Exploring the first occurrence of Scincomorpha lizard from Nenjiang Formation (lower Campanian) of Jilin, Northeast China: a follow-up study

MUHAMMAD Hassan, WU Wenhao and YANG Jinyue

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. Key Laboratory of Stratigraphy and Palaeontology, Ministry of Natural Resources, Beijing 100037, China

4. Core and Sample Center of Natural Resources, China Geological Survey, Sanhe 065201, Hebei, China

Abstract: In a region where lizard fossils are rare, this study explores a groundbreaking discovery of squamate lizard materials from the upper part of Nenjiang Formation (early Campanian) in Jilin Province,Northeast China.Despite the fragmentary nature of the dentaries, they exhibit distinct tooth morphological characteristics of squamate lizards, suggesting insectivorous and omnivorous diets for these small reptiles.This finding not only represents a significant addition to the squamate lizard fossil records from the Late Cretaceous in Northeast China but also contributes to our understanding of the paleogeographic distribution of squamate lizards.Furthermore, it sheds new light on the terrestrial vertebrate fauna of the Cretaceous Songliao Basin.

Keywords: squamates; Late Cretaceous; Nenjiang Formation; Songliao Basin

Introduction

The discovery of fossilized reptilian remains provides a unique window into the ancient ecosystems that once thrived on our planet.Squamates possess an extensive global evolutionary lineage that can be traced as far back as the Early Jurassic and, in some interpretations, possibly the Late Triassic (Evans, 2003;Datta & Ray, 2006; Evans & Jones, 2010; Hutchinsonet al., 2012).The Cretaceous marked a significant phase in the diversification of squamates, which include both lizards and snakes.The fossil records of Squamata span a wide distribution across North and South America (Nydam & Cifelli, 2002; Nava &Martinelli, 2011; Simõeset al., 2015; DeMaret al.,2017; Bittencourtet al., 2020; Viñola-Lópezet al.,2022), North Africa (Longrichet al., 2021; Longrichet al., 2023), Europe (Bolet & Evans, 2012; Makádi,2013; Evans & Bolet, 2016; Hernándezet al., 2022),Central and Eastern Asia, Mongolia (Gao & Nessov,1998; Conrad & Norell, 2007; Norellet al., 2008;Moet al., 2012; Parket al., 2015; Donget al., 2017;Donget al., 2023), documenting their existence from the Late Jurassic to the present day (Alifanovet al.,2000; Evanset al., 2004; Wick & Shiller, 2020).These fossils represent a diverse array of ecological niches,including terrestrial, scansorial, aquatic, and gliding forms (Donget al., 2023).China has yielded some of the most well-preserved lizard fossils.Notably,certain taxa likeYabeinosaurusandDalinghosaurusare represented by multiple specimens, providing valuable insights into their growth, variation, and lifestyles (Wang & Evans, 2011; Evans &Wang,2012).Many of these Chinese lizard fossils originate from western Liaoning and neighboring areas and are integral components of the renowned Jehol Biota.The world of paleontology has long been captivated by the diverse radiations of squamate reptiles, encompassing the remarkable diversity of lizards, snakes, and amphisbaenians.This group of terrestrial vertebrates,known for their rich fossil records, has offered invaluable insights into the evolution of reptilian life on Earth.At the heart of this evolutionary puzzle liesMegachirella wachtleri, a Middle Triassic reptile from the Italian Alps recognized as the oldest known stem Squamata, highlighting the ancient lineage of these fascinating creatures (Renesto & 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, Paramacellodidae(Alifanov, 2016; 2019).The earliest fossil record of scincomorpha is a paramacellodid from the Bathonian(Middle Jurassic) of Kirtlington, UK (Evans, 1993;Evans & Chure, 1998).Scincomorphs represent a successful lineage of squamates with a global distribution that spans from the Middle Jurassic to the present day.

