The first occurrence of Scincomorpha lizard from Nenjiang Formation (lower Campanian) of Jilin, Northeast China

YANG Jinyue, WU Wenhao, YU Kaifeng and SONG Qingqing

1. College of Earth Sciences, Jilin University, Changchun 130061, China;

2. Research Centre of Palaeontology and Stratigraphy, College of Earth Sciences, Jilin University, Changchun 130026, China;

3. Xinjiang Branch, Survey and Design Institute of Jiangxi Province,Urumqi 830000, China

Abstract: Fossil lizards are scarce in Jinlin Province, with only two jaws of Middle Eocene Acrodonta. Here the authors report the first scincomorph lizard materials from the early Campanian Nenjiang Formation in Nong’an, Changchun, Jilin Province. Three tooth morphologies of scincomorph lizards represented by fragmentary dentaries are tentatively identified as Scincomorpha. The morphology of the crowns indicates probably the insectivorous and omnivorous diets of these small lizards. It is the first lizard fossils discovered in the Late Cretaceous of Northeast China, extends the known paleogeographic distribution of scincomorph lizards,and provides new information about the terrestrial vertebrate fauna of the Cretaceous Songliao Basin.

Keywords: Songliao Basin; Nenjiang Formation; Scincomorpha; lower Campanian

Introduction

Squamata reptiles (lizards, snakes, and amphisbaenians) have the most diverse radiations among terrestrial vertebrates with a rich fossil record.Megachirella wachtleri(Middle Triassic, Italian Alps) is the oldest known stem Squamata (Renesto and Posenato,2003). Recent analysis with combined morphological and molecular data show that lepidosaurs originated before the Permian/Triassic extinction event, maybe in the Late Permian (Simõeset al., 2018; Boletet al.,2022). Scincomorpha is an infraorder of Squamata including skinks (Scincidae) and their close relatives.These include the living families Xantusiidae, Cordylidae, and Acontiidae, as well as the extinct taxa include Eoxantidae, Slavoiidae, Hodzhakuliidae, Globauridae,Ardeosauridae, Carusiidae, Parama-cellodidae (Alifanov,2016; 2019). The earliest fossil record of scincomorph is a paramacellodid from Bathonian (Middle Jurassic)of Kirtlington, UK (Evans, 1993; Evans & Chure,1998). The scincomorphs as a successful squamatas lineage with a global distribution range from Middle Jurassic to now.

Scincomorph lizards have been found during Jurassic to Cretaceous of China. Paramacellodid fossil fragments have been found in the Upper Jurassic of Xinjiang (Richteret al., 2010).Mimobecklesisaurus gansuensisLi, 1985 from the Lower Cretaceous of Gansu was also assigned to Paramacellodidae (Liet al., 2008; Gaoet al., 2021).Jeholacerta formosaJi and Ren, 1999 from the Lower Cretaceous of Hebei was identified as Lacertidae. ThenJeholacerta formosawas considered as a nomen dubium based on the lacking of obvious identification features (Evans & Wang, 2010).The following Scincomorpha incertae familiae were reported from the Cretaceous:Conicodontosaurus kanhsienensisYoung, 1973,C.djadochtaensisGilmore,1943,Teilhardosaurus carbonariusShikama, 1947,Pachygerys thlastesaGao and Cheng, 1999. Two species of Borioteiioidea are from Upper Cretaceous,includingTianyusaurus zhengiLüet al., 2008 andFuniusaurus luanchuanensisXuet al., 2014.Adamisaurus magnidentatusSulimski, 1972 from Upper Cretaceous of Inner Mongolia was arranged to Adamisauridae.Bainguissp., cf.B.parvusGao and Hou,1996 from the Upper Cretaceous of Inner Mongolia was re-examination with the aid of mCT scanning suggests that it was attributade toParmeosaurus, a scincoid lizard (Donget al., 2018).Sineoamphisbaena hexatabularisWuet al., 1993 from the Upper Cretaceous of Inner Mongolia was described as amphisbaenians, later it has been identified as Gilmoreteiidae(Kearney, 2003).

