and extant salamandroids sheds new light on the early evolution of the Salamandroidea, the most species-diverse clade in the Urodela. Cladistic analysis places the new salamander as the sister taxon to Beiyanerpeton, and the two taxa together form the basalmost clade within the Salamandroidea. Along with recently reported Beiyanerpeton from the same geological formation in the neighboring Liaoning Province, the discovery of Qinglongtriton indicates that morphological disparity had been underway for the salamandroid clade by early Late Jurassic (Oxfordian) time.
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Osteological variation is recorded among and within four of the most distinctive species of the Mexican salamander genus Chiropterotriton. Analysis of the data is consistent with the monophyletic status of the genus and documents previously unrecorded intraspecific and interspecific variation. Most of the recorded variation involves qualitative and quantitative proportional differences, but four fixed differences constitute autapomorphic states that affirm and diagnose some species (C. dimidiatus, C. magnipes). Osteological variation in 15 characters is analyzed with respect to predictions generated from four hypotheses: 1) phylogeny, 2) adaptation to specific habitats (the four species include cave-dwelling, terrestrial, and arboreal forms), 3) size-free shape, and 4) size. High levels of intraspecific variation suggest that the characters studied are not subject to rigid functional constraints in salamanders, regardless of size. The pattern predicted by the hypothesis based on size differences seen among these four Chiropterotriton species matches most closely the observed pattern of relative skull robustness. Since size change and heterochrony are often associated in plethodontid evolution, it is likely that changes in developmental timing play a role in the morphological transitions among these morphologically diverse taxa. Webbed feet, miniaturization, body shape, and an unusual tarsal arrangement are morphologies exhibited in species of Chiropterotrition that are shown to be homoplastic with other clades of tropical plethodontids. Although extensive homoplasy in salamanders might be seen as a roadblock to unraveling phylogenetic hypotheses, the homologous developmental systems that appear to underlie such homoplasy may reveal common and consistent evolutionary processes at work.
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Since the late 1990s, eight localities in volcanic shale-rich lacustrine deposits of Middle Jurassic through Early Cretaceous age in northern China (western Liaoning Province, northern Hebei Province, and southern Inner Mongolia) have yielded thousands of exceptionally well-preserved salamander specimens. With 10 species published and several new taxa yet to be named and described, the fossil samples from northern China represent the most species-diverse, individually abundant, and exquisitely preserved salamander fossil assemblage known from the Mesozoic Era. The stratigraphic range of the fossil record covers a geologic time span of roughly 40–45 million years from the Middle Jurassic (Bathonian) through the Early Cretaceous (Aptian). In contrast to the well-known stem-group salamanders Karaurus and Kokartus from the Middle to Late Jurassic of Middle Asia, the Chinese record contains the earliest known crown-group salamanders, including Jurassic representatives of both Cryptobranchoidea and Salamandroidea. The Chinese Mesozoic record includes numerous examples of virtually complete larval, juvenile, young adult, and fully grown adult individuals that collectively provide key information on the life histories and developmental patterns of the earliest known crown-group salamanders. Many specimens show preservation of soft tissue structures, including body outline, eye, liver, and external gill filaments. This kind of soft tissue preservation is unusual for fossil salamanders, so the Chinese Mesozoic specimens are important for furnishing otherwise unavailable information on the life history, diversity, and ecological adaptations of early crown-group salamanders.
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A fossil salamander is described based on specimens from a Lower Cretaceous deposit near Ningcheng, Inner Mongolia, China. Recently reported as Jeholotriton paradoxus, this form represents a neotenic salamander as indicated by larval features such as external gills, tooth-bearing coronoids, pterygoids with anteromedially oriented anterior processes, and underdeveloped maxillae, in combination with adult features such as extensive medial contact of the nasals and posteriorly directed tooth rows in the palate. This taxon is distinguished from other Mesozoic salamanders by having 15–16 presacrals, proximally expanded unicapitate ribs, vomers with large tooth patches anteriorly and longitudinal dentigerous bars posteriorly, large nasals with no anterior notch, frontals with no anterolateral extension, premaxillae with distinct alary processes, short transverse processes on the vertebrae, and phalangeal formulae of 2-2-3-2 for the manus and 2-2-3-3-2 for the pes. Including Laccotriton, Liaoxitriton, Sinerpeton and Chunerpeton, five taxa of Mesozoic salamander have now been found in China. Jeholotriton has especially well-preserved impressions of articulated skeletons, a type of preservation that is uncommon in Mesozoic strata throughout the world and provides important anatomical details of this early salamander. The great diversity of fossil salamanders from the late Mesozoic of northeastern China implies that East Asia was an important center for the early evolution of urodeles.
Version 1.5 of the computer program TNT completely integrates landmark data into phylogenetic analysis. Landmark data consist of coordinates (in two or three dimensions) for the terminal taxa; TNT reconstructs shapes for the internal nodes such that the difference between ancestor and descendant shapes for all tree branches sums up to a minimum; this sum is used as tree score. Landmark data can be analysed alone or in combination with standard characters; all the applicable commands and options in TNT can be used transparently after reading a landmark data set. The program continues implementing all the types of analyses in former versions, including discrete and continuous characters (which can now be read at any scale, and automatically rescaled by TNT). Using algorithms described in this paper, searches for landmark data can be made tens to hundreds of times faster than it was possible before (from T to 3T times faster, where T is the number of taxa), thus making phylogenetic analysis of landmarks feasible even on standard personal computers.
Over the past 10 years, there has been a wealth of discoveries of fossil salamanders from the Jehol Biota in northern China. These specimens have revealed many new species in addition to establishing probable divergence times and relationships among modern salamander families. Among these are numerous specimens of a neotenic species Jeholotriton paradoxus. In this study, we focused on this particular salamander species because its classification still remains enigmatic. The aim of this research was to determine the relationship of Jeholotriton with respect to other Jehol salamanders as well as modern salamander families. Although Jeholotriton has been described in previous studies, the discovery of new specimens and increasing knowledge of other Mesozoic salamanders has allowed for a more through description of the genus. Jeholotriton is known only from the Daohugou locality in Ningchen, south-eastern Inner Mongolia. It may be close to the base of the modern Urodele radiation, and might provide evidence of their transition from putative ancestors in the Permo-Triassic. We discovered that the fossil Kokartus (family Karauridae) and the living hynobiids (the most primitive group of modern salamanders) Ranodon sibiricus and Hynobius maculosus, as well as Dicamptodontidae tenebrosus all share some similarities with Jeholotriton. However, conclusive relationships could not be confidently established because of the unique combination of mature and larval characteristics in Jeholotriton. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164, 659–668.