張蜀康

(中國科學院古脊椎動物與古人類研究所，中國科學院脊椎動物演化與人類起源重點實驗室 北京 100044)

1 Introduction

Young (1959) described a large eggshell fragment collected in the middle part of Wangshi Group from Zhaotuan, Laiyang, Shandong Province.Due to its extremely large thickness of 5-7 mm, it was namedO?lithesmegadermus, which means a fossil egg with a thick eggshell (Young, 1959).In a review of the spherical dinosaur eggs from Laiyang, Shandong Province, Zhao and Jiang (1974) comparedO.megadermuswith other specimens and confirmed that it represented an independent oospecies.Zhao (1979) erected the Spheroolithidae based on the spherical dinosaur eggs from Laiyang.The large eggshell fragment was assigned to the type oogenusSpheroolithusof the Spheroolithidae.However, this assignment was uncertain due to the lack of comparative material (Zhao, 1979; Zhao et al., 2015).In a reassessment of the spherical dinosaur eggs housed at the Dalian Natural History Museum (DLNHM) from Changtu, Liaoning Province, Liu et al.(2013) described a half spherical egg with an eggshell thickness of 4.8-5.2 mm and considered it as a referred specimen ofSpheroolithusmegadermus.

Recently, a new type of dinosaur egg,Multifissoolithuswas reported in Yiwu, Zhejiang Province, China and Shimonoseki, Yamaguchi, Japan, including two oospeciesM.chianensisandM.shimonosekiensis.The eggshell ofMultifissoolithusis 3.21-4.70 mm thick, with roughly paralleled and wavy clefts on the outer surface (Zhang et al., 2019; Imai et al., 2020).This ootaxon shared several characteristics withSpheroolithussuch as a relatively thick eggshell and isolated eggshell units with triangular cunei near the inner surface of eggshell, but the clefts throughout the whole eggshell were not seen inSpheroolithus(Zhang et al., 2019).In this study, the eggshell microstructure in the tangential section of the large eggshell fragment from Laiyang, Shandong Province was studied for the first time, revealing the microstructural similarity between the large eggshell fragment from Laiyang and theMultifissoolithusfragment from Yiwu.Thus, the large eggshell fragment from Laiyang was reassigned toMultifissoolithus, and a new oospecies ofSpheroolithuswas erected based on the specimen from Changtu, Liaoning Province which indeed belongs toSpheroolithus.The revision of the large eggshell fragment from Laiyang also extended the chronological and spatial distribution of dongyangoolithid eggs.

2 Material and methods

A small piece of eggshell fragment was cut from the large eggshell fragment (IVPP V 2337) from Laiyang, Shandong Province and afterwards was embedded in EXAKT Technovit 7200 one-component resin.The eggshell fragment was cut radially and tangentially using an EXAKT 300CP cutting system.The thin sections were grinded and polished to the thickness of about 50 μm using an EXAKT 400CS variable speed grinding system with P500 and P4000 abrasive papers.The thin sections were observed and photographed under normal and polarized lights using a Zeiss Axio Imager A2 polarized light microscope.The large eggshell fragment and the thin sections were catalogued at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (IVPP).

3 Systematic paleontology

Dongyangoolithidae Zhang et al., 2019

Revised diagnosisSpherical to oval eggs, smooth outer surface with clefts and round pores, eggshell unit assemblages separated by large cavities in radial sections, a compact layer composed of cones near the inner surface of eggshell.

Multifissoolithus Zhang et al., 2019

Revised diagnosisEggs with a diameter of 8-10 cm randomly arranged in nest, eggshell 3.21-5.70 mm thick composed of cone-shaped or columnar eggshell unit assemblages, roughly paralleled and wavy clefts on the outer surface connecting to the large cavities between eggshell unit assemblages in the inner portion of the eggshell.Horizontal accretion lines throughout the whole eggshell.