In the study conducted by Yanget al.in 2022,new scincomorph lizard fossils from the Upper Cretaceous of Songliao Basin were described.The material described previously was fragmentary and consisted of only three dentaries.The RCPSVJ3011 and RCPS-VJ3012 squamate fossils have been identified as Paramacellodidae.RCPS-VJ3013 was assigned to Scincomorpha as of lack of skeletal material it was hard to assign it to a particular clade.Despite the fragmentary nature of the lizard materials,they exhibit three tooth morphologies that can be attributed to Scincomorpha.Ongoing studies on the remaining microvertebrate materials from this new fossil site are expected to enhance our understanding of the diversity and bioecosystem of the Songliao Basin in Northeast China during the Late Cretaceous.

This research paper embarks on a captivating journey into the past, exploring the first documented occurrence of Scincomorpha lizards within the Nenjiang Formation of the lower Campanian in the Songliao Basin, Jilin, Northeast China.Through a comprehensive analysis and examination of these fossilized remnants, this study seeks to shed light on the biodiversity of the Late Cretaceous Squamata fauna in the Songliao Basin.While not permitting detailed classification, the newly discovered reptile fossils from the Late Cretaceous Nenjiang Formation in the Songliao Basin represent a highly diverse group of squamate reptiles, holding significant scientific value.They provide valuable insights into our understanding of prehistoric ecosystems and the evolutionary history of these enigmatic reptiles.Furthermore, this study explores the similarities in Squamata fauna composition between this region and neighboring areas, such as North America, Central Asia, Mongolia,and Inner Mongolia during the Late Cretaceous.

1 Geological Setting

The fossils under consideration in this study were recovered from the upper part of the Nenjiang Formation in Songliao Basin, which is located in Gengjiawopu village, Fulongquan Town, Nong’an County, Jilin Province (Fig.1).The Songliao Basin is a significant geological region, covering parts of Heilongjiang, Jilin, Liaoning, and Inner Mongolia Autonomous Region.It is one of the world’s largest typical continental sedimentary basins and hosts a comprehensive sequence of Cretaceous continental strata (Wanet al., 2005; Houet al., 2018) (Fig.1 ).

Fig.1 Fossil location Songliao Basin (Yang et al., 2022)

The Upper Cretaceous sedimentary strata within the Songliao Basin consist of six distinct formations(Fig.2 Denget al., 2013; Xiet al., 2019).The Nenjiang Formation, within which the fossils were found, is divided into five members (Yeet al., 2002; Chen,2003).The ostracod specimens collected from the fossil layer are assigned to theTalicypridea augusta-Harbinia haplaassemblage, which is represented in the fourth-fifth members of the Nenjiang Formation(Yanget al., 2022).

Recent research has dated the Nenjiang Formation to the Late Santonian to Campanian stages (Wanget al., 2013; Xiet al., 2019; Yuet al., 2019; Yanget al., 2020).Therefore, the fossil-bearing layers in this region are inferred to be from the Early Campanian(Yanget al., 2022, Yuet al., 2023).

2 Methodology

Fig.2 Stratigraphic column of Nenjiang Formation in Songliao Basin (Yang et al., 2022.Fossil layer indicated by shadows)

The specimens under consideration consist of partial jawbones obtained from the early Campanian site known as Gengjiawopu in Jilin Province.These specific specimens are currently curated at the Research Center of Palaeontology and Stratigraphy (RCPS),Jilin University.The sediment, found to contain fossils with an estimated weight of approximately 2 tons, underwent a systematic collection process,and subsequently underwent a series of procedures.To separate the microvertebrate fossils, these rock samples were then washed through a series of sieves with mesh sizes of 10, 20, and 35, respectively.Following this sieving, the rock samples were allowed to air-dry within an indoor environment.They were then packed into sample bags according to the wet sieve size and labeled with sample numbers and their respective origins.In the final step, microvertebrates were meticulously separated under the illumination of a light microscope and systematically classified in a laboratory setting, using precision tools such as tweezers.The morphological characteristics of the specimens were documented by means of a Digital Microscope (Leica DVM6), and precise measurements were taken with the assistance of Image J software(Schneideret al., 2012).Furthermore, the specimens were subjected to scanning electron microscopy using a JEOL JSM-6700F instrument.Fossil photos are processed using the image editing software Adobe Photoshop and CorelDRAW.

