Pangerpeton sinensis, Wang & Evans Lissamphibia & Genus, 2006

Wang, Yuan, Evans Lissamphibia, Susan E. & Genus,., 2006, 1 H, 13 C, 15 N Chemical Shift Assignments for Mesd 12 - 155, Acta Palaeontologica Polonica 51 (1), pp. 127-130 : 127-130

Pangerpeton sinensis gen. et sp. nov.

Derivation of name: from the Latin, Sine, meaning Chinese .

Holotype: IVPP V14244 , a nearly complete skeletal impression ( Fig. 1 ), collections of the Institute of Vertebrate Paleontology and Paleoanthropology ( IVPP), Chinese Academy of Sciences , Beijing.

Type locality: Wubaiding Village,Lingyuan City, Liaoning Province, China.

Type horizon: Probably contemporaneous with the salamander−bearing horizon at Daohugou, Inner Mongolia, Late Jurassic/Early Cretaceous ( Wang 2004a, b).

Diagnosis.—A postmetamorphosed salamander with a short wide head and only 14 presacral vertebrae; longitudinal arrangement of vomerine teeth with at least two rows centrally; anterolateral pterygoid process short and tapering; short rib−bearers with single−headed ribs. Differs from Chunerpeton and Jeholotriton in lacking teeth on the palatopterygoid, and having longitudinal vomerine tooth rows rather than a row parallel to the marginal dentition ( Chunerpeton ) or a tooth patch ( Jeholotriton ); further differs from Jeholotriton in having the anterior process of the pterygoid directed anterolaterally rather than anteromedially, and the presence of ossified hyoid elements (unossified in Jeholotriton ); differs from Liaoxitriton , Laccotriton and Sinerpeton in the longitudinal rather than transverse arrangement of the vomerine teeth and in lacking ossified carpals and tarsals; differs from all other known Mesozoic salamander genera in the significantly shorter head and presacral series, and the presence of two ossified ceratobranchials ( Estes 1981; Milner 2000).

Description.— IVPP V 14244 preserves a ventral impression of the skeleton with associated skin traces showing the body outline. The specimen is almost complete, missing only the left hind foot and part of the tail ( Fig. 1A ). The head is proportionally short and wide ( Fig. 1B, C , Table 1), with the lower jaws together forming a semicircle. The snout−pelvis length (midline of premaxillae to the rear of the ischiadic plate) is 39.4 mm. The individual is postmetamorphic as the bone surfaces are finished and there are no traces of external gills. The description that follows is based on a high−fidelity cast taken from the impression.

Each premaxilla has a slender tooth−bearing alveolar portion and a short alary process with a distinct lateral curvature. Judging from the length and breadth of the alary processes, they probably overlapped the nasal anteriorly, although the articulation is not preserved. Each maxilla is long (c. three−quarters of lower jaw length) with a long, low facial portion, small teeth, and a slender premaxillary process. The palate is dominated by a broad, parallel−sided parasphenoid that expands posteriorly into short wings, each of which is perforated by an internal carotid foramen. In the anterior margin, just behind the jaw symphysis, is a depressed area that may be a membrane−filled fenestra, whereas on either side of the parasphenoid are anteroposteriorly elongated vomers. These are divided into two portions, most clearly seen on the right side. The anterior part is toothless, with an incurved medial expansion. Further back are at least two rows of teeth, although this is difficult to image as the vomer is tilted inwards. The more lateral row is continued some distance along the posterior bar of the vomer. The triradiate pterygoid has a long, curved posterolateral process, a shorter tapering anterolateral process, and a smaller anteromedial process. The anterolateral process is well separated from the end of the maxilla. Between the tip of the left posterolateral process and the coronoid−prearticular flange of the lower jaw, there is a rounded mass that may be an ossified or partially ossified quadrate. This would place the jaw articulation roughly on a line with the atlanto−occipital joint. Some of the dorsal skull bones are visible above the vomers and parasphenoid – notably frontals forming the orbital margins, orbitosphenoids between the frontals and parasphenoid, and short parietals above the expanded parasphenoid wings. On the left, anterior and posterior foramina perforate the orbitosphenoid, probably the optic and oculomotor foramina respectively ( Fig. 1C ). The squamosals lie above and parallel to the posterolateral processes of the pterygoids. The right bone is more exposed and reveals a short anterior ramus and a long slender ventral ramus. Elements of the braincase are clearest on the right and do not appear co−ossified. The prootic meets the anterior part of the parasphenoid wing, separated from the opisthotic by a cleft that represents the fenestra ovalis. The exoccipital condyles are widely separated, with their articular surfaces directed posteromedially.