Chilingosaurus chingshankouensisYoung, 1961 from the Qingshankou Formation (Late Cretaceous) in Songyuan City, Jilin Province used to be attributed to Varanidae (Anguimorpha). Later, Estes (1983) removed it from squamatas based on dentary morphology. Two jaws of Acrodonta from the Middle Eocene of the Huadian Basin were the only fossil evidence of lizards ever found in Jilin Province. No Late Cretaceous lizard fossils have been reported in Northeast China.

Here, we describe a new scincomorph lizard assemblage recently collected from the Upper Cretaceous of Songliao Basin, Northeast China. Although the lizard materials are fragmentary, they show three tooth morphologies can be attributed to Scincomorpha. With the ongoing studies on the rest of the microvertebrate materials from this new fossil site,the new discoveries will improve our knowledge of the diversity and bioecosystem of Songliao Basin,Northeast China during Late Cretaceous.

1 Geological setting

The fossils described here were collected from the the upper part of the Nenjiang Formation in Songliao Basin, which is located in a valley close to Gengjiawopu village, about 10 km southeast of Fulongquan Township, Nong’an County, Changchun City, Jilin Province (Figs.1,2). The Songliao Basin is one of the largest Cretaceous terrestrial sedimentary basins in the world, it is the most complete and continuous Cretaceous continental strata in China (Wanet al.,2005; Houet al., 2018). The Cretaceous strata in the basin consist of nine formations, from bottom to top including Shahezi (K1s), Yingcheng (K1y), Denglouku(K1d), Quantou (K2q), Qingshankou (K2qn), Yaojia(K2y), Nenjiang (K2n), Sifangtai (K2s) and Mingshui(K2m) formations (Fig.3; Denget al., 2013; Xiet al., 2019). The Nenjiang Formation can be divided into five members based on its lithofacies characters(Yeet al., 2002; Chen, 2003). Members 1-2 mainly consist of grey, dark-grey mudstones; members 3-5 are dominated by grey-green, grey, brown-red mudstones,siltstones and sandstones (Wanget al., 2011). Our preliminary study on the ostracod samples collected from the fossil layer shows the ostracod fauna can be assigned to theTalicypridea augusta-Harbinia haplaassemblage, which is the typical ostracod assemblage yielding in the 4-5 members of the Nenjiang Formation(Yeet al., 2002; Jinget al., 2011; Xiet al., 2019).

Fig.3 Stratigraphic subdivision of Upper Cretaceous strata in Songliao Basin (modified after Xi et al., 2019; Yu et al., 2022)

Based on high-resolution palaeomagnetic and SIMS U-Pb dates, the age of the Nenjiang Formation has been constrained to late Santonian to middle Campanian (Wanget al., 2013; Xiet al, 2019; Yuet al., 2019; Yanget al., 2020). Considering the upper part of the fifth member of the Nenjiang Formation normally has been eroded at the eastern Songliao Basin (Wanget al., 2011), the age of the fossil layer was estimated to be ~80.5-81.6 Myr (Fig.3; Yuet al.,2022).

2 Methods

The specimens are fragmental dentaries collected from the early Campanian locality of Gengjiawopu(Jilin Province). The specimens are housed in the collection of the Research Center of Palaeontology and Stratigraphy (RCPS), College of Earth Sciences, Jilin University. The fossil-bearing sediment (approximately 0.8 tons) was collected manually and then washed through 10 mesh, 20 mesh, as well as 35 mesh sieves.The microvertebrates were separated under a light microscope and sorted taxonomically in the lab, using tweezers. The morphologies were photographed by Digital Microscope (Leica DVM6) and measured using Image J (Schneideret al., 2012). In addition, the specimens were scanned using the scanning electron microscope (JEOL JSM-6700F). Lizard tooth crown terminology follows Wick and Shiller, 2020 (Fig.4).