Multifissoolithus megadermus (Young, 1959) comb.nov.(Figs.1-4)

O?lithes megadermusYoung, 1959, p.34-35; Zhao and Jiang, 1974, p.66, pl.4: 8

Spheroolithus megadermusZhao, 1979, p.332; Wang et al., 2012, fig.6

Spheroolithus megadermusZhao et al., 2013, p.4665, fig.2; Zhao et al., 2015, p.67, figs.44A, 45

HolotypeA large eggshell fragment (IVPP V 2337).

Locality and horizonZhaotuan, Laiyang, Shandong Province, Upper Cretaceous Jiangjunding Formation.

Revised diagnosisEggshell 5.7 mm thick composed of cone-shaped eggshell unit assemblages, secondary eggshell units in the large cavities between eggshell unit assemblages and the clefts and pores on outer surface.

DescriptionAfter removing the red sandstones on the outer surface of the large eggshell fragment, roughly paralleled and wavy clefts can be observed clearly with the naked eye (Figs.1, 2).The eggshell fragment is 5.7 mm in thickness.In radial views, horizontal accretion lines are distributed evenly throughout the whole eggshell.The cones form a dark compact layer with a light stripe and distinct radial microstructure near the inner surface of the eggshell.There are large cavities above the compact layer.A few cone-shaped eggshell unit assemblages extended to the outer surface of the eggshell from the compact layer.Near the outer surface, adjacent eggshell unit assemblages fuse together (Fig.3A, B).Meanwhile, secondary eggshell units are distributed randomly in the clefts between the eggshell unit assemblages and block some of the round pore openings on the outer surface (Figs.3C, 4A-C).In tangential views, the clefts become wider gradually towards the inner surface of the eggshell (Fig.4A-C).Near the inner surface of the eggshell, polygonal eggshell units were separated by large cavities (Fig.4D-F).Also, in the tangential view through the dark compact layer, all eggshell units are fused extensively with clear boundaries between each other.Only a few round and elongated pores were visible (Fig.4G, H).

Fig.1 Top (A) and lateral (B) views of the large eggshell fragment (IVPP V 2337) from Laiyang, Shandong Province Scale bars equal 1 cm

ComparisonThe large eggshell fragment can be easily assigned toMultifissoolithusof Dongyangoolithidae instead ofSpheroolithusof Spheroolithidae based on the roughly paralleled and wavy clefts on the outer surface, as well as the tangential sections through the outer portion (Figs.1, 2, 4A-C).By contrast, the eggshell units are fused extensively in the outer portion of the eggshells ofSpheroolithus, with only irregular cavities left between the eggshell units (Zhao and Jiang, 1974; Liu et al., 2013; Zhao et al., 2015; Zhu et al., 2021).Although the microstructure of the eggshell is similar to the other oospecies ofMultifissoolithus, its extreme thickness significantly exceeds that of all knownMultifissoolithuseggs (Zhang et al., 2019; Imai, 2020).Thus, a new combinationM.megadermusis erected based on the large eggshell fragment.

Fig.2 Enlargement of the roughly paralleled and wavy clefts and the round pores (white arrows) on the outer surface of IVPP V 2337 Scale bar equals 0.5 cm

Spheroolithidae Zhao, 1979 Spheroolithus Zhao, 1979

Revised diagnosisSpherical to subspherical eggs with 74-99 mm in length, eggshell 2.2-5.5 mm in thickness, cone-shaped eggshell unit assemblages separated by large cavities in the inner portion of eggshell, fusing extensively in the outer portion of eggshell, irregular cavities between eggshell units in the outer portion of eggshell, slender or branched eggshell units separated from each other near the outer surface of eggshell.