The lower jaw is composed mainly of the elongated dentary. This bears tiny conical teeth (6–7 per mm), but it is not clear whether or not they are pedicellate. The long coronoid−prearticular has a smooth continuous dorsomedial edge, in contrast to the bilobed condition in Chunerpeton and Liaoxitriton . On each side, the flange is perforated by a small foramen, but unlike Jeholotriton , it bears no teeth. On the left, the coronoid−prearticular flange is separated for a short distance from a thin sliver of bone that is exposed along the anteromedial aspect of the dentary. This sliver may be an anterior extension of the coronoid−prearticular, but this is uncertain.

Overlying the parasphenoid on the anatomical left side are two short ossified rods interpreted as the first and second hyoid ceratobranchials. These are not clearly discernable on the right.

There are 14 amphicoelous presacrals, one sacral, and up to five anterior caudals. The atlas is roughly the same length as succeeding vertebrae. It is hourglass shaped ventrally but appears to expand dorsally into flared lateral wings that are perforated in the narrowest region by pits or foramina. The anterior border forms a smooth curve, the apex of which is the interglenoid prominence. The remaining vertebrae are featureless, with no evidence of either a ventral keel or basapophyses. The rib bearers are short and support single−headed ribs with proximal and distal ends of similar width. The strongest ribs, supporting the pectoral girdle, are those of the third and fourth vertebrae, but the following two are longer. The ribs become noticeably shorter towards the sacrum, with the last being little more than a triangular stub. The sacral vertebra is similar in size to the presacrals, but the free ribs are angled backwards to support the pelvis. The first two or three caudal vertebrae are obscured by the ischiadic plate. The next two are visible but it is not possible to determine whether they had either free ribs or haemal arches. The rest of the tail is lost.

Both forelimbs are complete. The scapulocoracoid is a relatively small, flask−shaped bone composed of a rounded coracoid plate and a longer, narrower scapula with a slight proximal expansion. The proximal and distal heads of the humerus are only slightly expanded, with little development of a proximal crista ventrolateralis and no ossified joint surfaces. The radius and ulna are short (humerus, 5.7 mm; ulna, 3.6 mm) and the carpus is unossified. The manus consists of four widely spread digits with a phalangeal formula of 2:3:3:2 on the left and 2:2:3:2 on the right.

Each pelvis consists of a short, dumb−bell shaped ilium that is expanded proximally, and a longer, more massive ischiadic plate that is notched laterally in its midsection. A gap between the anteroventral edge of the ilium and the corresponding ischium suggests the presence in life of a cartilaginous pubic plate. The femur (6.2 mm) is slightly longer than the humerus but is of similar width. The tibia and fibula are short (tibia, 3.6 mm), the tarsals are unossified and the pes has a phalangeal formula of?:2:3:3:2.

The skeletal impression is surrounded by a soft tissue outline that reveals a broader body shape than the skeleton would suggest, with short, stumpy limbs and wide digits. The lateral edge of the body outline and parts of the surface are slightly irregular and the skin may have been warty.