Fig.4 Terminology of a schematic scincomorphan lizard tooth crown ( Wick & Shiller, 2020)

3 Systematic paleontology

Squamata Oppel, 1811.

Scincomorpha Camp, 1923.

Scincomorpha incertae familiae

Species 1 (Fig.5 a-b).

Fig.5 Scincomorpha incertae familiae lizards from Jilin Province

Referred Specimen: RCPS-VJ3011 (a fragmentary dentary).

Description:RCPS-VJ3011 is a dentary fragment preserving two pleurodont teeth which are tightly spaced and have some modest amounts of cementum at the tooth bases. It lost the subdental shelf and the sulcus dentalis. All crowns possess well-developed cristae medialis and distalis. The apex of the teeth are inclined such that the crista mesialis and distalis are asymmetrical. The tip of the main cusp is divided into a cuspis labialis and a lower cuspis lingualis. The cuspis labialis is formed by the sharp edged cristae mesialis and distalis, whereas the cuspis lingualis is formed by the striae dominantes anterior and posterior.Both cusps are connected by the straight ridge of the carina intercuspidalis. The tooth crowns are slightly convex and smooth labially and they are concave and striated lingually. The striations are only weakly developed on the smaller teeth. Tooth shafts are slender and cylindrical. The bases of the teeth expand somewhat labiolingually. A large circular resorption pit is at the base of the larger tooth.

Remarks:The tooth crown morphology of RCPSVJ3011 is the closest comparison to that of the specimens of the paramacellodidMimobecklesisaurus gansuensis(Li, 1985; Fig.2),Becklesiussp (Kosma,2004; Plate 14, Fig.7), andParasaurillus robustus(Evanset al., 2012). With scanning electron microscopy, the tooth tips are similar toBecklesiusandParasaurillus. The teeth are pointed, blunt, and fine striations on the lingual surfaces; cusps labialis and lingualis are connected by the straight ridge of the carina intercuspidalis. In addition, the teeth are homodont, cylindrical and closely spaced; the crowns are round to oval, lingually concave, cristae mesialis and distalis that are unequal in length giving the crown an asymmetrical shape; teeth bases are labiolingually expand, which make RCPS-VJ3011 similar to paramacellodid lizards. However, its status cannot be clarified until more complete material becomes available.

Fig.2 Geological setting of the lizard fossils locality

Species 2 (Fig.5 c-d).

Referred Specimen:RCPS-VJ3012 (a fragmentary dentary).

Description:RCPS-VJ3012 preserves four teeth positions and two complete teeth. Teeth are closely spaced, pleurodont and vertical. The complete tooth crowns are perfectly blunt, with well-developed cristae medialis and distalis. The culmen lateralis anterior and posterior turn lingually as they extend basally from the anguli mesialis and distalis. The weakly cuspis lingualis is formed by the striae dominantes anterior and posterior. The carina intercuspidalis between cuspis labialis and lingualis is very short and straight. Striations are restricted to the lingual surfaces of the tooth crowns. The tooth shafts are dumpy and cylindrical and the bases of the teeth expand labiolingually.

Remarks:The tooth morphology of the RCPSVJ3012 is identical to the tooth morphology of RCPSVJ3011. RCPS-VJ3012 differs from RCPS-VJ3011 in having developed anguli mesialis and distalis;culmen lateralis anterior and posterior turn lingually as opposed to basally; distinct striations and dumpy shafts. The crowns of RCPS-VJ3012 are similar toBecklesiussp. especially in tooth neck bulbous,teeth wide spaced and prominent cristae mesialis et distalis (Kosma, 2004; Plate 14, Fig.7). Although the differences exist among the RCPS-VJ3012 to RCPSVJ3011, the fragmentary nature of these materials do not provide enough support to fully exclude the possibility of an individual, ontogenetic variation and/or corrosion by the gastric fluids of predators. It cannot be also excluded that the different morphology might be a result of a different position along the tooth row in dentaries as well.

Species 3 (Fig.5 e-f).

Referred Specimen:RCPS-VJ3013 (a fragmentary dentary).