Fig.3 Eggshell microstructure of radial sections of Multifissoolithus megadermus comb.nov.(IVPP V 2337)A.radial section parallel to the clefts; lines at the left side indicate rough positions of Fig.4A-H in ascending order; B.enlargement of the dark compact layer near the inner surface of eggshell under polarized light, showing the radial microstructures of the cones; nucleation center is indicated by the white arrow; C.secondary eggshell units in the cleft between eggshell assemblages near the outer surface of eggshell (black and white arrows).Scale bars equal 1 mm in A, 0.5 mm in B and 0.4 mm in C

Spheroolithus quantouensisoosp.nov.(Liu et al., 2013:figs.6-8)

Etymology“quantou” refers Quantou Town, the locality where the specimen was collected.

HolotypeA half egg with secondary calcite crystals inside (DLNHM D154).

Locality and horizon1 km southwest of Quantou Railway Station, Changtu, Liaoning Province, Upper Cretaceous, upper portion of the second member of the Quantou Formation (Liu et al., 2013).

DiagnosisSpherical egg with 9.0 cm×8.0 cm in size, eggshell 4.8-5.5 mm thick with dark strips in the middle portion and horizontal dark lines in the outer portion.

ComparisonThis specimen is very similar toSpheroolithus spheroidsandS.chiangchiungtingensisexcept for the following features: the eggshell is much thicker than that ofS.spheroids(2.4-3.2 mm, Liu et al., 2013) andS.chiangchiungtingensis(2.20 mm, Zhao et al., 2015); in the outer portion of the eggshell, the eggshell units have more branches and lower density of horizontal dark lines thanS.spheroidsandS.chiangchiungtingensis.

Fig.4 Eggshell microstructure of tangential sections of Multifissoolithus megadermus comb.nov.(IVPP V 2337)A-C.through the outer portion of eggshell, the white arrows indicate the secondary eggshell units in the clefts and pores; D-F.through the inner portion of eggshell, showing the wavy eggshell units assemblages gradually split into isolated eggshell units; G, H.through the dark compact layer near the inner surface of eggshell, the white arrows in H indicate round and elongated pores Scale bars equal 0.5 mm

4 Discussion

Spheroolithuswas first reported in Laiyang, Shandong Province, China, including two oospeciesS.chiangchiungtingensisand ?S.megadermus(Zhao, 1979).In subsequent studies,Spheroolithuseggs were reported in Liaoning Province, China, Mongolia, Kazakhstan, Spain, U.S.A.and Canada (Hirsch and Quinn, 1990; Mikhailov, 1991, 1994, 1997; Zelenitsky and Hills, 1997; Bray and Hirsch, 1998; Lucas et al., 2012; Liu et al., 2013; Sellés et al., 2014), but all of them were reassigned toStromatoolithusbased on their ornamentation, pore canals and accretion lines except for those from Liaoning Province, China (Zhu et al., 2021).Thus, the distribution ofSpheroolithuswas restricted to the Shandong and Liaoning provinces in China.In recent studies,S.chiangchiungtingensiswas split into two oospecies, includingS.spheroidsandS.chiangchiungtingensis, whose holotypes were IVPP V 730 and IVPP RV 74002 (field number G5547), respectively (Zhao et al., 2013, 2015).?Spheroolithusmegadermusbecame an undoubted oospecies inSpheroolithusbased on the similarity between the large eggshell fragment from Laiyang, Shandong Province (IVPP V 2337) and the half egg studied in detail from Changtu, Liaoning Province (DLNHM D154) (Liu et al., 2013; Zhao et al., 2015).It is noteworthy to mention that this comparative study was merely based on the radial eggshell microstructure of V 2337 and DLNHM D154.The tangential eggshell microstructure of V 2337 was previously unknown due to the lack of tangential thin sections.Thus, the misassignment of V 2337 highlights the importance of the tangential eggshell microstructure in dinosaur egg identification.