3 Systematic Paleontology

Squamata Oppel, 1811.

Squamata indet.

Morphotype 1 (Fig.3a,b).

Referred Specimen: RCPS-VJ3014 (an incomplete jawbone, retaining two nearly complete teeth).

Fig.3 Squamata teeth, Morphotype 1, RCPS-VJ3014

Description: The RCPS-VJ3014 fragment represents a section of a jawbone, measuring 1.74 mm in length and 1.37 mm in height.The dentary exhibits a robust subdental shelf, whereas the Meckel’s groove is not preserved.The pleurodont teeth are robust and conical, closely spaced.The tooth crowns are rounded and blunt, with the smaller tooth bearing a small pit.Fine, roughly parallel longitudinal ridges extend from the base of the tooth to its crown.Additionally, the smaller tooth displays a circular resorption pit at its base.

Remarks: Specimen RCPS-VJ3014 exhibits prominent vertical ridges on the tooth’s shaft, which is similar to the lizards of Anguimorpha.However, in Anguimorpha, the tooth base has well-developed basal infoldings that extend to the dentine and the cementum forms a circumferential ridge around the base, distinguishing it from RCPS-VJ3014.Nonetheless, the overall morphological characteristics of RCPS-VJ3014 are commonly found in squamates.However, based solely on dental features, it is insufficient to assign it to a specific family or group within Squamata.Therefore, it is provisionally categorized under the order of Squamata.

Morphotype 2 (Fig.4a-c).

Referred Specimen: RCPS-VJ3015 (an incomplete jawbone and two teeth).

Description:

The specimen RCPS-VJ3015 consists of two teeth with different degrees of completeness, including one complete tooth and one with a tooth missing crown.The preserved jawbone measures 2.56 mm in length and 1.65 mm in height.The teeth are conical in shape with rounded crowns, displaying pronounced labiolingual compression and anterior-posterior expansion.Both teeth have anterior and posterior ridges that extend from the crown to the base.

Remarks: The teeth of specimen RCPS-VJ3015 are conical with prominently compressed crowns, resembling those of the middle and posterior teeth discovered in the Djadokhta Formation of Inner Mongolia,Isodontosaurus(Borsuk-Bialynicka & Moody, 1984).However, due to the poor preservation of the fossil, it is not possible to discern identification characteristics at the suborder level.As a result, it can only be classified within the order Squamata.

Morphotype 3 (Fig.5a, b).

Fig.5 Squamata teeth, Morphotype 3, RCPS-VJ3016

Referred Specimen: RCPS-VJ3016 (an incomplete jawbone, with two teeth of different levels of completeness preserved).

Description:

Specimen RCPS-VJ3016 consists of a jawbone fragment with an intact complete tooth and another tooth missing its crown.The jawbone measures 1.54 mm in length and 1.26 mm in height.The teeth are conical with pronounced labiolingual compression, resulting in a flattened appearance.A single ridge is present on one side of the teeth, extending from the crown to the base.

Remarks: The tooth morphology of specimen RCPSVJ3016 closely resembles that of specimen RCPSVJ3015, with the primary distinction being the presence of ridges on the anterior and posterior edges of the teeth.These variations are likely attributable to differences in tooth positioning along the jawbone,resulting in the observed morphological distinctions.

Morphotype 4 (Fig.6a-c).

Referred Specimen: RCPS-VJ3017 (an incomplete left mandible, with four relatively complete teeth preserved).

Description: RCPS-VJ3017 is a fragment of the left dentary.Its maximum total length is 3.56 mm.Meckel’s groove is fully open, becoming gradually ventrad and narrower anteriorly.At the level of the first anterior tooth, the dentary is obviously angled,i.e.the anterior region with the symphysis declines ventrally.While at the level of the second and third teeth, the dentary rises dorsally.The dentary symphysis is weakly developed and comparatively rounded.The subdental shelf is broad and the subdental gutter developed weakly.Two large mental foramina are preserved on the smooth lateral surface of the dentary.The anterior one is positioned at the level of the first tooth and is slightly larger than the posterior one,which is positioned at the level of the posterior part of the third tooth.