Description:RCPS-VJ3013 is a dentary fragment preserving two teeth. Teeth are pleurodont, slender,cylindrical, tightly spaced with lingually slightly flattened tooth crowns. The cuspis labialis and the cuspis lingualis are not separated. A blunt crown consists of cristae medialis and distalis. The large medial projection is present on the lingually. The tooth crown completely lacks striations and the base of teeth are not transverse expansion.

Remarks:The tooth crown morphology of RCPSVJ3013 is similar to hodzhakuliids and xantusiids,such asPlatynotoides altidentatus(Alifanov, 2016;Fig.4c) andXantusia vigilis(Kosma, 2004; Plate 13, Fig.15-16). The teeth bases are rounded, without widened transversely and flattened rostroccipitally and apical part flattened lingually makes it different from hodzhakuliids. The tooth shape is more similar toX. vigilis, different from the Upper Cretaceous xantusiidsContogenyswith chise-llike tooth crowns from late Maastrichtian to middle Paleocene, United States (Nydam, 2009). Pending the recovery of more diagnostic material, the new specimens can only tentatively be referred to Scincomorpha.

Fig.1 Fossil location (circle), Songliao Basin (after Qin et al., 2020)

4 Discussion

The materials described here are fragmentary and consist of only three dentaries. Although fragmentary nature of the new materials, they represent the first occurrence of scincomorph lizards in the Upper Cretaceous of Northeast China. The materials preserved only limited characters and can thus not be identified more exactly.

RCPS-VJ3011 and RCPS-VJ3012 are similar to Paramacellodidae according to the tooth implantation(pleurodont); structure of the tooth crowns (tooth apices are lingually concave and striations), and labiolingually expanded at their bases (Kosma, 2004;Alifanov, 2019; Gaoet al., 2021). The new specimens do not preserve rectangular osteoderms, so it is hard to attributed it to Paramacellodidae exactly. They can only be temporarily assigned to Scincomorpha until more diagnostic materials are available. Tooth structures of the RCPS-VJ3013 are too generalized to arrange it to particular clade without doubts, it can only tentatively be referred to Scincomorpha.

The apical angles of lizard teeth are thought to be related to diet (Kosma, 2004). Chisel-like tooth crowns help grind plants and are often found in herbivorous lizards, while sharp crowns help penetrate the exocuticle of the prey and are often found in insectivorous lizards. The absence of anguli indicates an increased proportion of insect prey. RCPS-VJ3011 has rounded crowns without anguli indicating an arthropod-dominated trophic structure which is analogous toParamacellodusHoffstetter, 1967. RCPSVJ3012 has rounded crowns with prominent anguli indicating a more omnivorous diet which is similar toBecklesius. The tooth shape of RCPS-VJ3013 is similar to that of the extant night lizardXantusia,indicating it may be an insectivorous species.

These lizard fossil assemblages cover a newly discovered site, including already discovered theropod dinosaurs (Yuet al., 2022), the ongoing studies on the rest of the microvertebrate fossils, together with the crocodyliform (Paralligator sungaricusSun, 1958 from Dehui County andRugosuchus nonganensisWuet al., 2001 from Fulongquan Town) contributing to a better understanding terrestrial vertebrate fauna of Songliao Basin during Late Cretaceous.

5 Conclusions

Three fragmental dentaries of scincomorph lizards are described from the early Campanian Nenjiang Formation, Jilin Province. The specimens represent at least two scincomorph lizard taxa, and the first occurrence of Upper Cretaceous lizards in Northeast China. Based on the morphology of the tooth, the scincomorph lizards are speculated with insectivorous and omnivorous diets. Together with the dinosaurian and crocodyliform fossils from the same formation in Jilin province, contributing to a better understanding of the terrestrial vertebrate fauna in Northeast China during Late Cretaceous.

Acknowledgements

We thank Dr. Yi Hongyu (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Science, China) for expertise and helpful advice on this work. We appreciate all members of the excavation team for fieldwork and microscopic collection of some specimens.