As described in the above, the features of the tangential microstructure in the outer portion clearly showed that V 2337 belongs toMultifissoolithus.On the other hand, the compact layer composed of the cones with radial microstructure was first noticed in dongyangoolithid eggs (Fig.3A, B).This layer was also visible in the eggshells ofM.chianensisandM.shimonosekiensis, due to the fact their inner surface was not weathered (Zhang et al., 2019:fig.3A, B; Imai et al., 2020:fig.4), and possibly seen in the eggshell ofDongyangoolithus nanmaensisdespite the weathering of inner surface (Jin, 2013:pl.X2, 3).However, this layer was not observed inSpheroolithus.In comparison with the large cavities in the inner portion of the eggshell, there were very few pores in this compact layer (Fig.4H).Due to the large cavities connected to the clefts on the outer surface, the gas conductance for most portions of dongyangoolithid eggshells would be relatively high.The compact layer near the inner surface could be the main conductive barrier of water vapor and respiratory gases.By contrast, either the middle or outer portion of the eggshells of other oofamilies has the lowest pore density (Zhao et al., 2015).

At present, dongyangoolithid eggs were reported in Zhejiang and Shandong provinces in China, Yamaguchi in Japan and the Sihwa Basin in South Korea (Kim et al., 2009; Zhang et al., 2019; Imai et al., 2020).Multifissoolithuschianensisfrom Yiwu andM.megadermusfrom Laiyang occur in the Lower Cretaceous Chaochuan Formation (119-104 Ma, Aptian-Albian) and the Upper Cretaceous Jiangjunding Formation, respectively (Zhao et al., 2013; Zhang et al., 2019).Multifissoolithusshimonosekiensisfrom Yamaguchi occurs in the Shimonoseki Subgroup (Aptian-Albian, Imai et al., 2020).The dongyangoolithid eggs from the Sihwa Basin occur in the Lower Cretaceous Sihwa Formation (119.8±2.3 Ma, 118.6±2.3 Ma and 125.5±2.4 Ma, Aptian, Kim et al., 2009).The geological ages of the Chaochuan and Sihwa formations, and the Shimonoseki Subgroup suggested that the dongyangoolithid eggs from Zhejiang Province in China, Yamaguchi in Japan and the Sihwa Basin in South Korea are almost contemporary, representing the earliest record of dongyangoolithid eggs in East Asia.The isotopic dating of the Jiangjunding Formation in Laiyang is still unknown.Although the lithologically similar Hongtuya Formation in Jiaozhou can be dated to 73.5±0.3 Ma (Campanian, Yan et al., 2003; Yan and Chen, 2005; Zhang et al., 2008), the hadrosaurineTanius sinensisfrom the same horizon ofM.megadermussuggest that the geological age of the Jiangjunding Formation is Coniacian-Santonian (Hu et al., 2001).Due to the absence of dongyangoolithid eggs in the overlaying Jingangkou Formation in Laiyang and other younger Upper Cretaceous deposits in China (Hu et al., 2001; Wang et al., 2012; Zhao et al., 2013), theM.megadermuscould represent the latest member for Dongyangoolithidae.

5 Conclusions

(1) The study of the tangential sections of the “Spheroolithusmegardermus” from Laiyang, Shandong Province suggested that this large eggshell fragment belongs toMultifissoolithusrather thanSpheroolithus.Due to its extremely thick eggshell, it now represents a new combinationMultifissoolithusmegardermus.The half egg housed at DLNHM represents a new oospeciesS.quantouensis.

(2) The compact layer near the inner surface of dongyangoolithid eggshells is the main conductive barrier of water vapor and respiratory gases.

(3) Dongyangoolithid eggs are currently reported in China, Japan and South Korea and their temporal presence ranges from the Aptian to Santonian in the Cretaceous.

AcknowledgementsThe author sincerely expresses gratitude to Prof.Zhao Zi-Kui (IVPP) for the constructive comments and feedback given to improve the manuscript, and Gao Wei (IVPP) for documenting the excellent images.This study is supported by grants from the National Natural Science Foundation of China (No.42072008, 41972021).