The dentary teeth are pleurodont, homodont,markedly inclined anteriorly (except for the last posterior teeth, which are straight or slightly inclined posteriorly).All teeth are closely spaced, labiolingually compressed.The second apex is recurved with a pit on the labial side.It cannot be fully excluded that the pit appeared due to occlusal tooth wear.The last posterior teeth crown presents the dorsoventral bevel.The shaft of the teeth is significantly swollen compared to the apices and has vertical striations weakly developed on the labial side (unfortunately, the third tooth has a broken apex).

Fig.6 Squamata dentary with teeth , Morphotype 4, RCPS-VJ3017

Remarks: Specimen RCPS-VJ3017 lacks characteristics that would permit suborder-level identification.Thus, it can only be classified within the order Squamata, with the suborder remaining undetermined.The specimen exhibits slight vertical striations on the labial side of the tooth shaft, while the lingual side remains smooth.The formation of these striations may be attributed to postmortal and diagenetic processes or corrosion of gastric fluids of vertebrate predators(Čerňanskýet al., 2016; Smithet al., 2021).The enamel of the specimens is undamaged, while the shaft is corroded with striations, similar to the Type 2 corrosion found in early Paleocene squamate lateral teeth from Wyoming, United States.This type of erosion is commonly observed in Turtle Graveyard(Smithet al., 2021; Fig.5).The striations on RCPSVJ3017 are consistent with the corrosion striations,which provides new evidence for studying the role of corrosion in fossils.

Morphotype 5 (Fig.7a,b).

Referred specimen: RCPS-VJ3018 (A single preserved tooth along with a portion of the attached jawbone).

Description:

RCPS-VJ3018 is a tiny jaw fragment bearing only one pleurodont tooth.The jawbone measures 1.78 mm in length and 1.98 mm in height.At the tooth base, there is a very small resorption pit preserved on the lingual side, and covered by cement.The tooth is cylindrical,blunt tip, without labiolingual compression.The dorsoventral bevel on the lingual side of the tooth crown results in a slight concavity on the lingual side.Some deep vertical striations developed on the shaft of the teeth.

Fig.7 Squamata tooth, Morphotype 5, RCPS-VJ3018

Remarks: Specimen RCPS-VJ3018 displays distinct erosion striations on the tooth shaft, akin to those observed on RCPS-VJ3017.The tooth crown of RCPSVJ3018 features a dorsoventral bevel, reminiscent of the tooth crown inPalaeoblanus tobieni(Schleich,1988).The presence of a resorption pit on the lingual posterior side of the tooth base is a characteristic shared with the Anguimorpha and Amphisbaenia(Gans, 1957).However, due to the poor preservation of the fossil, it is not possible to provide an exact classification for this specimen.

4 Discussion

This study further explores the work of Yanget al.2022, marks the first discovery of the Late Cretaceous squamate fauna in the Songliao Basin,shedding light on an area with limited prior research.Despite the fragmented preservation of the fossils,the fossils have successfully classified within the order Squamata, although the suborder remains undetermined.Furthermore, an intriguing aspect of this study is the analysis of tooth size, which implies that Late Cretaceous reptiles in the Songliao Basin were primarily small-sized, likely having an insectivorous and omnivorous diets.This dietary information adds a valuable dimension to our understanding of the ecological dynamics in the Late Cretaceous.

Comparatively, the Late Cretaceous reptile fauna in the Songliao Basin shares remarkable similarities with similar fauna found in other regions, such as North America, Mongolia, and Inner Mongolia, China.This finding raises important questions regarding biogeography and the potential connections between these regions during the Late Cretaceous.Further research could explore the mechanisms that allowed for these similarities, including continental drift or migratory patterns of these reptiles.