Discussion.—We compared the body proportions of Pangerpeton with those of other well−preserved Chinese Mesozoic salamanders ( Table 1). Skull width/skull length for Pangerpeton is 1.7, a higher value than that of other taxa. Thus Chunerpeton , Jeholotriton and Liaoxitriton all have proportionally narrower skulls than the smaller Pangerpeton . This cannot be an effect of small size or immaturity because younger representatives of each of the major Chinese taxa have proportionally longer, narrower skulls than full adults. The difference in proportions raises the question as to whether the skull is unusually short or unusually wide, or both. A comparison of skull proportions against a standard (one vertebral length) ( Table 1) shows that it is skull length (i.e., proportion B, Table 1) rather than skull width (proportion A, Table 1) that is significantly different in Pangerpeton . Thus skull width in Pangerpeton is within the range of other taxa, but the skull is significantly shorter. Unfortunately, the material referred to Sinerpeton and Laccotriton does not permit detailed measurements, but the published figures ( Gao and Shubin 2001) suggest the skulls have similar proportions to those of Liaoxitriton and thus also differ from Pangerpeton .

In order to determine the phylogenetic relationships of Pangerpeton with respect to other caudates, we constructed a data matrix ( Table 2) of 18 taxa and 72 characters (using mainly the characters listed by Gao and Shubin, 2003, and derived mainly from Duellman and Trueb 1986; Larson and Dimmick 1993; and Trueb 1993; see http:// spaces.msn.com/members/ywangivpp for character descriptions). In addition to modern families and Chinese fossil salamanders, we included two basal caudates, Marmorerpeton (Middle– Upper Jurassic, Europe, Evans et al. 1988) and Karaurus (Upper Jurassic, Central Asia, Ivakhnenko 1978), and two more derived taxa, Valdotriton (Lower Cretaceous, Spain, Evans and Milner 1996) and Iridotriton (Upper Jurassic, North America, Evans et al. 2005). We ran an analysis using PAUP * Version 4.0b10 for 32−bit Microsoft Windows ( Swofford 2001) on a PC computer. All characters were unordered and equally weighted, with DELTRAN tree optimization used to minimize reversals. The basal caudate Marmorerpeton was used as the outgroup taxon. A Branch−and−Bound search option resulted in 28 most parsimonious trees (TL = 167, CI = 0.6048, RI = 0.5823). As shown in the 50% Majority Rule Consensus Tree ( Fig. 2 ), Pangerpeton emerged as a possible sister taxon of the roughly contemporaneous Jeholotriton , and this clade forms an unresolved trichotomy with cryptobranchids on the one hand and all other included crown−group salamanders on the other. Pangerpeton is thus close to the base of crown−group Urodela (sensu Milner 1988; Evans and Milner 1996) either just outside it or just within. Further resolution requires more information on the skull and ear region. The only caudates lying basal to the trichotomy are the Jurassic Karaurus and Marmorerpeton .

Acknowledgments.—This joint work was supported by grants from the Royal Society of London−National Natural Science Foundation of China Joint Research Projects programme; Major Basic Research Projects of the Ministry of Science and Technology, China (G2000077700); and National Natural Science Foundation of China (40302008). Our thanks are due to Professor Christopher Dean ( UCL, University College London) for making the high fidelity cast of Pangerpeton .

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Yuan Wang [[e-mail]], Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 142 Xi−Zhi−Men−Wai St, P.O.Box 643, Beijing 100044, China;

Susan E. Evans [[e-mail]], Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, England.

Figures

Fig. 1. Pangerpeton sinensis gen. et sp. nov. A. Holotype specimen (IVPP V14244) from Wubaiding Village, Lingyuan City, Liaoning Province, China, probably contemporaneous with the salamander−bearing horizon at Daohugou, Inner Mongolia, Late Jurassic/Early Cretaceous, showing ventral impression of a nearly complete skeleton. B. High−fidelity cast of holotype showing skull, forelimbs and pectoral girdle. C. Line drawing of skull and atlas, taken from cast.

Fig. 2. Fifty percent Majority Rule Consensus of 28 most parsimonious trees resulting from a phylogenetic analysis using PAUP* (see character description in http://spaces.msn.com/members/ywangivpp, and matrix of taxon and character states in Table 2).