The individual specimen, RCPS-VJ3014 showcases intriguing morphological characteristics.The prominent vertical ridges on the tooth’s crown, resembling those found in the suborder of lizards known as Anguimorpha are noteworthy.However, the study rightly points out differences between this specimen and Anguimorpha regarding the basal involution and ridges around the tooth’s base.This leads to a broader discussion about the challenges in classifying specimens based solely on dental features, emphasizing the importance of considering multiple morphological aspects for a more precise classification.

Specimen RCPS-VJ3015, which bears similarities to teeth from the Djadokhta Formation of Inner Mongolia, namelyIsodontosaurus, is an exciting discovery.However, the poor preservation of the fossil limits the identification characteristics at the suborder level, restricting its classification to the order Squamata.This raises questions about the influence of preservation on the accuracy of taxonomic identification in paleontology and the need for innovative techniques to enhance the study of poorly preserved specimens.

Specimens RCPS-VJ3016 and RCPS-VJ3017 provide insights into the potential effects of tooth positioning and postmortal and diagenetic processes on morphological distinctions.RCPS-VJ3017 offers a a unique contribution through the observation of erosion striations on the tooth shaft, potentially influenced by postmortal and diagenetic processes or corrosion of gastric fluids of vertebrate predators.These observations provide novel evidence for the study of erosion effects in fossils and can expand our knowledge of the interactions between ancient reptiles and their predators or environmental conditions.

Finally, RCPS-VJ3018 exhibits features reminiscent of bothPalaeoblanus tobieniand the suborders Anguimorpha and Amphisbaenia.However, due to the fossil’s poor preservation, it is impossible to provide an exact classification for this specimen.

5 Conclusions

This study represents a significant step forward in our understanding of the Late Cretaceous reptile fauna in the Songliao Basin.Building upon the pioneering work of Yanget al.(2022) marks the first discovery of these ancient reptiles in a region that has seen limited prior research.Despite the challenging state of fossil preservation characterized by fragmentation,the fossils have been successfully classified within the order Squamata, providing valuable insights into the past biodiversity of the Songliao Basin.One of the most intriguing aspects of this research is the analysis of tooth size and morphology, which has implications for the dietary habits of Late Cretaceous reptiles in the Songliao Basin.The evidence suggests that these reptiles were primarily small-sized and likely had insectivorous and omnivorous diets.Furthermore, the discovery of similarities between the Late Cretaceous reptile faunas in the Songliao Basin and other regions,opens up essential questions related to biogeography and potential connections between these geographically distant areas.

RCPS-VJ3016 and RCPS-VJ3017 offer insights into the potential effects of tooth positioning and postmortal and diagenetic processes, which further our understanding of the complexities involved in interpreting fossil data.Additionally, the observation of erosion striations on the tooth shaft of RCPS-VJ3017, influenced by postmortal and diagenetic processes or corrosion of gastric fluids of vertebrate predators, contributes novel evidence to the study of erosion effects in fossils, expanding our knowledge of the interactions between ancient reptiles and their predators or environmental conditions.

This study significantly contributes to the knowledge of Late Cretaceous squamate fauna in the Songliao Basin, five fragmental dentaries of lizards were described.It prompts intriguing questions about biogeography, preservation challenges, and the complexities of taxonomic classification in paleontology.It underscores the importance of considering a wide range of factors and employing innovative techniques in the study of fossils, ultimately enhancing our understanding of prehistoric life and ecosystems.Lastly,RCPS-VJ3018, with its resemblances toPalaeoblanus tobieniand the suborders Anguimorpha and Amphisbaenia, underscores the impact of preservation on our ability to classify specimens accurately.In cases of poor preservation, challenges in taxonomy persist,emphasizing the need for further research to address these limitations.

Acknowledgments

We thank the quarry owner, Mr.Zhang Tingchang,for granting permission to excavate, and all excavation team members.We thank Dr.Krister Smith and Dr.Yi Hongyu for their constructive comments and suggestions.